--> Petrographic Differentiation of Fluvial and Tidally Influenced Fluvio-Estuarine Channels in Second Frontier Sandstones, Moxa Arch Area, Wyoming: Sequence Stratigraphic Implications, by S. A. Stonecipher, R. P. Diedrich, and L. G. Kessler, II; #90986 (1994).

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Abstract: Petrographic Differentiation of Fluvial and Tidally Influenced Fluvio-Estuarine Channels in Second Frontier Sandstones, Moxa Arch Area, Wyoming: Sequence Stratigraphic Implications

S. A. Stonecipher, R. P. Diedrich, L. G. Kessler, II

Petrographic examination of textural, compositional, and diagenetic properties of Second Frontier sands, Moxa Arch area, southwestern Wyoming has resulted in a reinterpretation of this gas productive interval, suggesting the presence of two non-synchronous channelized sequences. On the northern end of the Arch in the Lincoln Road area, Second Frontier sands were deposited in fluvial channels. In contrast, in areas such as the Wilson Ranch field to the south, Second Frontier sands appear to have been deposited in the fluvial input areas of tidally influenced estuaries. While traditional sedimentologic and paleontologic data on these sediments is ambiguous, distinctive variations in petrographic characteristics permit facies differentiation. For example, channel sands to the north conta n coarse, mica-derived, authigenic kaolinite which reflects early diagenesis in a meteoric regime, whereas channel deposits to the south contain only chlorite and illite suggesting a marine diagenetic setting.

The new interpretation would suggest that the "unconformity" at the base of the channelized sequences also changes character from north to south in the study area. A preexisting distributary channel at the southern end of the Arch caused downcutting and erosion here during a drop in sea level. Thus, the boundary between the channelized sequence and the underlying marine sediments in the Wilson Ranch area represents a lowstand surface of erosion (LSE). The lack of this earlier channel system to the north apparently prevented extensive incision and erosion there. As a result, the "unconformity" at the base of the later formed fluvial sequence in the Lincoln Road area would better be termed a diastem rather than an LSE.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994