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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 63 (1979)

Issue: 3. (March)

First Page: 471

Last Page: 471

Title: Seaward Primary Dip of Fall-in Beds, Lower Seven Rivers Formation (Permian), Guadalupe Mountains, New Mexico: ABSTRACT

Author(s): Neil F. Hurley

Article Type: Meeting abstract


Fall-in beds are shelf carbonate rocks which exist adjacent to the Capitan Limestone in a belt about 1 km wide, and which have basinward dips of 5 to 15°. Sedimentologic and structural-geopetal data gathered in field studies of the lower Seven Rivers Formation in North McKittrick Canyon show that tectonic tilting and/or compactional subsidence can account for only part of the basinward dip of fall-in beds, the remainder being primary depositional dip.

The dominant lithologies of fall-in beds are stromatolitic algal oncolite rudites and sand-sized, mixed skeletal-peloid grainstones. Rocks are tightly cemented with marine phreatic isopachous fibrous magnesium calcite. Fall-in beds lack features of the adjacent, shallower, but generally submerged shelf-crest facies such as fenestral fabric, pisolites, tepees, erosion surfaces, and shoaling cycles. An inferred energetic, subtidal marine depositional environment for fall-in beds is compatible with their significant basinward depositional dip.

Primary geopetal fabrics, although scarce in fall-in beds, have dips not exceeding a few degrees. The dip divergence between bedding planes and geopetal surfaces averages 8 ± 2°, a value which is inferred to be equal to the original depositional dip.

Proof of primary seaward dip in fall-in beds lends support to Dunham's marginal-mound hypothesis for the Capitan shelf. Also, primary dip in beds adjacent to the Capitan supports recent interpretations that the Capitan Limestone formed in a relatively deep (30 to 50 m), continually submerged shelf-edge position, and was not a true barrier reef.

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Copyright 1997 American Association of Petroleum Geologists