Provenance and Sedimentary Context of Clay Mineralogy in an Evolving Forearc Basin, Upper Cretaceous-Paleogene and Eocene Mudstones, San Joaquin Valley, California

: Mudstone samples from the Moreno (Upper Cretaceous-Paleocene) and Kreyenhagen (Eocene) formations are analysed using X-ray diffraction (XRD) and X-ray ﬂuorescence (XRF) to determine their mineralogy. Smectite (Reichweite R0) is the predominant phyllosilicate present, 48% to 71.7% bulk rock mineralogy (excluding carbonate cemented and highly bio siliceous samples) and 70% to 98% of the <2 µ m clay fraction. Opal CT and less so cristobalite concentrations cause the main deviations from smectite dominance. Opal A is common only in the Upper Kreyenhagen. In the <2 µ m fraction, the Moreno Fm is signiﬁcantly more smectite-rich than the Kreyenhagen Fm. Smectite in the Moreno Fm was derived from the alteration of volcaniclastic debris from contemporaneous rhyolitic-dacitic magmatic arc volcanism. No tuff is preserved. Smectite in the Kreyenhagen Fm was derived from intense sub-tropical weathering of granitoid-dioritic terrane during the hypothermal period in the early to mid-Eocene; the derivation from local volcanism is unlikely. All samples had chemical indices of alteration (CIA) indicative of intense weathering of source terrane. Ferriferous enrichment and the occurrence of locally common kaolinite are contributory evidence for the intensity of weathering. Low concentration (max. 7.5%) of clinoptilolite in the Lower Kreyenhagen is possibly indicative of more open marine conditions than in the Upper Kreyenhagen. There is no evidence of volumetrically signiﬁcant silicate diagenesis. The main diagenetic mineralisation is restricted to low-temperature silica phase transitions. The smectitic dominance of the phyllosilicate fraction in the Kreyenhagen Fm is known to the southeast of our study area, where 51 mudstone samples from boreholes were investigated [6], and a single outcrop sample was investigated in later studies [7,8], approximately 220 km and 55 km from our study area, respectively. Excellent outcrop [2,3] allows continuous sampling through the stratigraphy of both formations and enables the geological evolution of the San Joaquin Basin to be evaluated from the perspective of the ﬁne-grained sedimentary record. Speciﬁc attention is given to the origin of smectite in the context of volcanic activity and quiescence in the evolving Sierra Nevadan magmatic arc. In the present study, a broader evaluation of the lithostratigraphy and mineralogy of the San Joaquin Basin was performed. The provenance, weathering, and composition of source terrane were studied by X-ray diffraction (XRD) and X-ray ﬂuorescence (XRF). This approach is a useful tool for the investigation of ﬁne-grained sedimentary rock [9,10].


Introduction
Recently, the Moreno and Kreyenhagen Fms became a focus of global geological interest because they host the two largest and best-exposed outcrops of giant sand injection complexes, the Panoche Giant Injection Complex (PGIC) and the Tumey Giant Injection Complex (TGIC),~400 km 2 and >200 km 2 , respectively [1-4] ( Figure 1). As part of understanding the background geological setting of the mudstone-dominated host strata for the PGIC and TGIC, samples were collected and analysed from formal lithostratigraphic units in the Moreno Fm and informal units (Upper and Lower) in the Kreyenhagen Fm. Given the widespread occurrence and large outcrops of the Moreno and Kreyenhagen formations (henceforth Moreno Fm and Kreyenhagen Fm) in the San Joaquin Valley and their significance to petroleum systems, the paucity of mineralogical data in the public domain is surprising. According to Jay [5], the Kreyenhagen is "virtually unmentioned in the resource shale literature" despite producing significant volumes of hydrocarbons continuously since 1956. the resource shale literature" despite producing significant volumes of hydrocarbons co tinuously since 1956.
The smectitic dominance of the phyllosilicate fraction in the Kreyenhagen Fm known to the southeast of our study area, where 51 mudstone samples from boreho were investigated [6], and a single outcrop sample was investigated in later studies [7, approximately 220 km and 55 km from our study area, respectively. Excellent outcr [2,3] allows continuous sampling through the stratigraphy of both formations and enab the geological evolution of the San Joaquin Basin to be evaluated from the perspective the fine-grained sedimentary record. Specific attention is given to the origin of smectite the context of volcanic activity and quiescence in the evolving Sierra Nevadan magma arc. In the present study, a broader evaluation of the lithostratigraphy and mineralogy the San Joaquin Basin was performed. The provenance, weathering, and composition source terrane were studied by X-ray diffraction (XRD) and X-ray fluorescence (XRF). T approach is a useful tool for the investigation of fine-grained sedimentary rock [9,10].

