Contemporary Issues and Advancements in Coastal Eolianite Research

Abstract

This review of eolianite research—following a comprehensive overview earlier this century—looks at several areas of overlapping research in coastal eolianite deposition, including the following: tectonic setting; carbonate–sediment type and source areas (carbonate factories); the relationship between relative sea-level change and eolianite deposition; and the evolution of depositional models. Several locations are highlighted in order to emphasize the variety in the eolianite depositional record based on more recent research. In particular, a historical review of eolianite depositional models developed over the last 100-plus years indicates that purely high-stand models of the last century—developed on steep-sided carbonate platforms—do not adequately reflect the complex depositional histories found on other coastal shelves. More recent depositional models emphasize high-stand carbonate production followed by the mobilization of shelf sediments during subsequent relative drops in sea level.

1. Introduction

This review article is not intended as an exhaustive compilation of the literature devoted to eolianites since the research of Brooke [1] but is a presentation of the issues that continue to inform the directions that eolianite research has taken since the early 2000s. Eolianites reflect the environmental conditions in which they formed as uncemented dunes. Carbonate dunes are of scientific interest as they track with changes in sea level, while the composition of carbonate clasts reflects the offshore carbonate system/factory. Changes in clast composition through time also provide information related to changing climatic or oceanographic conditions. Pleistocene-age eolianites are of particular interest as they help constrain the magnitude and timing of environmental changes associated with the glacial–interglacial cycles over the past 800k+ years, helpful in predicting future changes to coastal systems. Eolianite cements also have the potential to track shifts in local-to-regional environmental conditions [2].

This review also highlights several localities that illustrate eolianite and dune deposition: (1) in an unusually high latitude area (the Outer Hebrides, UK); (2) in a transtentional tectonic setting (the Gulf of California) and; (3) in an area where the dominant bioclastic component is derived from rhodoliths (Cabo Verde). As modern dune systems provide a ready comparison or contrast to adjacent eolianite deposits, comparisons are made when possible. Locations discussed in the text are shown using a global Mollweide projection (Figure 1). Satellite images as well as coordinates for localities under discussion—determined using Google Earth Pro (7.3.6.10201)—will also be provided as an additional aid to visualization.

The terms eolianite and calcareous eolianite were introduced by Sayles in 1929 to refer to, respectively, any carbonate-cemented dune and those composed of largely calcareous material [3]. By common usage, the term eolianite now only refers to carbonate-rich dunes and is equated with dune calcarenite, a genetic term describing any detrital rock composed of >50% carbonate grains of sand size [4,5]. Regional synonyms for eolianite include aeolianite (UK), dunerock (South Africa), grès dunaire (Mediterranean), kukar (Israel), and miliolite (India and the Middle East) [1,6].

Carbonate-rich dunes form primarily along shorelines where offshore carbonate production (bioclastic or inorganic) contributes significantly to the aeolian sediments blown onshore. Factors that influence the development of eolianite dunes include the following: wind energy, offshore carbonate production, low siliciclastic input, climate (rate of cementation), tectonic setting, shelf profile (platform vs. ramp), and changes in sea level [2,7]. Eolianite deposits have been primarily identified from Quaternary rocks deposited in subtropical to tropical conditions, although there are exceptions, such as the carbonate-rich dunes of Scotland and Ireland.

Table 1. Climate, tectonic setting, and clast type for eolianite/dune localities in Figure 1.

Locality	Climate	Tectonic Setting	Clast	References *
Ascension Island (AS)	Subtropical	Intraplate	bioclasts	[8]
The Bahamas (BA)	Sub- to Semi-Tropical	Passive	ooids/peloids	[9,10]
Bermuda (BE)	Subtropical	Intraplate	bioclasts	[11]
Coorong Coast (CC)	Warm Temperate Dry	Passive	bioclasts	[12,13]
The Channel Islands (CH)	Warm Temperate Dry	Strike-slip	bioclasts	[14]
The Canary Islands (CI)	Warm Temperate Dry to Subtropical	Intraplate	bioclasts	[15,16]
The Cape Verde Islands (CV)	Subtropical	Intraplate	bioclasts	[15,17,18]
Dirk Hartog Island (DH)	Subtropical	Passive	bioclasts	[19]
Fernando de Noronha Island (FN)	Tropical	Intraplate	bioclasts	[20]
The Gulf of California (GOC)	Subtropical	Transtensional	bioclasts	[21,22]
Israel (IS)	Warm Temperate Dry	Divergent Rift	bioclasts	[23,24]
Llano Agua de los Burros (LAB)	Warm Temperate Dry	Convergent	bioclasts	[25]
Mallorca (MC)	Warm Temperate Dry	Extensional	bioclasts	[26,27]
The Madeira Islands (MD)	Warm Temperate Dry	Passive	bioclasts	[15,28]
The Outer Hebrides (OH)	Cool Temperate	Passive	bioclasts	[29]
The Persian Gulf (PG)	Tropical	Foreland Basin	ooids/peloids	[30]
The Pityusic Islands (PI)	Warm Temperate Dry	Extensional	bioclasts	[31]
Rhodes Island (RH)	Warm Temperate Dry	Convergent	bioclasts	[32]
Rottnest Island (RT)	Subtropical	Passive	bioclasts	[33]
The South African Coast (SA)	Warm Temperate Dry	Passive	bioclasts	[34]
Shark Bay (SB)	Subtropical	Passive	bioclasts	[35]
Sardinia (SD)	Warm Temperate Dry	Convergent	bioclasts	[36]
St. Helena (SH)	Subtropical	Intraplate	bioclasts/ooids	[8]
Shidao Island (SI)	Subtropical	Intraplate	bioclasts	[37]
Yorke Peninsula (YP)	Warm Temperate Dry	Passive	bioclasts	[38]

* References identify clast type in eolianites or dunes at the locality.

provides information (climate, tectonic setting, and type of clast) for localities with eolianites and modern dune systems that are described and discussed further in this review.

A comprehensive list of Quaternary eolianite localities is available, as are inventories of more ancient deposits [1,2,39]. The recognition of eolianites in rock records older than the Quaternary period has been a focus of several studies [40,41,42] as eolianites may be confused with adjacent near-shore or subtidal calcarenites in outcrop or drill core [39]. No single feature of eolian carbonates distinguishes them from subtidal carbonates [41]. Features used to distinguish eolian structures from their subtidal counterparts include relatively large-scale (>1 m) cross beds and well to very-well sorted allochems (<4 mm in size) that are also well-rounded and abraded (Table 1 in [41]; Table 2 in [42]). When present in dunes or sand sheets, low-amplitude wind-ripples (with a high ratio of wavelength to height) inversely grades pinstripe laminations, and adhesion ripples are diagnostic of eolian deposition [41,42]. The presence of rhizoliths and vadose cements that occur below exposure surfaces are also helpful in distinguishing eolian strata from subtidal carbonates [41,42].

