Late Cenozoic stratigraphy and structure of the western margin of the central San Joaquin Valley, California

Abstract

Late Cenozoic Stratigraphy Late Cenozoic deposits in the west-central San Joaquin Valley and adjacent foothills of the Diablo Range consist mainly of unconsolidated, poorly-sorted to well-sorted gravel, sand, silt and clay derived primarily from the Diablo Range and secondarily from the Sierra Nevada. Sedimentary structures, such as channeled contacts, laminated bedding, cross-stratification and clast-imbrication indicate that most of the deposits were transported and laid down by running water. These deposits are described and their facies relationships are illustrated in the 'Late Cenozoic Stratigraphy' section of this report (see Figures 17, and 26, and Table 9). Sediment shed from the Diablo Range accumulated primarily as a complex of coalescing alluvial fans on the piedmont slope of a San Joaquin Valley that at one time extended across the foothill belt to the present margin of the central Diablo Range; and as local fills within stream valleys of the Diablo Range foothills tributary to the San Joaquin Valley. These deposits are well exposed in Interstate-5 roadcuts, California Aqueduct and Delta-Mendota canal cuts, and stream banks along the many ephemeral and intermittent streams draining the Diablo Range. Sediment derived from the Sierra Nevada is confined primarily to the floodbasin of the San Joaquin Valley. It includes arkosic riverine and floodbasin deposits from the San Joaquin River and associated sloughs, as well as local ephemeral and perennial pond, swamp, oxbow-lake and lake deposits. These deposits are well-exposed in stream banks of the San Joaquin River and a few of the larger sloughs such as Salt Slough, Mud Slough and Kings Slough. Well-sorted, fine- and medium-grained, quartzose, cross-bedded sand, presumably derived from the Sierra Nevada, locally interfinger with or underlie fine-grained Coast Range alluvial-fan deposits. The sand probably originated by eolian reworking of Sierran alluvium from the floodbasin of the lower San Joaquin River or from fans of the northeastern San Joaquin Valley. These deposits are locally well exposed in Interstate-5 roadcuts, primarily between Orestimba and Garzas Creeks. The geomorphic character of the alluvium laid down by streams draining the Diablo Range reflects late Cenozoic uplift of the foothills and subsidence of the valley. Within the foothills and near the foothill-valley margin, the deposits form a sequence of inset stream terraces and nested alluvial fans. Valleyward, however, each deposit forms a veneer over older alluvial-fan deposits. Based primarily on geomorphic and pedologic indicators of relative age (see Figure 19 and Table 10), and to a lesser extent on lithologic and absolute age criteria, the late Cenozoic deposits are divided into five stratigraphic units. In order of decreasing age, these include the formally recognized Tulare Formation (Watts, 1894; Anderson, 1905) of late Pliocene and Pleistocene age, and the informally named Los Banos alluvium of middle and late Pleistocene age, San Luis Ranch alluvium of late Pleistocene and early Holocene age, and Patterson alluvium and Dos Palos alluvium of Holocene age. The Los Banos and San Luis Ranch alluvium are further divided into three and two members, respectively. Each of these members ranges in thickness from less than I m up to 15 m and thus represents, at least in part, a distinct period of aggradation. The lithology age and distribution of these units is described in the 'Stratigraphie Divisions' section of this report and is summarized in Figure 25 and Table 11. Plates 1 through 23 show the local distribution of these units on 7.5-minute Quadrangles. Mapping criteria are diagrammatically illustrated in Figure 19 and described in the 'Mapping Criteria' section of this report. Indirect evidence suggests that deposition of these units resulted primarily from climatic change rather than intermittent uplift of the Diablo Range. The units are recognized throughout 1500 Km