Geological Background
During the Upper Cretaceous, large sediment loads were deposited in the S Joaquin Basin, resulting from the rapid erosion of the Sierra Nevada arc. The basin gra ually extended due to migration of magmatic activity to the east combined with the f

Geological Background
During the Upper Cretaceous, large sediment loads were deposited in the San Joaquin Basin, resulting from the rapid erosion of the Sierra Nevada arc. The basin gradually extended due to migration of magmatic activity to the east combined with the formation of the trench to the west [11]. Most of the Moreno Fm examined in this study comprises predominantly fine-grained Maastrichtian slope deposits (Figure 2), but the uppermost unit, the Dos Palos Mbr, deposited in shallower water on the upper slope. In the outcrop area, a regional unconformity truncates the top of the Moreno Fm ( Figure 2). In the Eocene, rapid deformation of the basin occurred, and periods of uplift and subsidence ensued. The latter caused a regional marine transgression, associated with folding and thrusting at the basin margins. In the area of the TGIC outcrop, a regionally developed unconformity eroded deeply (in some cases >50 m) into the TGIC locally reworking shallow parts of the injection complex ( Figure 2). The Eocene-Oligocene boundary is not preserved. Further tectonic movements occurred in the Oligocene, creating normal and thrust faulting and anticlinal folding at the basin peripheries [12]. In the San Joaquin Basin, the Kreyenhagen Fm records approximately 16 million years of slope and basin sedimentation from Middle Eocene to Early Oligocene, an extension of the Sierra Nevada forearc that was created during the subduction of the Pacific plate beneath the North American plate. The formation is predominantly mudstone (also known as the Kreyenhagen Shale), up to 3000 m thick, which is present at outcrop and in boreholes but includes turbiditic and transgressive shallow marine sandstone. Much of the mudstone is bio siliceous and, in some areas are important hydrocarbon source rocks [7,8]. It contains sequences where opal CT is common. mation of the trench to the west [11]. Most of the Moreno Fm examined in this study comprises predominantly fine-grained Maastrichtian slope deposits ( Figure 2), but the uppermost unit, the Dos Palos Mbr, deposited in shallower water on the upper slope. In the outcrop area, a regional unconformity truncates the top of the Moreno Fm ( Figure 2). In the Eocene, rapid deformation of the basin occurred, and periods of uplift and subsidence ensued. The latter caused a regional marine transgression, associated with folding and thrusting at the basin margins. In the area of the TGIC outcrop, a regionally developed unconformity eroded deeply (in some cases >50 m) into the TGIC locally reworking shallow parts of the injection complex ( Figure 2). The Eocene-Oligocene boundary is not preserved. Further tectonic movements occurred in the Oligocene, creating normal and thrust faulting and anticlinal folding at the basin peripheries [12]. In the San Joaquin Basin, the Kreyenhagen Fm records approximately 16 million years of slope and basin sedimentation from Middle Eocene to Early Oligocene, an extension of the Sierra Nevada forearc that was created during the subduction of the Pacific plate beneath the North American plate. The formation is predominantly mudstone (also known as the Kreyenhagen Shale), up to 3000 m thick, which is present at outcrop and in boreholes but includes turbiditic and transgressive shallow marine sandstone. Much of the mudstone is bio siliceous and, in some areas are important hydrocarbon source rocks [7,8]. It contains sequences where opal CT is common.

Materials
Mudstone samples were collected from transects through the outcrop of the Upper Cretaceous to Lower Paleocene Moreno Formation (1) and the Eocene of the Kreyenhagen

Materials
Mudstone samples were collected from transects through the outcrop of the Upper Cretaceous to Lower Paleocene Moreno Formation (1) and the Eocene of the Kreyenhagen Formation (3) (Figure 3). Moreno Fm samples were exclusively from the Right-Angle Canyon locality (RAC, Figure 3a), the geology of which was most recently described in Grippa et al. [13]. Regional (Figure 3). Moreno Fm samples were exclusively from the Right-Angle C yon locality (RAC, Figure 3a), the geology of which was most recently described in Grip et al. [13].

Methods
All samples were analysed using X-ray fluorescence (XRF) and X-ray diffract