The most favorable conditions for the accumulation of carbonate eolianites are generally found between 50° N and 45° S, and more commonly where warm conditions support carbonate production and the trade winds blow [39,42]. With sufficient wind velocity, subtidal carbonate sediments exposed during tidal or higher-order cycles in sea level are winnowed and deposited as coastal dunes behind the beach. Under higher velocity conditions and a relatively low sediment budget, dunes may form inland of the beach with sediments sourced through the deflation of the beach or adjacent dunes [41]. With a sufficiently high sediment budget, a coastal dune system can develop into an oceanward prograding strand plain (a series of transverse sand dunes that pile up against the shore) [43]. Sand plains may develop where there is a high water table, coarser sediments, or vegetation that prevents dune formation [41].

A review of the distribution of eolianites [1] found that somewhat greater than 80% of eolianite deposits occur between 20 and 40° in both hemispheres, with the greatest extent and thickness of eolianite found along the southern coast of Australia and the southern and eastern coasts of South Africa, adjacent to major areas of carbonate production [1]. Preservation of carbonate dune deposits is enhanced in arid regions where there are seasonal rains that promote lithification by secondary cements in the form of crusts, calcrete horizons, and through pedogenic processes—the development of clay-rich soils that act as resistant caps (e.g., the alternating eolianite/paleosol allostratigraphy of Bermuda [3,44]).

2. Eolianite Cements and Climate

Eolianite cements have the potential to reflect changes in regional climatic and oceanographic systems [37]. As mentioned above, the cements that consolidate dune sands in the formation of eolianites depend on seasonal or intermittent rains that facilitate the diagenetic transfer of calcium carbonate from sediment to binding cements [45]. Seasonal changes in the water table also promote the formation of cements in eolianites (e.g., machair sand plains of the Outer Hebrides [46]). Variations in the dissolution of sediments within an eolianite sequence (alteration of high-Mg calcite to low-Mg calcite and dissolution of aragonite) should, in theory, reflect the volume of water that passed through the sediment in the process of cement formation (increase in void-filling calcite spar) as outlined in a study of vadose diagenesis in aeolianites near Warrnambool, SE Australia [47]. A more recent study illustrates potential issues with accepting the evidence of vadose diagenesis with an uncritical eye, showing that variation in diagenetic effects can occur across a single outcrop [48]. Multiple samples of eolianites from arid, semi-arid, Mediterranean, and subtropical settings were compared from more-exposed and less-exposed parts of individual outcrops with variations depending on particle size, percent cement, and sea spray [48]. It was concluded that local (meso-scale) factors commonly have stronger effects on outcrop diagenesis than regional (macro-scale) factors, such as climate [48].

An example of how eolianite cements may reflect changing climatic conditions is an analysis of drill cores extracted on Shidao Island (16°50′ N; 112°20′ E), Xisha Ids, China, which sits within a region affected by seasonal monsoons [37]. The drill core included five eolianite–paleosol cycles, deposited during MIS 3 and the transition to MIS 2. Analyses of the core, including stable isotope and major element geochemistry, particularly showed variations in the δ18O curve and Sr, Mg, Fe, and Al concentrations. In conjunction with the observation of changes in the bulk composition of the eolianite bioclasts, it was shown that more extreme diagenetic alteration of one of the eolianites (Unit 2) resulted in the bulk loss of bioclasts, lower Sr and Mg concentrations in the bulk rock analysis, and more negative δ18O values. It is also proposed that analysis of the core reflects dune formation occurred in a dry-cold climate with higher velocity winds deflating the upwind shelf during relatively low-stands in sea level. In addition, an increase in Fe and Al concentrations was linked to an increase in terrigenous dust. It was proposed that paleosols, interbedded with the eolianites, were formed in more humid environmental conditions [37].

3. Eolianite Dating Methods

Depending on the geological setting and potential age of an eolianite deposit, a variety of dating techniques have been used. An excellent example of how multiple geochronologic methods can be used to investigate the timing of eolianite deposition and consolidation is a study in the Channel Islands of California (USA) [14]. In conjunction with the mapping of stratigraphic relations with underlying marine terraces and related geosols, a combination of radiocarbon dating (14C), luminescence chronologic methods—including optically stimulated luminescence (OSL) and infrared stimulated luminescence (IRSL)—amino-acid geochronology, and previously reported uranium series (U/Th) datasets are used to constrain the ages of eolianite deposits (~80,000 to ~120,000 yr) and correlate them with the marine isotope stages [MIS] 4, 3, and 2 [14,49].

The increasing number of studies using luminescence techniques (OSL, TL, and IRSL) for dating coastal dune deposits, including eolianites, is illustrated by the review of ~200 studies from around the world and the extensive use of OSL measurements (n = 51) in a recent investigation of Lower to Upper Pleistocene coastal deposits (MIS 22 to MIS 2) on the Pityusic Islands of the Balearic Archipelago in the Western Mediterranean [31,50].

When eolianites do not include a siliciclastic component (e.g., Bahamas (32°18′ N; 64°46′), Western Australian coast), amino-acid racemization (AAR) analyses can be used to determine relative ages on whole-rock samples [51,52]. This method is particularly helpful if used in conjunction with (U/Th) analysis of suitably sized bioclasts from adjacent stratigraphic units [53,54,55]. Generating age data for eolianites can be difficult [55,56] but any model for eolianite deposition suffers if not supported by appropriately precise age data [56], particularly as models are becoming more nuanced with respect to variations in the fifth-order sea-level curve and more subtle stadial/interstadial variations [26,57,58,59,60].

4. Eolianites and Tectonic Setting

Eolianite sediments accrete and are preserved where accommodation space is available downwind of the source area for carbonate sand. Differences in coastal landscapes are, in part, reflective of a region’s tectonic history, as are the repose and spatial dimensions of offshore source areas where carbonates are produced [2,43]. The volumetrically largest eolianite deposits are found on tectonically passive margins such as coastal south-central Australia [1,12] and coastal South Africa [34,57,61,62] where complex shore–parallel coastal dune complexes are preserved adjacent to broad continental shelves with highly productive carbonate systems.

Though of much smaller volumes, dune sequences are also preserved in island areas that sit in stable settings such as Bermuda, where stacks of Pleistocene eolianites sit upon a slowly residing volcanic core astride the Bermuda Pedestal (Figure 2)—an extinct shield volcano [63]—or the Bahamas, where islands and smaller cays largely composed of oolitic eolianites are perched upon on an extensive carbonate platform, near the base of which are Upper Jurassic bioturbated limestones (Figure 3) [64].

An interesting counterpoint to stable or slowly subsiding passive margins is the Coroong to Mount Gambier Coastal Plain of Australia where Quaternary emplacement of volcanics has uplifted the region and preserved a series of shore-parallel belts of eolianite that reach 80–100 km inland along 300 km of coastline and represent almost 1 million years of sea-level high-stands (Figure 4) [13,56,65,66].

A region with a complex tectonic history where eolianites have been extensively studied is the Mediterranean Basin (see [1,31,36] and others listed in Table 1). As the basin touches or includes parts of the Eurasian, African, and Arabian Plate boundaries, eolianite localities within the region can be found in both extensional and convergent settings (Table 1) with eolianites preserved in both coastal plains (e.g., [58]) and rugged, cliff-sided coastlines (e.g., [27,36]).

An example from a tectonic region where eolianites have not been extensively studied is the Pleistocene eolianites (MIS 5e-5a) from a convergent tectonic boundary (subduction zone) described from the coastal region of the Atacama Desert (Llano Agua de los Burros) in Chile (27°55′ S; 71°05′ W), where the shelf drops precipitously toward the Peru–Chile Trench and wind velocities were sufficient to transport carbonate sediments over 3.5 km inland and over 250 m in elevation above the modern coastline [25].

Additional examples of eolianites from a less-common tectonic setting are found along the coast of the Gulf of California (GOC), an extensional/transtensional tectonic basin [21,22]. Initially, the Gulf of California region formed as part of a generally E-W extensional tectonic system in the Late Miocene (~12 to 6 mya). By 5 mya, the extension within the “Proto-Gulf” had rotated 30–50° to become a NW-SE transtensional system with the northern and southern parts of the Gulf of California opening at different rates. Eventually, the Baja California Peninsula became part of the Pacific Plate as the boundary with the North American Plate was transferred into the Gulf of California [67,68]. As a result of the shift to a transtensional tectonic system within the Gulf of California, fault blocks of Miocene-age volcanics along the Baja California coast have been uplifted along with Plio-Pleistocene rocky shore, eolianite, and other related deposits [22,67,69].

As the Gulf of California now extends for more than 1000 km, those parts of the coast that project into the Gulf are subject to high wind velocities due to fetch alone (Figure 5) [21,70]. Winter winds can sustain velocities in the range of 5–7 m/s over multiple days, with gusts of 8+ m/s not unusual [21,70]. As a result, a wide variety of modern dune field morphologies with varying percentages of carbonate material are also found perched on, ramped against, or in pockets along much of the western edge of the Gulf of California where accommodation space is available [71]. These modern dune systems provide a benchmark model for interpreting eolianites preserved along the coast of the Gulf [21,22,71]. A survey of 84 individual dune fields along the peninsular coast of the GOC indicates an average migration direction of S8° E within the modern wind field that blows directly down the axis of the GOC with an average azimuth of S24.5° E [70,71]. Plio-Pleistocene coastal and inland eolianites are best preserved at Punta Chivato and Isla Carmen [21,22]. These localities from the central region of the Gulf of California provide interesting examples of eolianite deposition in this rugged and seasonally windy environment.

At Punta Chivato, a narrow Plio-Pleistocene valley extends behind the north-facing beach of Ensenada el Muerto (Figure 6) [21,22]. Behind a cobble beach, the eroded remnants of an eolianite deposit extend 400 m to the south, in places crossing the 40 m contour while also rising 4 to 7.5 m above the arroyo stream bed. Cross-bedded calcarenite dipping SE at 32° is found along the eastern wall of the arroyo, while higher on the western wall, cross-beds are found dipping SW at 28° where wind-blown sediment collected against the arroyo walls (Figure 6b) [21]. Point counts of rock samples show 44% of the clasts as volcanic lithics with bioclasts (mollusks, coralline red algae, and echinoid) accounting for 56% of the sample, though dissolution of the clasts (resembling mollusk grain) suggests an undercount of the molluscan component [21]. Stratigraphic relationships with a presumed MIS 5e terrace outside of the canyon suggest a similar age for Punta Chivato eolianite [21].

The modern dune and beach at Punta Chivato are representative of many north-facing rocky promontories or island coasts in the Gulf of California and provide a modern example of dune deposition during the current high-stand in sea level. The sediments on the shore and in dunes plastered along the cliff fronts at Punta Chivato are primarily composed of mollusk fragments (50–86%) derived primarily from clams, such as the locally abundant infaunal bivalve, Megapitaria squalida. Along the coast behind Ensenada el Muerto at Punta Chivato, modern dune sediments are commonly found blown up and over the 20 m contour and in places over and around the 40 m and 60 m contours as well (Figure 6c) [21]. However, the modern coastal dune system is relatively starved of sediment and does not extend inland as extensively when compared with the Pleistocene eolianites preserved nearby, suggesting that the eolianite was deposited under a different set of conditions (e.g., sediment budget, relative sea level, coastal morphology, and wind velocity) [21,22].

To the south of Punta Chivato, on the north end of Isla Carmen, dune rocks are found stacked or draped over andesite ridges on the NW side of the island at Bahia Oto and the NE side of the island at Puerto de la Lancha (Figure 5 and Figure 7a–c) [22]. At low tide, the depositional sequence at Oto Bay can be seen to consist of a 2-m-thick conglomerate of andesite boulders eroded from the underlying Miocene basement volcanics, followed by 1.5 m of pink, coarse molluscan sands (beach deposit), which is in turn overlain by 4 m of beige-colored eolianite pock-marked by dissolution cavities and composed of 0.5 mm fragments of molluscan bioclasts and volcanic lithics. A Pleistocene faunal component of the basal conglomerate provides the only age limit on the overlying eolianite [22].

To the east at Bahia de la Lancha (Figure 7b), a succession of five fossil carbonate dunes stack to a height of ~70 m are exposed across 300 m of coastline (Figure 7d,e). Sand samples from an active dune deposited within a slot canyon cut into the north face of the eolianite suggest a bioclast component of about 68 percent, dominantly composed of mollusk fragments [22,67]. The base of the eolianite sequence drapes over faulted blocks of Miocene volcanics, which likely controlled the shape of the valley during deposition [22]. An Upper Pliocene-age estimate is assigned to the lowest dune rock unit [72].

An additional outcrop of interest is a karstic, eroded eolianite found approximately 2.5 km south of the Bahia Oto beach and banked behind and against the back of a 200-m-high, E-W trending ridge of andesite (Figure 7c,f). Only 16 m thick but extending 450 m across the valley, an extensive talus pile below the outcrop suggests that this falling dune was originally much more substantial in the past [22]. The relatively large volume of calcarenite preserved at Bahia de la Lancha and the inland eolianite deposit suggest a much larger accommodation space for beach and dune deposits than presently exists along the northern coast of Isla Carmen.

5. Carbonate Sediments Track Source Environments

The study of carbonate-rich dune systems has an additional advantage over siliciclastic-dominated eolian sandstones in that the bioclasts of the offshore marine environment are sampled with each successive depositional event. For example, a study documenting changes in the cool-water carbonate depositional system along the southern coast of Australia found that the coastal eolianites of the Yorke Peninsula dependably represented the upwind marine environment [38]. A similar analysis on the island of Bermuda documented a shift in the bioclastic composition of eolianites (coralline-rich to Halimeda-rich sediments) that reflected oceanographic controls (cool to warm conditions) consistent with other palaeozoological studies of the MIS 5e sea-level inflection from across the globe [11]. An interesting development in this relationship of eolianite to the upwind source area is the conundrum presented by the comparison of pre-Sangamon eolianite sediments that make up the windward islands and cays of the Bahamas with the modern bioclastic-rich carbonates found on the narrow shelf to windward. No oolites are presently found along the edge of the Blake-Bahama Escarpment while, the composition of dune ridges on perimeter islands or cays (e.g., Eleuthera, Cat, or Joulters) are primarily composed of oolites or oolitic-peloidal grainstones (Figure 3a,b) [9,10].

