Abstract

Upper Jurassic and Cretaceous sedimentary rocks more than 35,000 feet thick are exposed along the west side of the Sacramento Valley of California. To ascertain the framework in which the sediments were deposited, a detailed study of these rocks was made in the Cache Creek-Rumsey Hills area, and a general study was made at other localities.

The sedimentary section consists of interbedded sandstones, which are commonly graded, mudstones, and siltstones, with minor conglomerates and bentonitic rocks. Units that are dominantly sandstone alternate with units that are dominantly mudstone. The sequence is sparsely fossilferous, but approximate stage boundaries were determined.

Nine hundred paleocurrent measurements were made on sole marks, parting lineations, cross-bedding, and other structures. These indicate that paleocurrents in general flowed from north to south, parallel to the regional tectonic trend, the eastern shoreline of the basin of deposition, and the isopachous lines of the sequence. During Cenomanian and early Turonian time, however, currents moving toward the west were prominent.

The mineralogy of the sandstones was determined through the Cache Creek-Rumsey Hills section, and check samples from elsewhere in the belt were found to be similar. The sandstones include feldspathic, arkosic, and lithic wackes and arenites, with the main grain constituents being quartz, feldspar, and volcanic rock fragments. The K-feldspar content is greatest in the Upper Cretaceous rocks. Epidote is the dominant non-opaque, non-micaceous heavy mineral; apatite, sphene, hornblende, and zircon are also common. Six intervals of differing composition were established. In Interval I (Upper Jurassic), andesitic (?) volcanic detritus is dominant in the few samples studied; Interval II (Valanginian) contains abundant quartz and plagioclase; in Interval III (Hauterivian through middle Albian), quartz is dominant; Interval IV (upper Albian and Cenomanian) is characterized by K-feldspar and andesitic (?) volcanic rock fragments; Interval V (Turonian to upper Santonian) contains K-feldspar, plagioclase, quartz, and acidic volcanic detritus; and Interval VI (upper Santonian and Campanian) is characterized by K-feldspar, plagioclase, and quartz. The clayey fractions of sandstones and associated mudstones are similar, with chlorite dominant through much of the Lower Cretaceous, and mica and montmorillonite dominant in the Upper Cretaceous. Major diagenetic changes in the sandstones include widespread carbonate cementation and replacement, recrystallization of matrix, and alteration of plagioclase and biotite. Albitization of plagioclase and chloritization of biotite were found to vary with depth and the content of calcite in the sandstones. These alterations proceeded to a small degree at depths of burial estimated to be as shallow as 10,000 feet, they affected all noncalcareous strata buried from 20 to 30,000 feet, and were extensively developed in even the calcareous rocks that were buried 35,000 feet.

The ancestral Klamath Mountains and Sierra Nevada evidently were the sources of the detritus. Major events in the source areas, such as volcanism, pluton emplacement, and unroofing of plutons, apparently are reflected in the sedimentary rock column. The sequence was deposited below wave base, generally far from shore in an outer neritic-upper bathyal, probably miogeosynclinal, environment. Sedimentary structures and grading suggest deposition by turbidity currents. The sequence strikingly resembles other rock sequences which have been interpreted in the literature as “turbidites.”

In an attempt to explain the sedimentation, three models are considered, each consisting of an elongated north-south basin open to the west: (1) a southerly paleoslope trending parallel to the long axis and the eastern shoreline of the basin, with currents carrying sediment southward down the paleoslope; (2) a paleoslope to the west from the north-south trending shoreline, with currents moving down the paleoslope and subsequently deflected to the south by deep oceanic currents; and (3) a subsea fan with an asymmetric cross section that caused the deflection of westerly moving turbidity currents toward the south. Modern counterparts of the last model are present off the western coast of North America. This model is tentatively suggested as the one which best fits the data.

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