Methods
All samples were analysed using X-ray fluorescence (XRF) and X-ray diffraction (XRD) to determine chemical and mineralogical compositions of whole-rock samples (XRF and XRD) and clay fractions (XRD). Chemical analyses were carried out for major elements according to the procedure of Franzini et al. [14]. The sample preparation technique and the fusion procedure were those of Claisse [15]. A mixture containing 0.210 g of sample and 7.000 g of flux (50% Lithium tetraborate, Li 2 B 4 O 7 , and 50% Lithium metaborate, LiBO 2 ), corresponding to a 1:30 sample/borate dilution, was carefully homogenized in a 95Pt/5Au crucible using Claisse Fluxer-Bis! ® automatic apparatus (Malvern Panalytical, UK). Ammonium iodide anhydrous powder was added as a non-wetting agent. The mixture was fused at 1000 • C for 20 min while continuously stirring the melt. When the sample was completely dissolved and any reaction ceased, the melt was poured into 95Pt/5Au/2Rh plate and cooled slowly. After cooling the melt formed a glass disc (ø = 32 mm), which was directly analysed by ARL 9400 XP+ sequential X-ray spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) [16].
For the whole-rock XRD investigation, samples were gently crushed and then ground using a vibratory agate disc mill comminuting by friction. The particle size obtained was <5 µm. To separate the clay fraction (<2 µm), the whole sample was gently crushed (not ground) in an agate mortar, disaggregated in distilled water overnight, and then separated by settling in distilled water according to Stoke's law. Clay suspensions for quantitative analysis were saturated with Mg 2+ cations using 1 N MgCl 2 solution. Oriented mounts were prepared by settling clay suspensions (concentration of 5 mg/cm 2 [17]) on glass slides. Each specimen was analysed in an air-dried state, glycolated at 60 • C for 8 h and heated at 375 • C for 1 h [18].
XRD analyses were performed on whole-rock samples and clay fractions using a Rigaku Rint Miniflex powder diffractometer (Rigaku, Tokyo, Japan) with Cu-Kα radiation, sample spinner, and Cu anode at a voltage of 30 kV and a current of 15 mA. Mineralogical analyses of bulk samples were carried out on random mounts using side loading of bulk specimens, to guarantee a satisfactory reproducible density and random orientation [19]. Data were collected in a 2-70 • range of 2θ with 0.02 • step and a speed of 5 s/step. Data from the clay fraction were collected in the 2-33 • range of 2θ with a step of 0.02 • and a speed of 5 s/step. The content of clay minerals in <2 µm fractions was estimated by the peak areas on both glycolated and heated oriented mounts [20]. To distinguish between smectite and illite/smectite mixed-layer clay, the XRD patterns of glycolated clays were used as proposed by Moore and Reynolds [18]. The XRD patterns were processed using the WINFIT computer program [21].
The mineralogical composition was determined in two steps: (i) by XRD analysis using a Reference Intensity Ratio (RIR) method with quartz as an internal standard [20] and (ii) by combining XRD and XRF data using the vbAffina program (Microsoft Visual Basic 6.0) [22,23]. The VbAffina program requires as input data the major element composition of the bulk sample (SiO 2 , Al 2 O 3 , Fe 2 O 3 , MgO, CaO, Na 2 O, K 2 O, and LOI) and XRDP mineralogical data. All these data were processed using a least-squares procedure that minimizes the differences between chemical compositions calculated from the XRD-determined phase percentages that are introduced into vbAffina (i.e., XRDP results) and those determined using XRF. Stoichiometric compositions of quartz, calcite, dolomite, Na-plagioclase (albite), orthoclase, clinoptilolite, gypsum, opal, cristobalite, and kaolinite were used with the vbAffina program. Illite and smectite compositions were selected from a vbAffina database that contained the compositions of these minerals.

Bulk-Rock Mineralogy
Whole-rock analyses show that the samples are generally composed of quartz, feldspar (K-feldspar and plagioclase), cristobalite, and phyllosilicates ( probably representing opal A ( Figure 4). With one exception (RAC3) in which 58% of the bulk mineralogy is opal CT, samples from the Moreno Fm are dominated by phyllosilicates, and specifically, smectite. PGIC samples contain the highest amounts of quartz and cristobalite (Table 1), which accords with the chemical analyses that reveal a high proportion of SiO 2 ( Table 2). Samples from the Lower Kreyenhagen are characterised by the presence of clinoptilolite (Figure 4b), which although not present throughout, comprises 7.5% of the bulk rock volume in sample EO4-08 (Table 1). In the Upper Kreyenhagen, clinoptilolite is much less common ( Figure 4a) and absent in 8 of the 13 samples. Clinoptilolite is undetected in the Moreno Fm samples (Figure 4c). Opal CT is identified in several samples, and its content varies widely from a few percent up to~70% (Table 1). It is noteworthy that opal CT is present in varied abundance in both stratigraphic successions but absent in more than half of the samples. In the Kreyenhagen Fm, amorphous material attributed to opal A is present in the upper section of the Upper Kreyenhagen and comprises 28% and 24% in the youngest, probably least deeply buried samples (GKR-6 and TH-01, Table 1). It occurs only in one other Kreyenhagen sample (EO4-12) and is absent in the Moreno Fm.   Table 1. Sample locations are shown in Figure 3.
Potassium feldspar is pervasive in the Moreno Fm and more common than in the Kreyenhagen Fm, where occasionally absent (Table 1). Plagioclase is ubiquitous and always more abundant than potassium feldspar in the Moreno Fm, and generally more abundant than in the Kreyenhagen Fm (Table 1). Calcite and dolomite are absent in most  Table 1. Sample locations are shown in Figure 3. Table 1. Whole-rock mineralogy (wt %) for Kreyenhagen and Moreno formations estimated from X-ray diffraction (XRD) analysis. Qtz = quartz; Op CT = opal CT; Opal A? = amorphous material and probably opal A; Crist = cristobalite; K-feld = K-feldspar; Pl = plagioclase; Cal = calcite; Dol = dolomite; Clin = clinoptilolite; Gy = gypsum; Sm = smectite; Ill = illite; Kao = kaolinite; Σ Phy = total phyllosilicates.

