Abstract: An Evaluation of Extension in the Western Indian Wells Valley, San Bernardino County, CA, Using Geologic, Geophysical, and Downhole, Formation Microscanner® (FMS) Data

F. C. Monastero, D. Thorn, A. E. Sabin

The Indian Wells Valley (IWV) located at the southern terminus of the Sierra Nevada is considered by some as the westernmost valley in the Basin and Range province. Despite the fact that Cenozoic extension in the Basin and Range has been well documented, the IWV has never been explicitly recognized as an extensional feature. Interpretation of recently-acquired seismic reflection, gravity, and Formation MicroScanner® (FMS) images, and side wall cores supports an extension model for the basin. Seismic reflection data show that the sedimentary sequence proximal to the Sierra Nevada is nearly devoid of laterally extensive, coherent reflectors. This pattern suggests rapid, chaotic deposition of detritus from the nearby mountain range. Farther east, away from the base of th mountains, the seismic data show well-developed, more or less, flat-lying reflectors consistent with a distal lacustrine facies deposited in a lacustrine setting of pluvial lakes known to have existed in this valley during Plio-Pleistocene time. The data also clearly show a west-vergent reflector extending eastward beneath the IWV which has been interpreted as a detachment fault along which extension has taken place (Monastero and Katzenstein, in press).

More than 10,500 individual fractures, bedding planes, and textural features were described from FMS® images taken from a 3,048 m deep exploration hole designated SNORT #2 located in the western IWV. Analysis of these "events" in combination with core sample petrography reveals that the interval from 685 to 1856 m is composed of angular, medium- to coarse-grained, poorly-indurated granitic alluvium which lies directly on top of granitic basement. The composite attitude of bedding in this interval is north-striking at the bottom with a west dip in northeast-striking, west-dip at the top of the interval. The coarseness and angularity of the sediments supports the idea that they are indeed a proximal facies. Sedimentary structures as discerned from the FMS® images support the int rpretation of coalescing alluvial fan deposits. Top of the crystalline basement is located at 1856 m and is marked by a five-fold increase in fracture density. A zone of drilling mud loss was encountered at 2436 m depth. This coincides with a large increase in fracture density and formation "rugosity" suggesting a major fault zone through which fluids were produced on a flow test. Chlorite+epidote±calcite+pyrite±sericite found in the deep fractures indicates at least one episode of hydrothermal alteration although the timing cannot be established.

AAPG Search and Discovery Article #90958©1995 AAPG Pacific Section Meeting, San Francisco, California