Suggested models for the origin of the eolianite ooids include the transportation of ooids from the western basin side of the Bahamas Bank, while theoretical calculations suggest that the narrow shelf along the windward edge of the bank could possibly produce the necessary volume of ooids in the proper conditions. A plausible alternative narrative, related to shelf size and therefore ooid production volumes, is the proposal that scallop-shaped areas along the windward escarpment are evidence for the erosional collapse of what would have been a wider shelf in the past (Figure 3b) [9].

Connecting bioclasts and inorganic carbonate constituents of eolianites with their upwind, offshore carbonate “factories” leads to the evolving models for the classification of oceanic carbonate systems on a global scale. In the previous review of the global distribution of eolianites, Brooke [1] followed the works of James [73] who classified carbonate sediments as either heterozoan or photozoan associations. A new ecosystem approach to carbonate production introduced a tripartite division (tropical shallow-water, cool-water, and mud mound) of carbonate factories [74,75]. This conceptual framework is, in turn, modified further by splitting the Tropical (T) factory into a Photozoan-T factory (roughly 30° N–30° S)—bounded by the 18 °C winter limit—and a Biochemical factory (Persian Gulf), while the Cool-water (C) factory was subdivided into the Heterozoan-C (poleward of the Photozoan-T factory limits) and Photo-C (Mediterranean Sea) factories [76]. A predictive model for the distribution of the carbonate factories across the globe was then created using the main environmental controls of light, temperature, nutrients, salinity, substrate, and carbonate saturation [77] (Figure 8).

A more explicit description of the environmental controls on the carbonate factories that contribute to eolianite sediments is presented as part of a major synthesis of marine carbonate systems [7]. Not unexpectedly, the main environmental controls on the Photozoan-T factory are light and temperature (e.g., tropical reef areas and the Great Bahama Bank), while the Persian Gulf is considered a special case of the T-factory (Biochemical) where salinity and temperature are the controlling factors, with corals and other bioclast producers absent or limited to deep water areas [7]. The main environmental control on the Heterozoan-C factory is nutrients (e.g., southern coast of Australia), while the Photo-C (Mediterranean) is a variant controlled by light and temperature [7]. Local environmental controls that may have an impact on carbonate production include ocean currents, upwelling, shallow-water dynamics, and the input of terrestrial sediment and water [7].

6. Rhodoliths as Eolianite Bioclasts

Reflected in previous reviews and illustrated by many of the depositional environments described above, a majority of studies focus on the role marine invertebrates (mollusks, foraminifera, coral, echinoderms, and bryozoans) play in the production of bioclasts or the contribution of inorganic carbonate (e.g., ooids/peloids) to eolianite sediments [1,17]. The contribution to aeolian sediment by the Rhodophyta (coralline red algae) in the form of rhodoliths has become more apparent as there have been multiple studies since the last major review of eolianites [1,15,32,78]. Rhodoliths are unattached spherical to subspherical structures created by the concentric growth of precipitated calcium carbonate around a central core, often a small pebble or shell fragment. As algae are dependent on light for photosynthesis, rhodoliths thrive in relatively clear water [79,80]. A dependable rolling motion—as rhodoliths grow concentrically outward—means rhodolite beds are found below fair-weather base and above storm-weather base or within areas where currents are tidal or driven by surface winds [79,80]. For reference purposes, see studies of rhodolith-rich eolianite deposits that include images of rhodoliths in the field (see Figure 9.4 in [15]) and of rhodolith-derived bioclasts in thin-section (see Figure 6 in [17]).

Rhodolith beds play an important role in the ecosystems where they are found [79,80]. They are habitat modifiers, with hundreds of species (cryptic, soft-bodied, and hard-bodied) living within the beds or in the natural spaces created by the branches of individual rhodoliths. A habitat for polychaete worms, crustaceans, mollusks, and echinoderms, the hard surfaces of individual rhodoliths also support a variety of sessile organisms and borers [79,80]. Therefore, abundant rhodolith clasts in a dune or eolianite deposit indicate not just rhodoliths offshore but the entire ecosystem that rhodolith beds support [79,80]. In areas where rhodolith-rich deposits can be found representing significant time intervals (e.g., Pliocene and Pleistocene deposits in the Gulf of California), analysis of preserved rhodoliths can provide information on changes in community diversity or stability through time—as has been the case with corals [79]. Modern rhodolith beds have been reported from diverse areas such as, but not exclusively from, the following: the east coast of Brazil (2° N to 27° S); the shelf SE of Okinawa, Japan; the Western Australian coast centered on Shark Bay; the shelf outboard of the windward islands of the Caribbean; along much of the Gulf of California coastline and south to Puerto Vallarta in Mexico; along the Atlantic coastal areas of Northern Europe; as well as in the Mediterranean [79,80,81,82].

Thus, modern rhodolith beds are found within all the carbonate factory zones (Figure 8), with the exception of the Persian Gulf. Though species of algae that construct rhodoliths are found in tropical-to-polar latitudes, the low-latitude areas within the trade-wind zones of both hemispheres are where rhodoliths contribute most to beach and dune sediments [15,17,18,79,81]. Dunes or eolianites with high concentrations of rhodolith debris are found within the Gulf of California [65]; on the Macaronesian Islands of the North Atlantic [15,17,18,28,83]; Dirk Hartog Island, Shark Bay, Western Australia [19]; and Middle-Pleistocene eolianite on the island of Fernando de Noronha (3°51′ S; 32°25′ W) off the NE coast of Brazil [20] (Figure 1). A weakly cemented Late-Pleistocene mixed siliciclastic–algal sand dune is reported from the eastern Mediterranean island of Rhodes as well [32]. The paucity of eolianites reported with high concentrations of rhodolith debris suggests either that rhodoliths and rhodolith-rich dunes are in environments with low preservation potential or they have not been the focus of a sufficient number of studies to reflect their true presence in the geologic record.

Examples from the Cape Verde Islands off the Atlantic Coast of Africa (Figure 9) suggest that rhodoliths can be found as a major component in modern dune systems and fossil dune deposits, and may be present as an overlooked component of eolianites in other parts of the globe [15,17,18]. In the Cape Verde Archipelago, an extensive eolianite deposit is found on Maio, the easternmost island of the leeward islands, about 600 km west of Dakar, Senegal (Figure 1 and Figure 9). The Cape Verde Islands are a group of shield volcanoes with a varied geologic history related to the opening of the Atlantic Ocean. On Maio, a sizable portion of the landscape is composed of uplifted Jurassic- and Cretaceous-age deep-ocean deposits of black shale and pelagic limestone [17,84].

The deposit at Lomba Greija is a north–south-oriented prominence on the eastern side of the island, 2.25 km inland from the shore, near the village of Pilão Cão (Figure 9b) [17]. Approximately 800+ m, 40 m wide, and 127 m high, this Pleistocene-age deposit includes a basal marine conglomerate overlying Cretaceous basement rocks, back beach deposits, and a 35-m-thick eolianite that caps the sequence. Preserved slip faces dip 28–32° SW and SE with stoss slope beds dipping 8–10° NE indicating the local wind field that is the result of the interaction between the Pleistocene landscape and the regional aspect of the trade winds (Figure 10a,b) [17]. Sedimentological analysis of the Lomba Greija dune rock shows significant loss (29%) due to bioclast dissolution. Micritic matrix accounts for 38% of the sample, while only 5% of the eolianite is composed of basaltic and lithic grains. The remaining component of bioclasts has coralline red algae as the largest contributor at 74%, followed by 11% for bivalve mollusks and 6% for corals [17].

To the north and east of Maio, the island of Boa Vista has a relatively unstudied set of modern and fossil dunes (Figure 9a,c). The largest modern dune field falls within a wind corridor that originates on the NW edge of Boa Vista Island (NE of Sal Rei) and extends from the Boa Esparança coast south-west toward the Praia do Curralinho beach where aeolian sands re-enter the Atlantic Ocean [85]. In Figure 9c, the modern dune field can be seen to split around the airport with the barchan dunes of the eastern part of the dune field coalescing into barchanoid ridges as they are somewhat stabilized locally by vegetation, while the western lobe of the dune field somewhat skirts the bay at Cabeçadas. Barchnoid dunes that originate at Praia de Chaves beach, SW of Cabeçadas (Figure 10c), eventually exit along the SW corner of the island (Figure 10e). Other smaller dune fields can be seen perched on the more rugged coastline of the north-eastern, eastern, and southern side of the island [85]. A recent study of beach sands from across the Cabo Verde Archipelago, including beaches (n = 7) from the perimeter of Boa Vista, indicates that the modern carbonate fraction (dominantly bioclasts) composes roughly 70–90% of the Boa Vista sands analyzed [18]. Abundance data reported for the carbonate fraction of the sands show a relative predominance of red algae clasts (48–87%) with bivalve (11–44%), echinoderm (0–14%), and minor amounts of bryozoan plus foraminifera clasts (0–5%) accounting for the remainder of the bioclasts across all samples analyzed [18].

A recent preliminary study of dune systems on Boa Vista by a field party that included Dr. Markes Johnson of Williams College (Personal Communication, 6 September 2024) suggests that extensive eolianites underlay the modern dune deposits found across the island (Figure 10d,f).

7. Unusual Carbonate Dunes/Eolianites of Scotland and Ireland

An exceptional case of carbonate dune and sand plain deposition appears along the cool temperate Atlantic coasts of Scotland and Ireland (Figure 11a), well outside the usual subtropical-to-tropical distribution of eolianite and possibly the only carbonate-rich sediment facies within the zone of the last glaciation (MIS 2) [1,29]. In this region, carbonate-rich dunes form the predominantly western edge of a fertile, sandy, and carbonate-rich coastal plain ecosystem (machair in Scotland) and are found in the NW coastal Highlands and island areas of Scotland, as well as the NW coast of Ireland from Malin Head south to the Aran Islands and the northern coastline of Galway Bay [86,87,88,89].

From beach to hillside, the machair ecosystem typically includes carbonate-rich coastal dunes behind which is a sand plain stabilized by vegetation (non-grass) that has adapted to carbonate–sand environments [88]. In almost all cases, the carbonate dune and sand ramp portions of the machair ecosystems are underlain by eolianite cemented by seasonal fluctuations in the water table [88]. In Scotland, machair dunes and sand plains are found in the Hebrides from Islay north of Shetland. Along the Atlantic coast of the Outer Hebrides, the most extensive expression of the machair ecosystem is found along the western coasts of the islands SW of Harris (Figure 11b) [29,87,88,90].

The carbonate-rich dunes of the Outer Hebrides are considered to be part of an unusual barrier/lagoon system whose distribution is strongly controlled by the basal bedrock of the region (Proterozoic Gneiss) [90]. As the region was covered by a kilometers-thick ice sheet during the last continental glaciation, the system is relatively starved of siliciclastic sediments as there are no streams (historically or presently) that would carry post-glaciation sediments across the low-lying, low-angle, west dipping landscape of the region [29]. The continental shelf to the west of the Outer Hebrides is shallow dipping (0.2%) and broad in dimension (>50 km to ≈130 km wide) [29]. Glacially derived sediments deposited on the continental shelf are presumed to have been redistributed with the Holocene rise in sea level. Winds are highly energetic in the region, primarily out of the south-west, and averaging 6–8 m/s year-round [90]. Modern dune sediments are predominantly molluscan in origin with greater volumes of fragmented shell material pushed ashore during storm events [29]. Fragmented molluscan shell is usually reported as the dominant sediment type in the carbonate dunes of both Scotland and Ireland [29,89].

A study utilizing radiocarbon and optically stimulated luminescence (OSL) dating methods outlined in broad strokes a chronology of sand movement onto areas of the Uist Islands (Figure 11b) [91]. Initial accumulations were of quartzose sands (≈13,700 to 10,500 BP-cal) with significant accumulations of carbonate sand starting ≈ 9000 ago. Multiple episodes of sand/dune deposition occurred during the remainder of the Holocene with the last significant episode (600–200 BP-cal) associated with the latest Medieval Warm Period (MWP) and earliest Little Ice Age (LIA) [91]. Subsequent work from additional sites along the Uist Island Atlantic coasts suggests substantial variation in the sand incursion chronology but emphasized that the bulk of the machair (carbonate-rich) sand inventory was deposited between ≈5800–4200 (calendar years), a period known for coastal sand movement across NW Europe [29]. It is likely that the machair dune and sandplain ecosystem will disappear due to coastal erosion as the sea level continues to rise [88].

8. Development of Depositional Models

Though glacio-eustatic changes in sea level are not considered a requirement for eolianite deposition in much of the geologic record, it has become a primary focus in the development of Pleistocene eolianite depositional models [2]. In the first attempt at modeling the development of a carbonate dune, Darwin (Part 2, pp. 86–88, Superficial Deposits [8]) proposed that very fine material (bioclastic and oolitic) on the island of St. Helena (15°58′ S; 5°42′ W)—found between 600 and 700 ft above sea level—must have accumulated at a previous time when there was a marine shelf, “like that of Ascension” (7°57′ S; 14°22′ W). Darwin [8] recognizes that wind is the mechanism of deposition after reading about the violence of the trade winds and the description of similar deposits on the Canary Islands [16]. He also surmised that the relationship between land and water must have been different, though Darwin proposed “elevation of the land”, as the concept of eustatic changes in sea level was not yet conceived [8,92]. That percolating rainwater was the agent for the lithification of the calcareous material into rock was the final touch in his analysis [8]. Thus, almost all the components for a model for the origin, deposition, and lithification of carbonate-rich aeolian sediments were in place.

As part of a subsequent study on the geology of St. Helena, Daly [93] noticed offshore patches of calcareous debris, which led him to propose that a drop in sea level of a moderate speed would allow for the mobilization of the sand deposits beachward with the wind moving them further landward. He also makes the case for a 5-m eustatic sea-level drop related to the last glaciation, compiling and discussing data from around the globe and suggesting that this type of analysis will be useful in the study of “earlier oscillations” [93]. Thus, the case for sea-level drop and the deflation of shelf sediments in the formation of carbonate-rich dune sands is conceptually realized.

However, it is the subsequent work by Sayles [3] on the islands of Bermuda where the case for the link between the alternating sequence of five eolianite formations and intervening paleosols with Pleistocene glacio-eustatic sea-level cycles is made. He proposed that dune development was triggered by exposure of lagoon sediments as global sea level dropped due to continental glaciation, coupled with increased winds and a loss of vegetation as cooler climatic conditions prevailed. Intervening paleosols were proposed to have developed as relict weathering features during interglacial periods (Figure 2b) [3]. Observations made by Sayles [3] that have implications on subsequent models proposed for the relationship between eolianite genesis and the recession of relative sea levels are: (1) that modern dunes are rare on Bermuda (interglacial period) and; (2) there is a relatively older group of eolianites that form the core of Bermuda, while younger dune sets form the perimeter of the island. In a more recent review of the models proposed for eolianite genesis on Bermuda, it is noted that the unstated but implied aspect of Sayles’ low-stand model for eolianite development is that the extensive lagoon (~12 km wide) along the north-western border of the island group (see Figure 2), as well as other perimeter shelf areas, act as a sink for calcareous sediments generated during high-stand conditions prior to becoming the source material for Pleistocene dunes during subsequent sea-level fall [94].

Contrary to the model proposed by Sayles [3], a rise in relative sea level (RSL) is proposed by Bretz [95] and endorsed by others [96,97]. Bretz proposed that younger, perimeter eolianites along Bermuda’s north shore were developed as rising sea levels progressively moved sediments out of the lagoon, onto the beach, and landward no further than behind the beach. In this proposed sequence of events, lagoon sediments are washed to the beach, winnowed by winds as they are blown into dunes, and cemented while they sit just behind the beach. In this way, the dunes are preserved as eolianites sitting above or adjacent to their source beach, preserved as a subtidal calcarenite.

Key to this model is overriding the energy-dampening effect of the reef tract outboard of the lagoon [95]. Sea levels must rise quickly enough to outpace reef catch-up so that waves capable of transporting sand from the lagoon to the beach are not impeded by the reef [95,97]. Vacher [97] suggests different modes of eolianite formation for the north and south coasts of Bermuda as the reef structures associated with two sides of the island are very different. Dune-building episodes along the north side of the island—which faces the lagoon (see Figure 2b)—occur only during sea-level rise, while eolianite deposition along the high-energy south coast occurs only when relative sea level exposes the slow-growing boiler reefs (composed of algae and vermetids, not corals) and associated sediments along the comparatively narrow (~1 km wide) shelf. North- and south-shore eolianite deposition would, therefore, be active or stalled depending on the RSL and not at the same time (Figure 19 in [97]). A further requisite criterion for this depositional model is the prompt cementation of the carbonate dunes by rainwater, preventing further landward movement and preservation of the foredunes adjacent to their beach source [97].

The expectation that younger eolianites could not overtop or impinge on the older, smaller core of eolianites that make up the backbone of Bermuda is an underlying assumption for both the Vacher depositional model and a preferred stratigraphy for Bermuda’s eolianite sequences [97]. In support of the idea that younger eolianites were cemented near the beach, Vacher [97] illustrates what is described as a typical cross-section for a dune on Bermuda: a foreset wedge draped or rolled over by a windward topset wedge with the intervening sub-horizontal surface merging with the foresets to landward. Vacher implies that this structural configuration shows vertical dune growth by aggradation and not the recycling of older sediments, as would be expected if the dune were moving landward (Figure 10 in [97]).

In laying out a preferred stratigraphy for Bermuda, Vacher [97] rejects the proposed previous stratigraphic schemes [3,96], instead arguing that analytical error allows for the inverting of radiometric ages and, therefore, the reordering of associated marine units and eolianites into a “preferred” stratigraphic order [97]. Additionally, this stratigraphic scheme allows for “missing” eolianites to be attributed to removal by subsequent high-stand events as opposed to being emplaced and preserved inland after overtopping the older eolianite core of the island [97]. It should be noted that age determinations for Bermudian eolianites are difficult to achieve as there is no suitable quartz fraction available for analysis. Therefore, radiometric age determinations from bioclasts preserved the marine units (beach calcarenites) that underlay eolianites must be used in determining the time-stratigraphic ordering of the island’s dunes. As a consequence, organizing the eolianites of Bermuda in a refined time-stratigraphic order has been an evolving process, complicated by the fact that only a portion of Bermuda Island’s geological record is found at any single locality. For example, the idea that “terra rossa” paleosols might be island-wide depositional markers [96] was later shown to be erroneous [98].

As part of their thorough and extensive structural analysis of Bermuda’s eolianites, Rowe and Bristow [59] show that the typical dune of Vacher [97] actually mirrors the architecture of a sub-critical climbing transverse dune [99]. In addition, their structural analysis of Bermuda’s eolianites using ground-penetrating radar provides ample evidence that younger dunes did “transgress” or advance inland over the older eolianitic core of the island [59]. If Bermuda’s Pleistocene dunes were not emplaced as transverse coastal dune sets, a re-evaluation of the stratigraphic and age data used to assemble the geologic story of Bermuda’s eolianites with respect to changes in Pleistocene sea levels becomes necessary.

Building on an earlier, detailed facies analysis of the interbedded coastal dune, beach, and paleosol deposits of the Belmont Formation (MIS 7) [53], Rowe and Bristow [94] review previous U-series age determinations for coral fragments collected from shoreface and foreshore calcarenites at the base of the upper three (Belmont, Rocky Bay, and Southampton) of Bermuda’s formations and provide new age determinations of their own. One of the primary results of the re-evaluation of facies relationships and age data is the adjustment/positioning of the three formations and their member eolianites in relation to the [MIS] 7.1, 5e, and 5a peaks of the Pleistocene eustatic sea-level curve [94] and a consequential depositional model. Rowe and Bristow [94] conclude that, in all three cases, eolianite deposition occurred during the drawdown in sea level following an RSL rise and high-stand, during which carbonate sediments accumulated on the lagoon and shelf, a refinement of Sayles’ original model.