Mineralogy of the <2 µm Clay Fraction
The clay fraction (<2 µm) comprises mainly smectitic clay, with small amounts of illite and kaolinite ( Figure 6). Smectite content ranges from 92% to 98% (mean 95.3%) in the Moreno Fm and from 70% to 95% (mean 88.9%) in the Kreyenhagen Fm (Table 3). In the Kreyenhagen Fm, the Upper and Lower Kreyenhagen have averages of 88.9% and 86.1%, respectively; illite is present in all samples ranging from 1% to 11% (Figure 6a,b). In the Moreno Fm, illite is uncommon, ranging from 1% to 8% (mean 1.9%), and slightly more common

Mineralogy of the <2 µm Clay Fraction
The clay fraction (<2 µm) comprises mainly smectitic clay, with small amounts of illite and kaolinite ( Figure 6). Smectite content ranges from 92% to 98% (mean 95.3%) in the Moreno Fm and from 70% to 95% (mean 88.9%) in the Kreyenhagen Fm (Table 3). In the Kreyenhagen Fm, the Upper and Lower Kreyenhagen have averages of 88.9% and 86.1%, respectively; illite is present in all samples ranging from 1% to 11% (Figure 6a,b). In the Moreno Fm, illite is uncommon, ranging from 1% to 8% (mean 1.9%), and slightly more common Potassium feldspar is pervasive in the Moreno Fm and more common than in the Kreyenhagen Fm, where occasionally absent (Table 1). Plagioclase is ubiquitous and always more abundant than potassium feldspar in the Moreno Fm, and generally more abundant than in the Kreyenhagen Fm (Table 1). Calcite and dolomite are absent in most samples and, where present, form diagenetic cement. Where calcite and dolomite occur, chemical data show high concentrations of CaO and MgO, respectively ( Table 2).
Gypsum is present in one sample only (GKR-2) as 1%, consistent with the percentage of CaO (Table 2). Phyllosilicates in bulk samples from the Moreno Fm and Kreyenhagen Fm are predominantly represented by smectite (probably montmorillonite) or mixed layers illite/smectite R0 with low illite content (<10%). Illite and kaolinite are present in small amounts. Smectite content is in the range~50% to 70% for most samples. Exceptions with much lower smectite content (12% to 28%, Table 1) are enriched in opal CT and, in one case (EO4-04), dolomite cement. Illite is pervasive in low proportions ranging from 5% to 8% (Table 1), while kaolinite is significantly less common and absent in nine samples (Table 1).

Bulk Rock Chemistry
The chemical data have limited major element data variability (  [24], which is indicative of significant leaching in the source terrane. Further evidence of a leached source terrane is recorded by TiO 2 concentration that averages between 0.68% and 0.85%, slightly higher than average values from the Upper Continental Crust. Al 2 O 3 content has little variability and an average content of 12% and 16% in the Lower Kreyenhagen and the Moreno formations, respectively. In accord with the mineralogical data (Table 1), the highest Al 2 O 3 concentrations are where smectite and kaolinite predominate. In all samples, Al 2 O 3 concentration is comparable with the average values of the Upper Continental Crust [24].
As expected from the high chemical indices of alteration (CIA) values (Table 2), sample compositions are close to the A vertex and the smectite compositional field ( Figure 5). The combined presence of K-feldspar and illite shift the compositional field approximately 10% toward the A-K axis. The plots exclude all carbonate-rich samples. The chemical data have limited major element data variability ( Table 2). In the Lower Kreyenhagen, some exceptional CaO and MgO concentrations are caused by the presence of calcite and dolomite. SiO2 variability is associated with the concentration of bio siliceous opal. Na2O and K2O are present in low concentrations (1-2%); however, where clinoptilolite and plagioclase, and clinoptilolite and K-feldspar occur, the concentration of Na2O and K2O are higher, respectively. Excluding samples with carbonate minerals, Na2O, K2O, and CaO concentrations are lower than their average concentration in the Upper Continental Crust [24], which is indicative of significant leaching in the source terrane. Further evidence of a leached source terrane is recorded by TiO2 concentration that averages between 0.68% and 0.85%, slightly higher than average values from the Upper Continental Crust. Al2O3 content has little variability and an average content of 12% and 16% in the Lower Kreyenhagen and the Moreno formations, respectively. In accord with the mineralogical data (Table 1), the highest Al2O3 concentrations are where smectite and kaolinite predominate. In all samples, Al2O3 concentration is comparable with the average values of the Upper Continental Crust [24].
As expected from the high chemical indices of alteration (CIA) values (Table 2), sample compositions are close to the A vertex and the smectite compositional field ( Figure 5). The combined presence of K-feldspar and illite shift the compositional field approximately 10% toward the A-K axis. The plots exclude all carbonate-rich samples.  Table 2 excluding those enriched in carbonate. Figure 5. The A-CN-K plot of samples from Table 2 excluding those enriched in carbonate.

