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Deposits from Wave-Influenced Turbidity Currents: Pennsylvanian Minturn Formation, Colorado, U.S.A.

Lamb, M. P. and Myrow, P. M. and Lukens, C. and Houck, K. and Strauss, J. (2008) Deposits from Wave-Influenced Turbidity Currents: Pennsylvanian Minturn Formation, Colorado, U.S.A. Journal of Sedimentary Research, 78 (7). pp. 480-498. ISSN 1527-1404. doi:10.2110/jsr.2008.052.

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Turbidity currents generated nearshore have been suggested to be the source of some sandy marine event beds, but in most cases the evidence is circumstantial. Such flows must commonly travel through a field of oscillatory flow caused by wind-generated waves; little is known, however, about the interactions between waves and turbidity currents. We explore these interactions through detailed process-oriented sedimentological analysis of sandstone event beds from the Pennsylvanian Minturn Formation in north-central Colorado, U.S.A. The Minturn Formation exhibits a complex stratigraphic architecture of fan-delta deposits that developed in association with high topographic relief in a tectonically active setting. An ~20–35-m-thick, unconformity-bounded unit of prodelta deposits consists of dark green shale and turbidite-like sandstone beds with tool marks produced by abundant plant debris. Some of the sandstone event beds, most abundant at distal localities, contain reverse-to-normal grading and sequences of sedimentary structures that indicate deposition from waxing to waning flows. In contrast, proximal deposits, in some cases less than a kilometer away, contain abundant beds with evidence for deposition by wave-dominated combined flows, including large-scale hummocky cross-stratification. We interpret the majority of these event beds as a record of deposition from hyperpycnal flows, i.e., turbidity currents generated directly from highly concentrated river plumes, which accelerated and decelerated in response to a rising and falling flood discharge. Additional support for this interpretation includes the following: (1) a variety of sole marks including flute and gutter casts, as well as tool marks made by relatively large (up to tens of centimeters across) woody debris (i.e., groove, prod, and chevron marks); (2) consistent unimodal orientation of sole marks and abundant ripple cross-stratification, which indicate strong downslope-directed flow; (3) a well documented sedimentological framework for the formation of fan-delta deposits adjacent to nearby highlands; and (4) plant fossils typical of middle- to high-elevation habitats that are abundant in the turbidite beds but absent in underlying and overlying shoreline and marginal marine deposits, which have a separate floral assemblage. Differences in grain sizes, vertical stratification sequences, and bed thicknesses between outcrops are interpreted to result from the spatial distribution of wave effects, the time history of hyperpycnal flows, and the interaction of these processes. The latter varied both spatially and temporally and produced a wide range of bed types, which are incorporated into a new conceptual model for storm-influenced hyperpycnal flows.

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Lamb, M. P.0000-0002-5701-0504
Additional Information:© 2008 SEPM Society for Sedimentary Geology. Received 11 September 2007; accepted 17 January 2008. This project was funded by National Science Foundation Grants to Paul Myrow (EAR-0309693) at Colorado College and Jeff Parsons (EAR-0309887), Department of Oceanography, University of Washington. M.P. Lamb thanks Jeff Parsons for financial support and advisement during the course of this study. We also thank reviewers Tom Hickson, Piret Plink-Bjorklund, and Takeshi Nakajima for their suggestions on a previous draft. Thanks to Philip Allen, Kyle Straub, Peter Talling, and John Southard for reviewing this manuscript.
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Subject Keywords:hummocky cross-stratification; driven sediment transport; continental-shelf; river mouths; bed configurations; bedding sequences; oscillatory-flow; rocky mountains; underflows; dynamics
Issue or Number:7
Record Number:CaltechAUTHORS:LAMjsr08
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:13596
Deposited By: Tony Diaz
Deposited On:03 Mar 2009 01:00
Last Modified:08 Nov 2021 22:39

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