--> Abstract: Tidal-Fluvial Versus Estuarine Bars: Upper Cretaceous Sego Sandstone, Utah, Compared with Modern-Ancient Analogues, by Shuji Yoshida, Ron Steel, Robert Dalrymple, James MacEachern, and Kerrie Bann; #90039 (2005)

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Tidal-Fluvial Versus Estuarine Bars: Upper Cretaceous Sego Sandstone, Utah, Compared with Modern-Ancient Analogues

Shuji Yoshida1, Ron Steel1, Robert Dalrymple2, James MacEachern3, and Kerrie Bann4
1 The University of Texas at Austin, Austin, TX
2 Queen's University, Kingston, ON
3 Simon Fraser University, Burnaby, BC
4 University of Alberta, Edmonton, AB

Existing tidal sandbar models assume an elongated, flow-parallel geometry and accretion in the direction of the dominant flow, and have been applied to numerous ancient tidal sandbodies of that formed in estuarine, tide-dominated deltaic and tide-dominated shelf environments. However, recent studies reveal that an increasing number of these so-called tidal sandbars formed as laterally accreting, tidal point bars within the tidal-fluvial zone, extending over 10's to 100's km inland from the coeval shorelines. Models that ignore this complexity can lead to unrealistic paleogeographic reconstructions, with an error range of proximal-distal positioning easily exceeding 100's of kilometers, and water-depth estimations exceeding 10's of meters.

These tidal point bars have a significant reservoir potential, as is the case in the Lower Cretaceous McMurray Formation in Alberta. Reliable identification is crucial for the exploration and exploitation of hydrocarbon reserves, because, unlike their marginal-marine to open-marine counterparts, these tidal point bars have (1) hemi-circular, tightly-curved external geometries, (2) strong contrasts in lithologic and biogenic heterogeneities between laterally adjoining sandbars, and (3) small dimensions with internal lateral accretion directions changing from one sandbar to another over short distances (< 200 m).

We have used the outcrops from the Upper Cretaceous Sego Sandstone, Utah, to address these issues. Previous researchers have interpreted this unit to contain elongated tidal sandbars within estuaries or tidal deltas. We suggest, however, that many of these tidal sandbars are of tidal-fluvial origin, based on architectural element analysis on several 3-D outcrops.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005