Mineralogy of the <2 µm Clay Fraction
The clay fraction (<2 µm) comprises mainly smectitic clay, with small amounts of illite and kaolinite ( Figure 6). Smectite content ranges from 92% to 98% (mean 95.3%) in the Moreno Fm and from 70% to 95% (mean 88.9%) in the Kreyenhagen Fm (Table 3). In the Kreyenhagen Fm, the Upper and Lower Kreyenhagen have averages of 88.9% and 86.1%, respectively; illite is present in all samples ranging from 1% to 11% (Figure 6a,b). In the Moreno Fm, illite is uncommon, ranging from 1% to 8% (mean 1.9%), and slightly more common

Mineralogy of the <2 µm Clay Fraction
The clay fraction (<2 µm) comprises mainly smectitic clay, with small amounts of illite and kaolinite ( Figure 6). Smectite content ranges from 92% to 98% (mean 95.3%) in the Moreno Fm and from 70% to 95% (mean 88.9%) in the Kreyenhagen Fm (Table 3). In the Kreyenhagen Fm, the Upper and Lower Kreyenhagen have averages of 88.9% and 86.1%, respectively; illite is present in all samples ranging from 1% to 11% (Figure 6a,b). In the Moreno Fm, illite is uncommon, ranging from 1% to 8% (mean 1.9%), and slightly more common

Mineralogy of the <2 µm Clay Fraction
The clay fraction (<2 µm) comprises mainly smectitic clay, with small amounts of illite and kaolinite ( Figure 6). Smectite content ranges from 92% to 98% (mean 95.3%) in the Moreno Fm and from 70% to 95% (mean 88.9%) in the Kreyenhagen Fm (Table 3). In the Kreyenhagen Fm, the Upper and Lower Kreyenhagen have averages of 88.9% and 86.1%, respectively; illite is present in all samples ranging from 1% to 11% (Figure 6a,b). In the Moreno Fm, illite is uncommon, ranging from 1% to 8% (mean 1.9%), and slightly more common in the Kreyenhagen Fm, ranging from 1% to 11% (mean 4.5%), with 3% and 5.8% mean illite in the Upper and Lower Kreyenhagen, respectively. 9.68, respectively, and much greater than in the Moreno Fm. In the Kreyenhagen Fm (Table 3), the clusters of high kaolinite content occur in an ~40 m thick interval at the base of the sampled section (samples EO4-01 to -04), and in an ~25 m interval directly below (EO4-09) and above the transition from the Lower to the Upper Kreyenhagen (EO4-10 and -11). A less pronounced kaolinite enrichment occurs in an ~60 m interval. Samples with either low or no kaolinite content typically contain significant amounts of opal CT; for example, in EO4-06, EO-08, GKR1, GKR4, and RAC3 (Table 1).  Table 1. Sample locations are shown in Figure 3.
In the Moreno Fm, kaolinite is 4% or less of the <2 µm fraction in samples from the Tierra Loma and Marca Mbrs (Table 3), while in the overlying Dos Palos Mbr, it constitutes 5% or more of the <2 µm fraction (compare RAC2 and RAC8, Figure 6c). Kaolinite's cumulative mean is 3% with a standard deviation of 2.65. In the Kreyenhagen Fm the kaolinite concentration varies significantly, with nine samples with 4% or less, and eight samples with 10% or more ( Table 3). The mean concentration of kaolinite for the entire Kreyenhagen Fm and the Upper and Lower Kreyenhagen individually is 7.9%. The standard deviation for the Upper and Lower Kreyenhagen are significantly different, 6.63 and 9.68, respectively, and much greater than in the Moreno Fm. In the Kreyenhagen Fm (Table 3), the clusters of high kaolinite content occur in an~40 m thick interval at the base of the sampled section (samples EO4-01 to -04), and in an~25 m interval directly below (EO4-09) and above the transition from the Lower to the Upper Kreyenhagen (EO4-10 and -11). A less pronounced kaolinite enrichment occurs in an~60 m interval. Samples with either low or no kaolinite content typically contain significant amounts of opal CT; for example, in EO4-06, EO-08, GKR1, GKR4, and RAC3 (Table 1).

