Walther's Law Caveats in Clinoformal Settings: An Example from the San Andres Formation, Last Chance Canyon, Guadalupe Mountains, NM
Mark D. Sonnenfeld and Alan J. Scott
Facies modeling requires the routine application of Walther's Law. Inferring horizontal distributions of strata from vertical facies successions requires lateral translation through space of an inclined, often clinoformal depositional profile. Outcrop belts such as the Guadalupe Mountains and published world wide seismic data have shown that clinoform geometries and their attendant facies associations are in a nearly continuous state of dynamic evolution. Sequences, systems tracts, base-level hemicycles, stacking patterns, and geometric descriptions are, in part, attempts to discretize such evolution so that precise applications of Walther's Law avoid crossing important unconformable or hiatal surfaces.
The facies architecture of Last Chance Canyon's San Andres strata has previously been depicted on cross-section. To better portray evolving 3-d facies associations, we present a series of block diagrams, each of which represents, in part, a Walther's Law inference. We discuss the strengths and weaknesses of the time-honored block diagram approach in the context of an experiment wherein we apply Walther's Law over subtly different rock intervals corresponding to four important methods of sequence stratigraphic subdivision: a) a full 4th-order depositional sequence; b) systems tracts; c) bipartite stratal division based on increasing versus decreasing inferred accommodation/sediment supply ratios; and d) subdivision of the stratal record into landward-stepping, landward vertically stack d, stratigraphically rising seaward-stepping. stratigraphically falling seaward-stepping, and seaward vertically stacked stacking patterns. Comparison with more direct facies models based upon preserved shelf-margin clinoform profiles representing 5m to 100m paleo water depth, suggests that rigorous five-fold stacking pattern analysis yields the highest resolution depiction of facies tract evolution.
The same experiment can be applied with other approaches used to depict dynamic facies associations including: histograms, proportion curves, and "snake" diagrams. Snake diagrams graphically superimpose key facies trends on inferred shelf and basin accommodation curves. Multiple graphic approaches seem necessary to capture "moving target" facies associations; however, approaches such as snake diagrams seem more effective at the exploration scale, whereas histograms and proportion curves are more effective at the development scale. Block diagrams can be effective at any scale, but for exploitation relevance require the high degree of temporal resolution attainable through physical stratigraphic stacking-pattern analysis.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California