56-9 Interaction of Local and Regional Fluvial Systems in the Pliocene Olla Formation, Southern California, USA

Abstract:

Fluvial systems stratigraphy can record windows of Earth’s history, with systems ranging from small, local drainage networks to large, continental catchments that can be deposited across varying temporal scales. Spatial interaction between large- and small-scale fluvial networks can lead to deposits with lateral interfingering of sediments. The ultimate scope for this study is to understand how system interfingering is controlled by autogenic sediment storage and release cycles versus allogenic climate and tectonic forcing. The coeval deposition of two systems makes disentangling the controls that govern the architecture of these fluvial deposits challenging, due to the complex nature that the spatial and temporal distribution of the interbedded local and regional sediment presents. For this study, a detailed facies and stratigraphic analysis was conducted of the fluvial Pliocene Olla Formation in the Fish Creek Vallecito Basin (FCVB) of Southern California. The Olla Formation constitutes a unique case where exposure of local sediments deposited from a transverse braided river system (L-suite composition) are interbedded with regional sediment from the axial Colorado River (C-suite composition). Fieldwork, including two stratigraphic sections summarizing over 580 m of section, over 2500 outcrop photos, and satellite imagery show clear vertical alternation and relatively minor grain mixing between the two fluvial systems. C-suite intervals are dominated by paleochannels and floodplains of the Colorado river. Typical C-suite characteristics consist of very fine – fine grain size, quartzose composition, red mudstone layers, high density low angle – parallel lamination, and clear fining upward cycles. L-suite intervals are dominated by floodplains, sheetfloods and bar scale channelized flow. Typical L-suite characteristics consist of medium – coarse grain size, arkosic composition, dark brown – grey mudstone layers, large tangential crossbeds (0.5 – 2 m thick) and poorly-developed thickening and coarsening upward cycles. Through our ongoing work, we aim to quantify the degree to which the interfingering of L- and C-suite deposits varies vertically and laterally in the stratigraphy. The results of this study on stratigraphy and controls on fluvial mixing of large and small fluvial systems represents an important tool for understanding basin stratigraphy. Especially during the tectonically active phase, many basins develop a large basin-axial fluvial system which also receives sediments from basin-transversal smaller fluvial systems.

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