WARME, JOHN , Colorado School of Mines, Golden, CO

ABSTRACT: Anatomy of an Anomaly: The Catastrophic Devonian Alamo Impact Breccia, Nevada

One must understand the expectable, in order to recognize the anomalous. Facies models, stratification sequences, sequence stratigraphy, and seismic stratigraphy provide tools to interpret sedimentary rocks. Using these tools, and by experience, rock analysts learn what to expect in sedimentary basins. Without such background and expectations, important anomalous occurrences are misinterpreted or completely overlooked. This theme is illustrated using three stratigraphic anomalies. All three are ancient catastrophic carbonate mass flows. Two are expectable, occurring along rift fault lines and therefore readily understood. The third, the Alamo Breccia, seems totally anomalous, and requires another level of investigation.

The Late Devonian Alamo Breccia is a sedimentary megabreccia discovered in 15 different mountain ranges across southern Nevada. It occurs anomalously within the shallow-water carbonate platform facies of the Upper Devonian Guilmette Formation. Before tectonic dismemberment by thrusting, it covered a north-south distance of 200 km and an area of several thousand square kilometers across the carbonate platform. The Breccia ranges in thickness from 130 m near its center to a featheredge at its periphery. Iridium concentrations, shocked quartz, ejecta spherules, displaced fossils, and other evidence all confirm that the Breccia formed from a meteorite or comet impact. Internally it contains a chaotic ejecta bed and displaced blocks as much as 500 m long, overlain by as many as 5 graded beds deposited by tsunamis. The Alamo Event was large enough to rear-range the Devonian platform paleogeography, but not sufficient to cause direct extinctions.

The known flux of asteroids and comets with Earth-crossing orbits predicts that many similar deposits must exist in the stratigraphic record. They may serve as significant reservoirs, seals, or seismic markers, as well as extinction horizons. The anomalous Alamo Breccia is a well-exposed example that represents a model for such deposits.

AAPG Search and Discovery Article #90910©2000-2001 AAPG Distinguished Lectures