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HERRIN, MATTHEW, Department of Earth and Environmental Science, New Mexico Institute of Mining and Technology, Socorro, NM 87801, USA

ABSTRACT: Outcrop and Petrographic Observations of Deformation Bands in Poorly Lithified Sand: Lower Santa Fe Group, Central New Mexico

Deformation bands which cut poorly lithified, lithic-rich sand of the Lower Santa Fe Group are characterized by reduced porosity and grain size as well as an altered composition and fabric relative to the parent sediment. Displacement of each formation band is accommodated across 1.5-2.5 mm wide zones that are slightly more indurated than the adjacent undeformed sand. Fault zones accommodating net displacements of 0.5-3 m are composed of ~30-100 deformation bands arranged in parallel and anastamosing or inosculating arrays. Petrographic studies indicate that deformation bands exhibit a five-fold reduction in macroporosity relative to the parent material. Porosity reduction is accompanied by a five-fold increase in the proportion of clay and silt-sized material relative to framework grains. Preferential breakdown of volcanic lithic fragments and feldspars during deformation is believed to be responsible for the generation of this fine-grained matrix. In situ and laboratory-based based permeability studies have shown that singlephase permeability within these deformation bands may be up to four orders of magnitude lower than that of the undeformed sand. This demonstrates a need to better understand the development, distribution, and fluid flow properties of small-displacement faults in unlithified or poorly lithified sand reservoirs.

AAPG Search and Discovery Article #90915©2000 AAPG Rocky Mountain Section, Albuquerque, New Mexico