INVESTIGATING THE RESPONSE OF FLUVIAL FAN DEPOSITION TO CLIMATIC CHANGES: ESTABLISHMENT OF NEBRASKAN GLACIAL AND POST-GLACIAL STAGE LITHOFACIES AND ARCHITECTURAL ELEMENTS OF THE SAN JOAQUIN RIVER FLUVIAL FAN, CA

Climate affects stream discharge and sediment supply in fluvial fans. Braided rivers record the intensity of glacial episodes in the characteristics of lithofacies and architectural elements. Previous regional studies suggest that 1) river discharge, bedrock scouring and rock avalanches (gravels) increase during glaciation; and 2) river discharge and alpine sediments decrease as channel incision (sand) and over-bank flooding increases during an interglacial (Janda, 1966, Weissmann, 2002). This study describes the lithofacies and stratigraphic architecture of the Turlock Lake Formation (Early to Middle Pleistocene) exposed by successive quarry excavations. The formation records fan deposition during and after the Nebraskan glacial stage.

Using Miall’s lithofacies and within-channel architectural elements classification models (2010), we identified 11 lithofacies in 14 measured stratigraphic sections (extending >10m laterally and >3m vertically). Poorly sorted, well-rounded clasts and arkosic sands distinguish matrix supported (Gmm) from clast supported (Gcm/Gci) gravel-dominated lithofacies. Imbrication and crudely bedded gravels defined two additional (Gh/Gp) gravel-dominated lithofacies. Sand-dominated lithofacies exhibit cross, planar, trough and ripple laminations, or are massive sand and silty clay (Sp-Sl, St, Sr, Sm, respectively). Associations of lithofacies and geometry of deposits were used to identify architectural elements in terms of their genetic significance. Gravel-dominated lithofacies are interpreted as forming within gravel bars with interbedded sandy bedforms on a multi-channel flood plain. Sand-dominated lithofacies are interpreted as aggrading channel bars, sandy bedforms and abandoned channels. The lithofacies, architectural elements, and fining upward aggradational cycles identified in this study, are consistent with 1) high energy stream discharge transporting a large volume of eroded river valley till to the proximal fan during glacial advance and 2) decreasing stream discharge and incipient floodplain formation during glacial retreat.