Abstract
The lower member of the Animas Formation (McDermott Member) is a volcaniclastic sequence derived from a north-northwest source (San Juan-La Plata area). It consists of purple andesitic debris flows, green fan-delta sandstones and mud rocks, and dark gray conglomerates with clast compositions indicating that the Precambrian core of the source uplift was exposed. The upper member is a sand-dominated alluvial plain sequence deposited by southwest-flowing braided streams. It includes green-gray-brown carbonaceous mudstones and pebbly sandstones containing clasts of mudstone, andesite, and detritus from Precambrian and Mesozoic sources in the Brazos-San Luis uplift to the east and northeast. by the end of Animas deposition, the San Juan sag (then a northeastern extension of the San Juan basin) was a broad, southwest-plunging synclinal downwarp bounded by hogback monoclines to the north and east. An erosional period followed Animas deposition; the greatest thickness of Animas was preserved along the axis of this synclinal feature. Bright-red sandy mudstones and yellow-gray pebbly sandstones and cobble conglomerates comprise the proximal alluvial-fan deposits of the Blanco Basin Formation. They unconformably overlie Precambrian through Paleocene rocks and clast compositions reflect these sources. Renewed uplift and segmentation of the Brazos-San Luis uplift resulted in the shedding of detritus southwestwardmore » into the San Juan sag and eastward into a narrow, asymmetrical, north-trending wrench basin within the uplift. Following Blanco Basin deposition, the last Laramide event is represented by the separation of the San Juan sag from the San Juan basin by uplift of the Archuleta anticlinorium.« less
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