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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 57 (1973)

Issue: 5. (May)

First Page: 960

Last Page: 960

Title: Depositional Environments of Entrada Formation (Jurassic), Northeastern Utah: ABSTRACT

Author(s): E. P. Otto, M. Dane Picard

Article Type: Meeting abstract


In northeastern Utah the Entrada Formation contains 2 lithologic units; a lower pale yellowish-orange, fine- to medium-grained sandstone; and an upper moderate reddish-orange, very fine-grained silty sandstone. The upper unit is present only locally and where present interfingers with the lower unit. The rocks of the lower unit were deposited in 2 environments, beach and dune. The upper unit is a shallow-water marine deposit.

The beach sequence is usually the thickest part of the lower unit. These deposits are characterized by horizontal stratification, disturbed bedding, and small- to medium-scale wedge- and tabular-planar cross-stratification mostly of low Previous HitangleNext Hit. In the backshore zone channels containing medium-scale trough cross-stratification parallel the shoreline. Ripple marks and burrowed structures are uncommon in the lower unit.

The dune sequence is characterized by medium- to large-scale wedge-planar cross-stratification of high Previous HitangleNext Hit. The cross-strata are tangential to the lower bounding surface. Trough cross-stratification is present, but not Previous HitcommonTop. Most cross-stratification is faint and indistinct on weathered surfaces, suggesting that water probably reworked the sand after deposition.

The upper silty sandstone unit is "structureless," except for sparse horizontal stratification.

Dominant paleocurrent directions, based on 275 measurements, are west and southwest. Nine of 11 locations show a unimodal distribution, and 79% of these measurements are between 151 and 300°. The other 2 locations show bimodal distributions; southeast-northwest and southeast-northeast.

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Copyright 1997 American Association of Petroleum Geologists