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Stratigraphic architecture and avulsion deposits of a low net-sand content fluvial succession: Lower Wasatch formation, Uinta Basin
Sendziak, Kassandra L.
Sendziak, Kassandra L.
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2012
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2012
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Abstract
This study documents the stratigraphic architecture of channel and floodplain strata of low net-sand content fluvial deposits in outcrops of the lower Wasatch Formation, Desolation Canyon, Uinta Basin, Utah. The lower Wasatch Formation has a net sand-content of 0.27 and contains predominantly floodplain strata (79% in the field area). Three types of crevasse splays are recognized in this field area based on their physical relationship to adjacent channel-belt strata. Associated coeval splays are laterally adjacent and are physically connected to a channel-belt element, indicating that the crevasse splay was deposited coeval with the channel fill. Unassociated splays are spatially isolated from channel-belt elements and are interpreted to represent a failed avulsion. Associated non-coeval splays underlie the channel-belt element and are interpreted to be genetically related to the overlying channel-belt element, and therefore are a record of a successful avulsion. Three distinct types of associated non-coeval splays are identified in this study area based on physical, observable characteristics: type I, type II, and type III. A conceptual model is proposed that describes longitudinal changes in associated non-coeval splay deposits where type I, type II, and type III splay units represent proximal, medial, and distal positions in splay deposits relative to the source channel, respectively. Decreases in the following characteristics of splays occur with increased distance from the source channel: (1) thickness of splay unit, (2) thickness and abundance of splay beds, (3) net-sand content, (4) grain size, and (5) erosion. The occurrence of floodplain and channel-belt strata in a vertical transect through the outcrop are evaluated to: (1) determine whether the dominant avulsion style is aggradational or incisional, and (2) relate channel story type (i.e. downstream versus lateral accreting) to avulsion style. The correlation between the abundance of splay beds and the abundance of overlying channel-belt elements is interpreted to indicate that the succession resulted from predominantly aggradational avulsion processes. The occurrence of splay beds below channel-belts containing predominantly downstream-accreting stories is interpreted to indicate that these channels resulted from predominantly aggradational avulsion processes. The lack of splay beds below channel-belts containing predominantly lateral-accreting stories is interpreted to indicate that these channels resulted from predominantly incisional avulsion processes.
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