--> ABSTRACT: High Resolution Lacustrine Sequence Stratigraphy: An Example From The Green River Formation, Uinta Basin, East-central Utah, by David Keighley, Stephen Flint, and John Howell; #90906(2001)

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David Keighley1, Stephen Flint2, John Howell2

(1) St Mary's University, Halifax, NS
(2) University of Liverpool, Liverpool, United Kingdom

ABSTRACT: High Resolution Lacustrine Sequence Stratigraphy: An Example From The Green River Formation, Uinta Basin, East-central Utah

At certain stratigraphic levels in a 200m thick study succession of the middle Green River Formation, central Nine Mile Canyon, Utah, regionally mappable surfaces show 10m of truncation over 12km and form subtle angular unconformities in places. The surfaces are interpreted as sequence boundaries and juxtapose 5 - 15m thick amalgamated braided fluvial channel sandstones (late lowstand/early transgressive systems tract) on deep lake oil shales, interpreted as maximum flooding surfaces. These 'Type A' sequence boundaries bound sequences that show a distinct asymmetry, being dominated by 5 - 80m thick transgressive systems tracts. Highstand systems tracts are <4m thick due to truncation by overlying sequence boundaries, and may be removed completely. 'Type B' sequence boundaries juxtapose subaerial floodplain strata containing isolated channel sandstones on shallow lacustrine-dominated strata lacking evidence of deep-water deposition and are not regionally mappable as unconformities.

Type B sequence boundaries are interpreted as due to higher frequency, lower magnitude lake level falls than Type A boundaries, driven by the drying phase of Milankovitch orbital eccentricity climate cycles. Type A sequence boundaries mark more pronounced base-level falls when the pluvial part of the cycle was more pronounced and Lake Uinta had merged with an adjacent basin to form a much deeper lake. The merging permitted the establishment of a new threshold at higher elevation following lake level balancing. More frequent deep lake deposits up section suggests that pluvials became more pronounced with time and increasing up section angularity of Type A sequence boundaries suggests increased basin margin tilting.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado