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Kevin M. Bohacs1, Marybeth Wegner2, Steve Hasiotis1
(1) Exxon Production Research Company, Houston, TX
(2) University of Wisconsin, Madison, WI

Abstract: Slope and basin-floor strata deposition, geometry, and continuity: Insights from Ichnofossil analyses

Diagnostic ichnofossil assemblages characterize each type of the wide range of slope and basin sandstone and mudstone strata. Types of slope and basin strata and their geometry and lateral continuity strongly correspond, providing another tool for estimating lateral extent from vertical core samples. Ichnogenera mostly belong to either the Nereites or Cruziana ichnofacies, portraying close alternation of high and low bottom-energy levels and sedimentation rates. Ichnofossils reveal details of depositional processes, such as organic-matter availability and relative duration of hiatuses, difficult to interpret from other data.

Examples from the upper Brushy Canyon and lower Cherry Canyon Formations (Upper Permian, Delaware Mountains, Texas) illustrate these relations. Three large-scale facies assemblages of Channel/Intrachannel, Inter-Channel Complex, and Extra-Channel Complex can each be subdivided into three facies with different geometry and continuity.

Within the reservoir-prone Channel/Intrachannel facies assemblage, we recognize potential baffle or seal facies of Channel Base, Channel Margin, and Channel Top. The most laterally extensive Channel-Top facies has the deepest tiering, moderate diversity, and highest abundance, whereas the short-extent Channel-Margin facies has only moderate depth of tiering, but higher diversity and moderate abundance of ichnofossils.

Within the seal-prone Extra-Channel Complex facies assemblage, the wedge-shaped Distal-Overbank facies (deposited laterally from active sediment conduits) contains a moderate diversity and abundance ichnofossil assemblage with moderate depth of tiering. In contrast, the blanket-shaped Distal-Sheet facies (deposited in unconfined downdip reaches) has a lower diversity and abundance assemblage in shallow tiers. The Downdip Drape has a monospecific, low-abundance assemblage of very small individuals in shallow tiers, indicating inhospitable bottom conditions.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana