New Techniques in Defining Allocyclicity in "Dry" Alluvial Fan Sequences

Dan Walker, Jeff Grigsby

Allocyclicity in alluvial fan sedimentation can be attributed to changes in three extrinsic factors: (1) tectonism, (2) climate, and (3) eustatic sea level. This study examines the viability of two methods for identifying allocyclic variations in "dry" alluvial fan sequences. These include large-scale (> 100 m) vertical trends in the ratio of trough cross-stratified to horizontally laminated sediments (reflecting progradation or retreat of the midfan environment), and the petrographic ratio of volcanic rock fragments to feldspar grains. Variations in climate should be reflected by this ratio due to the higher susceptibility of rock fragments to chemical weathering.

Midfan and distal fan alluvial sediments comprise 923 m of the Hayner Ranch and Rincon Valley Formations (Miocene) at San Diego Mountain, New Mexico. These sediments were derived exclusively from volcanic and sedimentary source rocks, and were deposited in a closed basin, eliminating eustatic sea level change as a possible allocyclic variable. Analysis of the vertical trends in average maximum clast size results in the delineation of two allocyclic trends. These trends are also reflected in the ratio of trough cross-stratified to horizontally laminated sediments. The ratio of volcanic rock fragments to feldspar grains lacks these trends, indicating a near constant climate. This apparent uniformity in climate is in agreement with other observed climatic indicators. These relationships llow a high degree of confidence in the interpretation of the two allocyclic sequences as being tectonic in origin, and suggest that the use of these factors may be valid criteria for determining allocyclicity in similar alluvial fan deposits.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.