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Description
The Pliocene Fish Creek Sturzstrom is a volumetrically significant (300x106 m3) deposit of a subaqueous long run-out “landslide.” It exhibits a curious distribution of major rock types including pegmatitic granodiorite/tonolite, pegmatitic mica schist and gneiss, and pegmatitic leucocratic granite. Although it traveled 11 km from the Fish Creek Mountains to the foot of the Vallecito Mountains, the breccia consists of countless lithologically similar domains often with abrupt boundaries indicating little particle-by-particle mixing. Erosion into the breccia sheet reveals that domains are scale independent (from cm to km). Typically a domain will consist of pervasively fractured clasts of a given composition that are shattered and separated by an intervening matrix composed primarily of comminuted fragments of the same composition Often exhibiting “three dimensional jigsaw-puzzle fabric” the fragments remain spatially connected and allow the observer to visually “put the pieces back together”. Geochemical study of 16 samples using major, minor and REE indicate that plutonic clasts and their surrounding matrix are virtually identical in composition; both plot within the Harper Creek and Cuyamaca Reservoir gneiss (metaplutonic) fields. Log normal distribution plots suggest a chemical affinity between schist clasts and their corresponding matrix; both are similar to Julian Schist average data. Normalized to immobile aluminum, pegmatitic dike clasts contain only 3 elements with similar abundance to those of the surrounding matrix indicating that their respective chemical compositions diverge. Results obtained from a point-count procedure requiring up to 22 decisions per grain are consistent with the geochemical data and suggest that both matrix and clasts in a given domain are lithologically similar. For example, the composition of large (cm's across) plutonic rock fragments with Q48K20P32 are similar in composition to the single mineral sandstone fraction with Q44K22P34 found in the surrounding matrix. The persistence of lithologically similar domains within the sturzstrom many kilometers from its source indicates little mixing within the mass during transport. Compelling evidence for a southeast to northwest paleoflow direction is the progressive and irreversible loss of internal sturzstrom fabric. On the apron of the Vallecito Mountains near the toe of the deposit, the jigsaw-puzzle fabric was destroyed as lithological domains were dispersed and the fragments were disseminated throughout the matrix.
