Print this pageP. Ted Doughty1, Douglas K. McCarty2, Jean Hsieh2
(1) Eastern Washington University, Cheney, WA
(2) Texaco Upstream Technology, Houston, TX
ABSTRACT: Controls on the Development of Clay Smearing Fault Seals Along Synsedimentary Faults in Unconsolidated Sediments: a Field Example from the Calabacillas Fault, New Mexico
New geologic mapping and measurements of fault zone materials along the Calabacillas fault provide new information on the development of clay smears and the fault seal potential of synsedimentary normal faults. The Calabacillas fault is a Quaternary growth fault that offsets interbedded unconsolidated mud, sand, and gravel of Pliocene-Pleistocene age along the western margin of the Rio Grande Rift near Albuquerque, New Mexico. The fault zone exhibits an external damage zone of small-throw faults that surround an internal zone where one or two slip surfaces have accommodated most of the fault displacement and anastomose around large fault bounded slivers. Such features are typical of more "brittle" faults formed in consolidated sediments. Clay gouge is present nearly everywhere along the 2,000 meter strike exposure of the fault, ranging in thickness from 1 cm to 380 cm. Only eight holes (< 2-5 m in length) were observed. The clay gouge is developed in alternating bands of clay-rich and sand-rich gouge that formed by the attenuation of footwall lithologies; there is little evidence of wholesale mixing. Source beds with the highest smectite equivalent content have been injected and smeared along the fault zone, whereas source beds with lower smectite equivalent content are truncated and do not contribute to the clay gouge. Thus the clay mineralogy and resulting ductility of the mud source beds controls the formation of clay smears. Current fault seal algorithms will overestimate the potential for fault seal unless they can account for the presence of noncontributing low smectite source beds.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado