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Abstract: Stratigraphy and Provenance of Conglomerate Beds in the Puente Formation near Puddingstone Reservoir

MCLARTY, ANDREW, California State Polytechnic University, Pomona, CA; JONATHAN NOURSE, California State Polytechnic University, Pomona, CA

South and west of Puddingstone Reservoir, 1:6000 mapping characterizes conglomerate beds associated with the Puente Formation. Objectives were to determine stratigraphic position and possible source areas through clast population analysis. Mapped stratigraphic units reveal three conglomerate members exposed on both limbs of an east-northeast trending syncline. The oldest conglomerates rest unconformably on the mid Miocene Glendora Volcanics. Two or possibly three younger conglomerate units are also present, spatially associated with siltstones, sandstones, and shales. Some of these finer clastic beds of the Puente Formation have yielded Mohnian fossils (Mull, 1934). Pebble counts reveal that the two lower conglomerate units contain mainly volcanic clasts derived from local Glendora Volcanics. The uppermost conglomerate unit is dominated by clasts from eastern San Gabriel basement terrain.

The basal conglomerate is matrix supported with clasts ranging from 2-25cm and contains of 53.6% rhyolite-dacite, 21.7% gray porphyritic andesite, 18.3% aphanitic andesite, 6.0% dark gray basalt andesite, and 0.4% rotten porphyritic basalt clasts. The medial conglomerate unit consists of an upper and lower conglomerate beds separated by a sandstone horizon. Clasts from the medial conglomerate member typically range in diameter from 2-30cm and consist of 46.8% porphyritic andesite, 26.6% andesite, 9.2% flow-banded rhyolite, 6.9% rhyolite-dacite, 6.9% basaltic andesite, and 3.5% basalt-vesicular basalt. Flow-banded rhyolite distinguishes the lower horizon and porphyritic andesite the upper horizon. The uppermost boulder-cobble conglomerate unit contains 33.3% Mylonite, 25.3% granite, 13.8% diorite-gabbros, 9.2% porphyritic andesite, 6.9% quartzite, 5.7% gneiss, and 5.7% aphanitic andesite clasts.

 

AAPG Search and Discovery Article #90911©2000 AAPG Pacific Section and Western Region Society of Petroleum Engineers, Long Beach, California