The model developed for Bermuda by Vacher [97] was also applied to the extensive eolianite deposits of the Bahamas by Carew and Mylroie [100,101], who subdivided eolianite deposition into three phases (transgressive, stillstand, and regressive) corresponding to the runup, maximum flooding, and regression of water from the Bahamian carbonate banks during high-stands in sea level (Figure 3). As both Bermuda and the Bahamas occur on steep-sided platforms, carbonate production would be restricted to times when advancing, stillstand, or regressing sea levels occur without exposure of the bank [2,100]. Eolianites developed in early phases of the runup to sea-level high-stands would have low preservation potential as the continually encroaching beach would cannibalize these dunes as well as potentially erode eolianites deposited during earlier high-stand events [100]. Paleosols would have been developed during low-stands when the banks were exposed. In addition, source beach and eolianite would move in concert and be cemented relatively quickly, as with the “Vacher Model” proposed for Bermuda [97,100,101].

Given the 1000+ km distance from Little Bahama Bank to the Caicos Bank to the south-east, it is not surprising that the geologic record for the Bahama Banks is complex. As noted by Kindler and Hearty [55], linking depositional events to specific sea-level events is challenging. As such, the stratigraphic column for the Bahamas has evolved since the initial depositional history proposed by Garret and Gould on New Providence Island [102]. A more recent synthetic column that reflects more recent studies on Mayaguana Island is provided by Kindler and Hearty, who also review key studies—focused primarily on outcrops from San Salvador, Eleuthera, and Great Inagua—that have advanced the physical stratigraphic understanding of the Bahamas up to now [55].

Representative of the multi-method approach needed to unravel the complex sequence of depositional events across the Bahamas is the work of Kindler and Hearty establishing the Whale Point Formation (WPF) [55]. Utilizing morphostratigraphic, stratigraphic, sedimentological, and petrographic methods anchored by amino-acid racemization (AAR) analyses from 11 islands across the island banks, Kindler and Hearty reposition eolianites and related paleosols—previously thought to be related to the (MIS) 5e maximum in sea level—to the younger (MIS) 5a high-stand. Adding to the evolving thinking around depositional models for the emplacement of eolianites, they found that the WPF eolianites advanced landward and progressively away from their beach source areas, similar to what was found on Bermuda by Rowe and Bristow [55,94]. In addition, Kindler and Hearty proposed that the two eolianites and the scant intervening protosol of the WPF represent deposition during phases of RSL drop, related to two sub-peaks in the MIS 5a high-stand record, with carbonate sediments produced during the preceding, sequential high-stands [55].

In contrast to the advancing eolianites described from Bermuda and the Bahamas, seaward prograding dunes are less commonly recognized and only from the Late Holocene. In the Bahamas, strand plains of Late Holocene age (MIS) 1, created by multiple beach ridges that sequentially accrete as the system progrades oceanward, are described on San Salvador, Eleuthera, and both the Great and Little Exuma Islands [100,103,104]. Grouped together in the Hanna Bay Member of the Rice Bay Formation, eolianites are found above or inboard and laterally adjacent to foreshore and backshore deposits, evidence that dunes either prograded oceanward over beach deposits or that the beach deposits were the source area for the ridges forming the strand plain [104].

The range of whole-rock radiocarbon ages obtained from the Hanna Bay Member suggests deposition during and somewhat beyond the Medieval Warm Period (MWP) [105], similar to the time frame for the deposition of sands in the Outer Hebrides of Scotland [93]. Rejecting the idea that deposition of the intertidal and eolian deposits of the Hannah Bay Member was driven by a decelerating rise in eustatic sea level, it is proposed that more ephemeral sea-level high-stands, related to increased storminess during the MWP, resulted in the oceanward progradation of these strand plains [103,104,105,106]. It is also surmised that these strand plain deposits will not survive continued sea-level rise [104]. This prediction reflects the generally accepted proposition that many interstadial or interglacial variations in the past may have resulted in eolianite formations that were eroded away during subsequent and more significant sea-level high-stands [97,100,104].

9. Depositional Models on Coastal Shelf vs. Rimmed Platform

As described above, depositional models for eolianite deposition have been—since the initial work of Darwin [8] and Daly [93]—developed in reference to steep-sided and rimmed platforms (Bermuda and the Bahamas), where deposition of sediments can only occur when the RSL overtops the edge of the platform [96,97,100]. Therefore, it is necessary to compare eolianite deposition on coastal ramps since the high-stand depositional model—developed on rimmed and steep-sided platforms—in some cases has been unilaterally applied to eolianites deposited on coastal shelves [107,108].

In contrast to carbonate platforms, deposition can occur on a coastal ramp with sea-level regression as the carbonate factory is able to track with the shoreline [2]. The limiting factors in this depositional system become the width/profile of the downwind shelf area as this delimits the volume of sediment that can be stored in the offshore environment and the extent of the RSL drop relative to the edge of the local shelf. Examples of broad coastal shelves with substantial carbonate production include the south-west corner of the Persian Gulf [30], the southern tip of South Africa (Figure 12) [34], and along the southern and western coasts of Australia [33,66].

Strand plains, composed of multiple shore-parallel beach ridges that accumulate sequentially oceanward from the coast, have commonly been linked to high-stand sea levels along these broad shelves [1]. For example, the low-angle, broad (150+ km) coastal shelf off of the Coorong Coast Plain of Australia (Figure 4) has a cool-water carbonate factory with a high rate of bioclast production [56,108]. The extensive eolianite ridges produced over the last 800k+ years are interpreted as being shoreline barriers correlated with successive Pleistocene interglacial sea-level highs [13,56,65]. Uplift of the region due to Quaternary neotectonism has led to the elevation and spatial separation of these paleoshorelines after deposition, though several of the ridges are compound structures of multiple, stacked dune sets that complicate deciphering the depositional chronology [13].

In comparison, the more tectonically stable coastal shelf of Western Australia, near Perth and Rottnest Island, gently deepens to around 75 m before any significant shelf break. As along the southern coast of Australia, the onshore eolianite ridges have very complex depositional histories as earlier ridges have been eroded, reworked, or overstepped by younger preserved dune systems (see Figure 7 in [33]). These composite deposits are mirrored by offshore structures that appear to be drowned eolianite ridges representing relict coastal barriers. Comparable eolianite ridges with similarly complex depositional histories are also found on the coastal plains of Israel [109] and South Africa [110,111], while analogous submerged aeolianite structures are found along other parts of the Western Australian, Southeastern Australian, and South African shelves [111,112,113,114]. Taken as a whole, there is the suggestion that stable, gently to modestly deepening shelves will record a more sensitive eolianite depositional record, with the formation of carbonate dunes occurring at times other than peak high-stands [33].

