In four large Oligocene calderas in the western San Juan Mountains – Lake City, Silverton, San Juan, and Uncompahgre – spectacular breccias are intermixed with thick intracaldera ash-flow tuffs that accumulated during caldera collapse. These breccias are divided into two intergradational types: (1) mesobreccia in which numerous small clasts are visible within single outcrops and (2) megabreccia in which many clasts are so large that the fragmental nature of the deposit is obscure in many individual outcrops.

In general, mesobreccia occurs as thin tabular deposits locally interlayered with upper parts of the intracaldera ash-flow accumulations; it is readily interpretable as resulting from small- to medium-sized rock falls and rock slides from the caldera walls. In contrast, megabreccia is dominant in the lower part of the caldera-filling sequence and contains only minor intermixed ash-flow material. Megabreccia is difficult to distinguish from pre-collapse caldera floor in places, but local lenses of welded tuff near the deepest stratigraphic levels exposed within the calderas indicate that these rocks are mostly megabreccia that resulted from major slumping and caving of caldera walls during the initial stages of caldera collapse. An especially large megabreccia unit within the San Juan and Uncompahgre calderas is here named the Picayune Megabreccia Member of the Sapinero Mesa Tuff.

Megabreccias similar to those in the western San Juan calderas occur in other eroded collapse structures in the western United States, and the presence of such deposits may be useful guides to the roots of caldera structures in deeply eroded, highly altered, or structurally complex volcanic terranes.

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