Abstract

South and southeast of Clear Lake, California, approximately 35,000 ft of clastic sedimentary strata of the Great Valley sequence ranging in age from Late Jurassic to Late Cretaceous crop out in irregular belts trending from northwest to southeast. Together with overthrust lower Tertiary beds, these rocks form a thrust complex that rests structurally upon the Franciscan assemblage of late Mesozoic age along the Soda Creek thrust and is overlain unconformably by late Cenozoic strata. A number of subsidiary thrusts, that are discordant to bedding, break the Great Valley sequence into at least three and possibly more than four successive thrust plates or slices. In all exposed cases, younger strata overlie older strata along the thrusts. The principal structurally intact stratigraphic components of the thrust complex are:

A conformable Tithonian to Valanginian succession, 6900 ft, dominantly mudstone with 1600 ft of lithic sandstone and conglomerate in the upper part.

Lower Cretaceous strata, 4500 ft, in part Aptian, dominantly thin-bedded quartzose sandstone and mterbedded mudstone.

Three structurally isolated “mid”-Cretaceous successions, two of massive sandstone and one of mudstone, total of 5700 ft exposed.

A conformable Upper Cretaceoussuccession, 17,500 ft, in large part Campanian, composed of five mappable units including massive sandstone (3700 ft), siltstonc (3600 ft), massive sandstone (4000 ft), mudstone (1000ft), and alternating sandstone and mudstone (5200 ft) in ascending order.

A conformablemarine Paleocene and Eocene succession (5500 ft).

The lithologic characteristics, petrologic variations, and paleocurrent indicators oi the strata in the Great Valley sequence near Clear Lake are comparable to those of correlative parts of the Great Valley sequence along the western side of the Sacramento Valley. Emplacement of the Great Valley sequence and the lower Tertiary beds above the Franciscan assemblage by regional thrusting was probably complete by Oligocene time from evidence elsewhere in the Northern Coast Ranges. The thrusts themselves and the strata of the thrust complex were later folded and cut by younger normal and reverse faults during Cenozoic deformations.

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