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.