Mineralogy of the <2 µm Clay Fraction
The clay fraction (<2 µm) comprises mainly smectitic clay, with small amounts of illite and kaolinite ( Figure 6). Smectite content ranges from 92% to 98% (mean 95.3%) in the Moreno Fm and from 70% to 95% (mean 88.9%) in the Kreyenhagen Fm (Table 3). In the Kreyenhagen Fm, the Upper and Lower Kreyenhagen have averages of 88.9% and 86.1%, respectively; illite is present in all samples ranging from 1% to 11% (Figure 6a,b). In the Moreno Fm, illite is uncommon, ranging from 1% to 8% (mean 1.9%), and slightly more common

Mineralogy of the <2 µm Clay Fraction
The clay fraction (<2 µm) comprises mainly smectitic clay, with small amounts of il kaolinite ( Figure 6). Smectite content ranges from 92% to 98% (mean 95.3%) in the Fm and from 70% to 95% (mean 88.9%) in the Kreyenhagen Fm (Table 3). In the K hagen Fm, the Upper and Lower Kreyenhagen have averages of 88.9% and 86.1%, tively; illite is present in all samples ranging from 1% to 11% (Figure 6a,b). In the Fm, illite is uncommon, ranging from 1% to 8% (mean 1.9%), and slightly more c

Opaline Phases
The scanning electron microscopy (SEM) analysis reveals micro-textural variations that confirm the presence of opal A and differentiate it from opal CT. In the Upper Kreyenhagen, broken diatom tests exhibit pristine, box-work, shell micro-structure in a groundmass of comminuted bio siliceous debris (Figure 7). During sample preparation for XRD analysis, the opal A is assumed to disintegrate into finer-grained particles that are X-ray amorphous. By contrast, in the Moreno Fm, there is no preservation of shell tests or their micro-texture. Diatom morphology is sometimes preserved as crystalline fills of their internal geometry (Figure 8). XRD analyses of the clay fractions show an XRD pattern that is much closer to low tridymite rather than low cristobalite, with strong reflections at 4.33 Å and at 4.10 Å and weak, broad reflections at 2.49 Å and 2.31 Å.
Minerals 2021, 11, x FOR PEER REVIEW 12 of 18 micro-texture. Diatom morphology is sometimes preserved as crystalline fills of their internal geometry (Figure 8). XRD analyses of the clay fractions show an XRD pattern that is much closer to low tridymite rather than low cristobalite, with strong reflections at 4.33 Å and at 4.10 Å and weak, broad reflections at 2.49 Å and 2.31 Å .

Origin of Smectite
Mudstone of the Moreno and Kreyenhagen fms are dominated by smectite. Except where opal CT or dolomite occur, it is three to more than six times as abundant as any other mineral present (Table 1). Grim  During the Upper Cretaceous, deposition of the Moreno Fm coincided with active volcanism in the palaeo-Sierran magmatic arc [33] and erosion of substantial volumes of clastic detritus into a forearc basin [34], the site of the present-day San Joaquin Valley ( Figure 1). Alteration of volcaniclastics from rhyolitic or dacitic composition to form smectite is inferred and supported by the high average SiO 2 content (66.8%, Table 2). This accords with the occurrence of quartz, cristobalite, and opal CT (Table 1). No evidence exists for direct deposition from airborne volcaniclastics and tuff is unrecognized in our study area. Despite the prevalence of smectite (Tables 1 and 3), high (mean 84.44) CIA values [25] persist and are indicative of significant leaching of the source terrane. The widespread contemporary volcanic activity makes alteration of volcaniclastics the likely primary source of smectite; however, prior to marine deposition, we suggest that smectite was likely to be retained and formed in pedogenic settings in which it was stable [28] but where further leaching continued to modify the bulk chemistry of the detritus. The low standard deviation of CIA in the Moreno Fm is a measure of the homogeneity of the bulk chemistry ( Table 2).
Contemporary volcanism was absent in the palaeo-Sierra Nevada during the deposition of the Kreyenhagen Fm (Eocene), although it did occur further east on the "Nevadaplano" [35]. As with the Moreno Fm, the absence of tuff makes direct deposition from volcaniclastics unlikely, and weathering processes and soil formation along the western margin of the Sierra Nevada are more plausible origins for most of the smectite. High CIA values prevail, averaging 84.33 and 78.14 in the Upper and Lower Kreyenhagen, respectively. These are indicative of a moderately high level of leaching of the source terrane that coincides with the Paleocene-Eocene thermal maximum followed by almost 10 Ma years of the Eocene hyperthermal [36]. Red soils/paleosols with locally abundant smectite developed along the western margin of the North American continent recording a trend of tropical to sub-tropical climate from Baja California [37] to Oregon [38]. The Kreyenhagen Fm is typically deficient in alkali and alkaline earth cations and is notably ferriferous; Upper and Lower Kreyenhagen with 4.44% and 3.97% mean Fe 2 O 3 , respectively, relative to 2.56% Fe 2 O 3 in the Moreno Fm (Table 2). Local concentrations of kaolinite (10-24%, Table 3) record erosion of more deeply weathered contemporary terrane. Iron oxide stain along cleavage planes in feldspar in Kreyenhagen sandstone is evidence of the erosion of deeply weathered source terrane [4]. Higher concentrations of Fe 2 O 3 are consistent with the generation of smectite by weathering of the granitic or dioritic-granodioritic substrate and probable soil formation rather than by alteration of volcaniclastic debris.