Another example that illustrates, albeit obliquely, the contrast in depositional models for coastal shelf systems as compared to rimmed platforms is the contrasting approaches taken in establishing the timing of eolianite accumulation represented in the outcrops of the Tamala Limestone along the south-western coast of Western Australia. Hearty and O’Leary use U/Th age determinations of corals to calibrate a series of whole-rock amino-acid racemization (AAR) ratios [107]. AAR is a technique developed in the Bahamas that requires another dating method to anchor any progression of the AAR data [115]. In evaluating their U/Th results, Hearty and O’Leary question the veracity of the age data that did not fall within the age range (too old and too young) indicating the 5e high-stand as they support an “a priori” assumption that eolianites only develop during sea-level high-stands [97,107]. A similar approach is taken by Lipar and Webb in support of the “high-stand Model” for eolianite deposition of the Tamala Limestone [35]. U/Th age data are reported (±20 to 50 ka) with the measurements not falling in the range of high-stand peaks described as potentially problematic [35]. Similar to the high-stand model of Vacher for Bermuda, early cementation of the eolianites due to the corresponding warm, wet climate is a component of the model proposed for the Carnavon shelf eolianites [107].

In contrast, Vimpere and others use detailed sedimentological and stratigraphic analyses to support the idea that the eolianites of the Tamala Limestone represent landward-advancing dunes [60]. They use a climate model to support the idea that the Tamala Limestone eolianites developed during glacial periods, when the climate would have been drier and windier. Also, they plot the available age data against the Lisiecki and Raymo MIS stack [116], illustrating that the preponderance of the data does not support the “high-stand” model for eolianite deposition [60]. In addition, they note that sea levels control sediment production with generally lower production rates on ramps compared to rimmed platforms [117], while sediment availability is controlled by climate [2,30]. As the Tamala Limestone eolianites developed adjacent to the 50 to 100 km wide Carnavon shelf, carbonate production is not limited by sea level compared to a rimmed platform [60].

The process of refining depositional models requires placing eolianites within a stratigraphic framework where the sequence and relationships between lithofacies can be understood so that the relative sea-level position can be established [2]. In an effort to standardize the language used to describe the relationship between eolianite deposition and RSL, more than twenty studies were interpreted and categorized in terms of sequence stratigraphy nomenclature [58]. Of the studies from coastal ramp or shelf areas, about three-quarters of the studies connected eolianite accumulation to high-stand systems tract (HST) conditions or related variations in timing (e.g., approaching or exiting high-stand conditions). The remainder could be categorized as being emplaced during either the falling-stage systems tract (FSST), approaching a low-stand, or low-stand systems tract (LST) conditions.

Overall, there is the suggestion that where sediments accumulate in the offshore shelf-to-beach system, the transect may influence the perceived timing of dune/eolianite emplacement onshore when evaluating age data. If the source sediments for eolianite accumulate predominantly in a seaward prograding beach face, then the distribution of the age data recovered from the dune system will reflect emplacement during high-stand conditions. In contrast, if sediments are primarily stored in offshore accommodation space, the mobilization of this source material during an RSL drop will overwhelm any signal from a preceding high-stand beach.

For example, in their study of eolianites along the southern coast of the Persian Gulf, Wiliams and Walkden found that modern carbonate-rich sediments are sequestered outboard of a coastal barrier complex with no active dune accumulation behind the beach [30]. Eolianites found 85 km downwind from the modern beach were interpreted as sourced from the Gulf during sea-level fall after the MIS 5e high-stand. In this case, it is worth noting that the eolianites are thought to have been decoupled from their source area due to high aridity preventing quick cementation [30].

Another example is from the cliff-lined coast of Pollenca Bay, Mallorca, where eolianites (MIS 5c/b, 4, 3) are preserved along the southern edge of the bay. Sediments preserved in the eolianite are sourced from the adjacent bowl-bottomed bay, exposed during relative drops in sea level—not from the coastal beach [26,27]. Studies of similarly rugged coastal areas of the Western Mediterranean show that high-volume storage of sediments also does not occur in the near-shore but on the adjacent shelf during high-stand conditions, with sediment then mobilized, deposited, and preserved as eolianite after being exposed during the subsequent sea-level drop [31,36,118].

10. Conclusions

Since the publication of the extensive review on eolianites by Brooke [1], the study of eolianites has become increasingly multi-disciplinary with stratigraphic, petrologic, and sedimentological analyses enhanced by the application of ground-penetrating radar, the use of climate models, and the creative application of dating techniques.

There has been an increase in the use of luminescence methods of dating eolianites in mixed carbonate–siliciclastic environments, while U/Th, the amino-acid racemization, and radiocarbon dating techniques predominate in depositional settings where minor to no siliciclastics are present. The dating of Plio-Pleistocene and particularly post-Sangamon (Eemian) eolianite deposits will require more precise and accurate analysis in teasing apart the complex depositional record related to interstadial/interglacial fluctuations and the concomitant cyclical generation of sediment and eolianite.

There has been a re-evaluation of the “high-stand” model, which is developed on the steep-sided carbonate platforms of Bermuda and is a dominant model in the literature. Many of the key precepts of the “high-stand” model, such as the early cementation of dunes next to their beach sources, have not been upheld by more recent studies that point to regression exposing sediments accumulated during the preceding sea-level rise. On broad coastal shelves, the depositional histories are more complex as both carbonate factory and shoreline positions shift with changes in relative sea levels, leading to complex depositional histories where younger eolianites have overstepped, cannibalized, or coalesced with older eolianites. The link between regression and exposure of offshore sediment reservoirs is also asserted for rugged, cliff-walled coastlines where accommodation space on the beach is limited. Establishing eolianites in a sequence stratigraphic framework allows for a better understanding of the role that relative sea level plays in depositional models.

The most extensive eolianite deposits of the Pleistocene age (volumetric and time span) are found on tectonically passive margins associated with highly productive offshore carbonate factories. Eolianites preserved in the Mediterranean Basin are found in more varied coastal environments, reflecting the region’s complex tectonic history. More recently, eolianites have been identified in the transtensional Gulf of California basin and along the convergent margin of coastal Chile.

The recognition, refined definition, and geographic distribution of global carbonate factories have been a significant advancement in the understanding of where carbonate bioclastic and inorganic sediments (eolianite source sediments) are produced. Primarily controlled by light and temperature, the Photozoan-T factory includes tropical (~30 N to 30 S) localities (e.g., the Great Bahama Bank) with the Persian Gulf subregion (Biochemical Zone) an area of high salinity and temperature. The volumetrically large eolianite deposits along the southern coasts of Australia and South Africa fall within the Heterozoan-C factory, which lies poleward of the Photozoan-T factory and is primarily controlled by nutrients. The Mediterranean Sea (Photo-C factory) is a variant of the Heterozoan-C factory, with the primary controls of light and temperature. Eolianite clasts continue to be a source of information concerning offshore depositional systems of the past, tracking shifts in and within carbonate factories. In depositional settings with sufficient wave action and relatively clear light conditions, rhodolith clasts are a substantial or dominant component in coastal eolianites. Rhodoliths are found in all regions with carbonate deposition, with the exception of the Persian Gulf (Biochemical factory).

There have been few studies that focus primarily on eolianite cements; however, alterations in eolianite cements may reflect environmental changes, particularly in regions where seasonal rain patterns are affected by shifts in climate.