Abstract
The Great Valley Sequence of Upper Jurassic to Upper Cretaceous strata is composed of clastic detritus eroded from the Sierran-Klamath belt, which was the site of a late Mesozoic magmatic arc. The sequence accumulated mainly as a prismatic wedge of turbidites in an asymmetric trough within the arc-trench gap. Stratigraphic variations in detrital sandstone mineralogy define five petrologic intervals, or nearly synchronous petrofacies, useful for regional correlation. By contrast, local lithofacies are markedly lenticular. The clastic debris in the sequence was derived mainly from volcanic and plutonic rocks in varying proportions. The parameters used to define the petrofacies include: (a) content of quartzose grains (Q), feldspar grains (F), and unstable lithic fragments (L) expressed as volumetric percentages of the Q-F-L population; (b) ratios of plagioclase to total feldspar (P/F) and volcanic rock fragments to total unstable lithic fragments (V/L), and (c) content of mica (M) expressed as volumetric percentage of framework sand grains. The most distinctive characteristics of the petrofacies recognized, and their approximate time-stratigraphic spans, are as follows: (1) Stony Creek (Tithonian-Neocomian): quartz-poor and feldspatholithic sandstones with high P/F, high V/L, low M; (2) Lodoga (Aptian-Albian): quartz-rich sandstones with high P/F; (3) Boxer (Cenomanian): two distinct variants with moderate P/F, one quartz-rich and the other quartz-poor and feldspatholithic; (4) Cortina (Turonian-Coniacian): two similar variants with moderate P/F, one with Q-F-L proportions nearly equal and the other quartz-poor and feld-spatholithic; (5) Rumsey (Santonian-Campanian): quartz-rich lithofeldspathic with low P/F and high M. The nature of the petrofacies and their age can be related to the petrology and timing of dated intrusive episodes with inferred volcanic accompaniments in the Sierra Nevada. The Stony Creek petrofacies contains mainly volcanic debris erupted during the Yosemite magmatic epoch, and the overlying Lodoga petrofacies contains mainly plutonic debris emplaced during the same epoch, but only exposed to erosion by subsequent dissection. The Boxer and Cortina petrofacies contain mixed and intercalated volcanic and plutonic debris derived mainly from rocks erupted or emplaced during the Huntington Lake magmatic epoch. The slight differences between the Boxer and Cortina petrofacies may indicate that plutonic contributions were partly from older rocks for the Boxer, but volcanic contributions were essentially the same for both. The Rumsey facies contains mainly plutonic debris, with subordinate volcanic debris, derived from igneous rocks of the Cathedral Range magmatic epoch. Except locally or slightly, petrofacies boundaries do not transgress time-stratigraphic boundaries in the region. A close linkage among major magmatic, tectonic, and depositional events in arc-trench systems is implied.