Opal CT
Textural differences between opal CT from the Moreno Fm ( Figure 7) and opal A from the Upper Kreyenhagen ( Figure 8) show how opal A dissolution occurs by the disappearance of the intricate highly micro-porous diatom fragments and growth of coarser individual crystals of opal CT that sometimes preserve diatom test morphology but none of the original micro-porous structure. In a micro-textural context, the transition of opal A to opal CT is demonstrably not a solid-state transition. Growth of opal CT is manifest as an increase in bulk density in the sedimentary rock in which it occurs and is accompanied by a visible change from a substantial proportion of sub-µm micropores in diatomaceous fragments (Figure 7b) to less but larger (~0.5 µm to 2.5 µm) pores (Figure 8d). Although we have not attempted a quantitative evaluation of this change in micro-texture, it is apparent that the pore-size distribution in opal CT is skewed to larger pore sizes than in opal A, implicitly enhancing pore connectivity. It is worth noting that XRD analyses of the clay fractions show an XRD pattern that is much closer to low tridymite rather than low cristobalite, with strong reflections at 4.33 Å and 4.10 Å and weak, broad reflections at 2.49 Å and 2.31 Å. In contrast, low cristobalite has an XRD pattern with a very strong reflection at 4.05 Å and with medium-strong reflections at 3.14 Å, 2.84 Å, and 2.48 Å [39]. Despite these differences, most authors continue to identify opal CT as a precursor of cristobalite rather than tridymite, even though recent [40] and older work [41] presented spectroscopic evidence in support of opal CT as a precursor of tridymite rather than cristobalite.

Clinoptilolite
Global occurrence of clinoptilolite in sedimentary rocks is concentrated in strata of Upper Cretaceous to Eocene age [42]. When present in the Lower Kreyenhagen (seven of ten samples), clinoptilolite has a mean abundance of 3.63% whereas it is mainly absent from the Upper Kreyenhagen. Clinoptilolite was known in the Moreno Fm and associated with an early diagenetic alteration of smectite [43], but it is undetected in this study. The general relationship between the co-occurrence of clinoptilolite with opal CT and smectite [42,44] is not present in five of the seven clinoptilolite-bearing samples; smectite is abundant, but opal CT, opal A, and cristobalite are absent (Table 1). Clinoptilolite forms in normal salinity marine conditions in which opal A or CT are freely available and magnesium concentration is low, conditions typical of open oceans rather than where ocean circulation is restricted. In this context, it could be that the Lower Kreyenhagen deposited in more open marine conditions than the Upper Kreyenhagen. An alternative origin for clinoptilolite is from igneous rocks, often as alteration products of ignimbrites and tuffs, and as vesicular crystals [45][46][47]. This may be a potential contributory source for clinoptilolite in the Lower Kreyenhagen. In common with the origin of smectite, the lack of adjacent volcanic sources during the Eocene diminishes the likelihood of significant volcanic provenance.

Sedimentology and Diagenesis
Content of bio siliceous silica relative to phyllosilicates, and specifically smectite, is the main sedimentological variation present: in the Moreno Fm, the Marca Mbr is a thick, pale grey, regionally developed bio siliceous mudstone [2,13]; in the Kreyenhagen Fm, the Lower and Upper Kreyenhagen are differentiated by their clay mineral and bio siliceous content, respectively [3]. Deposition of the Dos Palos Mbr records a shallowing upward and from the medial part of the Marca Mbr into the Dos Palos Mbr kaolinite content has a significant increase, averaging 4.6% compared with 1% in the underlying mudstone ( Figure 3a, Table 3). In the uppermost four samples (RAC-6, -7, -8, and -9), Fe 2 O 3 is enriched along with alkali and alkali earth elements while SiO 2 is less concentrated ( Table 2). In the absence of any silicate diagenesis, the increase of kaolinite and Fe 2 O 3 records increased terrestrial input with other chemical variations responding to a slight gradual coarsening and increased feldspar content. Independent evaluation of diagenetic grade in the Moreno Fm estimated the thermal maximum to be <50 • C [48], and we have no evidence to contradict that in this study.
Localized diagenetic calcite and dolomite cement and opaline silica transformations are identified and gypsum is present in the Kreyenhagen Fm [4]. High silica content and deficiency in alkali and alkaline earth cations is characteristic ( Table 2) and attributed to derivation of clastic material from siliceous magma. The Lower Kreyenhagen is significantly enriched in alkali and alkali earth elements and depleted in Si 4+ relative to the Upper Kreyenhagen and the Moreno Fm. When the four samples with diagenetic carbonate cement from the Lower Kreyenhagen are excluded from the statistical analysis, Mg 2+ and Ca 2+ remain significantly higher than in Upper Kreyenhagen and Moreno samples, thus reinforcing the significance of the chemical difference of the Lower Kreyenhagen.

