Upper Mesozoic subsea fan deposits in the southern Diablo Range, California: Record of the Sierra Nevada magmatic arc

Near Coalinga, California, about 8.5 km of feldspathic and lithic siliciclastic strata of the Great Valley sequence is exposed along the west side of the San Joaquin Valley. There are four distinctive lithofacies in the section. These include: (I) graded, thin-bedded mudstone and fine-grained sandstone; (II) graded, moderately thin-bedded sandstone; (III) graded and nongraded, thick-bedded, commonly scoured and amalgamated sandstone; and (IV) normally and inversely graded and nongraded, thick-bedded conglomerate. Lithofacies I and II consist of low- and intermediate-energy turbidite deposits, including distal fan, overbank, and, possibly, contourite deposits. Lithofacies III consists of high-energy turbidite, subaqueous debris-flow, and grain-flow deposits, most of which are restricted to fan channel systems. Lithofacies IV contains predominantly subaqueous debris-flow deposits confined in fan channels. Primary sedimentary structures indicate that sediments were transported from the east and northeast, and the facies pattern indicates deposition mainly on a subsea fan. Furthermore, the sequence of facies is suprafan lobe over midfan over upper fan, indicating that the deposit is retrogradational toward the source, the Sierra Nevada magmatic arc. Seven stratigraphically delimited petrofacies units, defined by the quartz, K-feldspar, plagioclase, and total lithic fragment QFL percentages,[1][1] and the ratio of metamorphic to volcanic rock fragments (M/V), occur in the sand-size fraction. Throughout the section, the QFL percentage of total quartz is about 30, and that of K-feldspar is below 7 except for the uppermost petrofacies unit. The following units are recognized in terms of the aforementioned parameters: (1) Joaquin Ridge, 1.5 km (Campanian), 20% K-feldspar, low plagioclase (25%), low lithics (20%), and low M/V ratio (0.13); (2) Upper Los Gatos Creek, 1 km [Santonian-Coniacian(?)], high plagioclase (43%), low total lithics (21%), and moderate M/V ratio (0.59); (3) Lower Los Gatos Creek, 1 km (Turronian), moderate plagioclase (31%), total lithics (33%), and M/V ratio (0.78); (4) Studhorse, 1 km (Cenomanian), moderate plagioclase (27%) and total lithics (43%), and high M/V ratio (1.35); (5) Grabast, 1.5 km [Albian(?)], moderate plagioclase (27%), total lithics (40%), and M/V ratio (0.63); (6) Center Peak, 0.7 km [Aptian(?)-Albian(?)], low plagioclase (22%), high total lithics (51%), and low M/V ratio (0.19); and (7) Gravelly Flat, 0.7 km (Upper Jurassic–Lower Cretaceous), moderately high plagioclase (33%), moderate total lithics (44%), low M/V ratio (0.16), and also extensive epidotization. The Lower Los Gatos Creek and lower units contain diagenetic matrix including chlorite and, locally, laumontite. In addition, the detrital petrology records events associated with the evolution of the Sierran batholith — in particular, the three late Mesozoic magmatic epochs. The detrital petrology indicates that extensive volcanism preceded, or accompanied, the start of each magmatic epoch. As emplacement of the plutons proceeded, the volcanic cover was generally stripped off, exposing older terranes as well as the terrane produced by that particular magmatic episode. Plutons of the Yosemite epoch were emplaced under and into a sedimentary-volcanic terrane, whereas the plutons of the Huntington Lake intrusive epoch were emplaced in a metamorphic terrane. Lastly, the K-feldspar–bearing sandstones of the Joaquin Ridge petrofacies record unroofing of plutons of the Cathedral Range epoch, plutons which were the most potassic and which were emplaced in an older plutonic terrane. [1]: #fn-1