Litho-Kinematic Facies Model for Large Landslide Deposits in Arid Settings

J. C. Yarnold, J. P. Lombard

Reconnaissance field studies of six large landslide deposits in the S. Basin and Range suggest that a set of characteristic features is common to the deposits of large landslides in an arid setting. These include a coarse boulder cap, an upper massive zone, a lower disrupted zone, and a mixed zone overlying disturbed substrate. The upper massive zone is dominated by crackle breccia. This grades downward into a lower disrupted zone composed of a more matrix-rich breccia that is internally sheared, intruded by clastic dikes, and often contains a cataclasite layer at its base. An underlying discontinuous mixed zone is composed of material from the overlying breccia mixed with material entrained from the underlying substrate. Bedding in the substrate sometimes displays foldin and contortion that die out downward.

Our work suggests a spatial zonation of these characteristic features within many landslide deposits. In general, clastic dikes, the basal cataclasite, and folding in the substrate are observed mainly in distal parts of landslides. In most cases, total thickness, thickness of the basal disturbed and mixed zones, and the degree of internal shearing increase distally, whereas maximum clast size commonly decreases distally. Zonation of these features is interpreted to result from kinematics of emplacement that cause generally increased deformation in the distal regions of the landslide.

Spatial zones within landslide deposits are frequently associated with particular lithofacies in the underlying substrate. Proximal landslide deposits are commonly associated with near-source coarse-grained substrate sediments. In contrast, features typical of the distal margin often overlie lacustrine or floodplain sediments. A combination of the internal kinematics zones and associated substrate lithofacies allows the construction of a litho-kinematic facies model that can aid in reconstructing paleogeography and the geometry of ancient landslides.

AAPG Search and Discovery Article #91024©1989 AAPG Pacific Section, May 10-12, 1989, Palm Springs, California.