Abstract
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 μm 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 μm 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.
Highlights
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 km2 and >200 km2, respectively [1,2,3,4] (Figure 1)
With one exception (RAC3) in which 58% of the bulk mineralogy is opal CT, samples from the Moreno Fm are dominated by phyllosilicates, and 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 SiO2 (Table 2)
Summary
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 and >200 km, respectively [1,2,3,4] (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 ( 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. T approaapcphroiaschaius saeufsuelfutlotoololfoforrtthe iinnvveestsigtiagtiaotnioonf fionfef-ginraei-ngerdasiendeidmesnetdarimy reonckta[9ry,10r]o. ck [9,10]
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