Smectite and Hydrocarbon Generation
Unsurprisingly, given the known hydrocarbon reserves in the area [5,49], most of the very few published papers on the clay mineralogy of Paleogene strata in the San Joaquin Basin are associated with hydrocarbons. Previously, smectite in the Kreyenhagen Fm was believed to promote oil expulsion efficiency and was investigated using hydrous pyrolysis to compare natural and thermal maturation of samples [50]. Using the same samples, Lewan et al. [7] concluded that oil expulsion efficiency from smectitic mudstone was reduced by 88% compared to that of mudstone impregnated with kerogen. The reduction in expulsion efficiency was thought to be due to the kerogen in the interlayer region of the smectite structure being converted to bitumen that on heating converted to pyrobitumen through a crosslinking reaction. This actively inhibits oil generation and expulsion efficiency by changing the pore system from water-wet to bitumen-wet. Again, using the same samples, this result was confirmed by Clauer et al. [8], who monitored the changes in chemistry, mineralogy, and K-Ar isotope ratios. Mineralogical changes were monitored by XRD and consisted of recording inhibition of swelling of smectite layers and promotion of illite layers following the impregnation of the pore system and interlayer region of the smectite structure by pyrobitumen after heating to temperatures above 365 • C for 72 h.
Data from our <2 µm samples show that smectite in Kreyenhagen samples (Figure 6b,c) is dominated by randomly interstratified mixed-layer illite-smectite (I/S) where the S component exceeds~85% in a Reichweite R0 arrangement. Samples show evidence of inhibition to swelling after treatment with ethylene glycol, a behaviour attributed to the <2 µm fraction being Mg-saturated [51]. When heated at 375 • C for 1 h it produces a very broad 10 Å reflection, which is asymmetric toward the high angle side. This XRD characteristic is caused by the difficulty of removing interlayer water associated with the interlayer Mg 2+ cation [51]. Thus, inhibition of swelling in smectite need not be associated with the adsorption of pyrobitumen in the interlayer space although our data neither confirm nor deny the concept that formation of a pyrobitumen-smectite complex in the interlayer space [7,8] may inhibit oil generation. Further investigation is required in order to resolve this issue.

Conclusions
Evolution of the palaeo-Sierra Nevada provided remarkably uniform smectite-dominated fine-grained sediment input to the forearc basin during the deposition of the Upper Cretaceous and Paleogene Moreno Fm and the mid-to-late Eocene Kreyenhagen Fm. Deposition of the Moreno Fm was concurrent with volcanic activity in the magmatic arc, and the alteration of rhyolitic or dacitic volcaniclastics is the likely primary source of smectite. The absence of tuff means that there is no direct evidence of volcaniclastic deposition in the marine forearc basin. Volcaniclastics are likely to have been incorporated into terrestrial sedimentary systems, possibly pedogenic, and later reworked into marine environments.
Smectite is prevalent in the Kreyenhagen Fm but in the absence of concurrent Sierran magmatism. Weathering of granitic or dioritic-granodioritic source terrane is inferred during the extended period of sub-tropical climate in the late Paleocene and early Eocene. Local periods of kaolinite enrichment are present that record erosion of more deeply weathered terrane. Chemical data (high CIA) confirm significant leaching of source terrane.
Diatomaceous opaline silica is the main variable constituent in the fine-grained sediment budget. Opal CT is locally common in the Moreno Fm and the prevailing constituent of the Marca Member. In the Upper Kreyenhagen, opal CT is common and opal A is preserved in the youngest parts of the section.
Clinoptilolite is consistently present in small quantities in the Lower Kreyenhagen. but the common general relationship between co-occurrence of clinoptilolite, opal CT, and smectite is not unsustained; opal CT is typically absent where clinoptilolite occurs. Otherwise, the occurrence of clinoptilolite is entirely consistent with the strata of this age globally and indicative of open oceanic conditions.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author. Some data availability is restricted by confidentially agreements with research sponsors.

Conflicts of Interest:
There are no conflicts of interest.