SOREGHAN, GERILYN S.¹ and KATHERINE A. GILES^2
1University of Oklahoma, Norman, OK
²New Mexico State University, Las Cruces, NM

Abstract: Facies, Stratal Geometry, and Growth History of Pennsylvanian Bioherms, Western Orogrande Basin, NM

Detailed study of a high-relief (>100 m), exhumed algal mound of the western Orogrande Basin (NM) indicates that it consists of stacked high-frequency cycles (sequences) bounded by surfaces of paleo-subaerial exposure. Facies include (1) various boundstones in the mound core, (2) packstones on flanks, and (3) auxiliary facies formed off-mound regions. Stratal cyclicity in this system was the result of high-frequency, high-amplitude glacioeustasy. Paleorelief on subaerial exposure surfaces records glacioeustatic amplitudes >80-100 m; preserved paleoslopes >50° indicate that mounds were cemented rapidly and not easily eroded, even at lowstands.

Timing of stratal accretion within a given cycle varied as a function of both spatial and temporal position within the bioherm. Accretion in mound-nucleation stages produced late-fall cycles both on- and off-mound. During "acme" growth, accretion occurred during rise and maximum inundation, pacing short-term (glacioeustatic) accommodation and producing an anomalously thick, near keep-up cycle in the mound core. Subsequent cycles of the core accreted during maximum inundation to incipient fall and display thicknesses reflecting long-term accommodation. In a shallow-water biohermal system capable of accretion rates commensurate with production of short-term accommodation space, very thick near keep-up cycles will be laterally offset owing to short-term accommodation limits in any given locality.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas