Late Cenozoic stratigraphic units, northeastern San Joaquin Valley, California

Abstract

Recent mapping of late Tertiary and Quaternary deposits in the northeastern San Joaquin Valley has shown the need for clarification, revision, and subdivision of some of the previously mapped stratigraphic units. We propose a uniform nomenclature for deposits younger than the Mehrten Formation in this region. The Laguna Formation (late Pliocene) is now recognized as far south as the Merced area, where it is overlain by the North Merced Gravel (Pliocene or Pleistocene). The China Hat pediment of Hudson (1960, p. 1552) is interpreted as the uppermost gravel member of the Laguna Formation and is believed to represent an old fan deposit rather than a pediment. The Laguna, Turlock Lake, Riverbank, and Modesto Formations are lithologically similar but may be distinguished and subdivided on the basis of soil profile development, topographic position and expression, local lithologic differences, unconformities, and associated buried soils. The Turlock Lake Formation is subdivided into two units (lower, upper) and the Riverbank Formation into three units (lower, middle, upper), separated by unconformities and well-developed buried soils. The Modesto Formation is subdivided into two informally designated members, separated by a minor unconformity and weakly developed buried soil. The lower (oldest) member of the Modesto is in places associated with two geomorphic surfaces, and as many as four terraces showing slightly different soil development are associated with deposits of the upper (youngest) member. Post-Modesto deposits are informally subdivided into four stratigraphic units, designated Post-Modesto I, II, III, and IV. Most of the stratigraphic units discussed are believed to represent separate alluvial episodes, recorded by fill terraces opening westward onto alluvial fans. Substantial time intervals between periods of aggradation are represented by buried paleosols. Some of the properties of relict and buried paleosols are summarized and their relations to the proposed stratigraphic units are described.