AN INVESTIGATION OF SHAY'S DELTA (BALDWIN LAKE, CA) AND ITS HOLOCENE DEPOSITIONAL HISTORY

Along the southern shore of Baldwin Lake at the mouth of Shay's Creek is a fan-shaped featured interpreted as a stranded delta (hereafter Shay's Delta). The depositional history of Shay's Delta was investigated using a combination of geophysical, sedimentological, geomorpological, and chronological data. Ground penetrating radar (GPR) data reveal two stratigraphic sequences: 1) a lower sequence (I) of weak, thin reflectors that are horizontally continuous, indicating high attenuation material, such as clay. Sequence (I) is interpreted as the prodelta of the oldest, imaged phase (late-Glacial to early Holocene) of deltaic deposition; and, 2) an upper sequence (II) of stronger, hummocky and wavy, sub-parallel reflectors, which are fairly discontinuous. Sequences (II) is interpreted as alternating parasequences of the delta plain /delta front/prodelta; it represents the most recent, but not modern, phase (mid-to-late Holocene) of active deltaic deposition. A trench and several cores capture the entirety of sequence II and the upper portion of sequence I. Six sedimentological analyses from the trench and core sediments were measured (i.e. magnetic susceptibility, total organic matter, total carbonate, percent sand, percent silt, percent clay). A statistically robust facies model based on these sediment analyses was developed using multivariate statistics. A depositional cross section along our trench/core transect was developed using this facies model. The cross section reveals six episodes of transgression-regression of the delta, including the dominant depositional change between Sequence I and II. Geomorphological data (total station and DEM data) reveal a step-decrease in the delta surface elevation, which is interpreted to represent two primary phases of deltaic deposition similar to the GPR data (i.e. Sequence I and II). This step-decrease in surface elevation is coeval to the boundary between Sequence I and II and matches the sedimentological facies interpretation (see below). In all, these data suggest a major drop in base level from (i.e. lake level) ca. 5,400 cy BP, which resulted in the down cutting of Shay Creek into Sequence I and the progradation of the more recent Sequence II deposits.