New Mexico Geological Society Annual Spring Meeting — Abstracts

New observations in the Holt Mountain 7½-minute quadrangle near Glenwood, New Mexico are clarifying the volcanic stratigraphy and origin of the Eocene (~34 MY) Cooney Tuff (Ratté and McIntosh, 2001, The Cooney Tuff and Mogollon Caldera- A Thesis Looking For A Student: New Mexico Geology, v. 23, no. 2, p. 55). A third mappable member, long suspected, but previously undefined, is now confirmed to underlie the middle Whitewater Creek and the upper Cooney Canyon members. This lower member, provisionally designated the South Fork Member, for outcrops south of the junction of the South Fork and main Whitewater Creek, consists of as much as 300 meters of silicic ash-flow tuff (ignimbrite) and a total of 50 to 100 meters of interlayered basaltic lava flows; and is interpreted as an early, episodic member erupted prior to the major caldera-forming eruptions of the Whitewater Creek and Cooney Canyon Members.

Whereas, the Whitewater Creek Member is a simple cooling unit, 100 to 200 meters thick, whose catastrophic eruption is interpreted to have caused the initial subsidence of the proposed Mogollon caldera, the overlying Cooney Canyon Member consists of a dozen or more compound cooling units, several tens of meters thick, erupted during continued subsidence. Several thin, <1 to ~10 meters thick, dark-colored, fine to coarse-grained volcaniclastic sandstone layers, which are interbedded with the ignimbrites of the Cooney Canyon Member, have long presented problems as to their origin and provenance either as true waterlain sedimentary deposits, or volcanic pyroclastic deposits. New petrographic evidence in the form of unbroken bubble-wall shards, pumice, and possible accretionary lapilli, provide seemingly indisputable evidence of a pyroclastic fall origin for at least some, if not all, of these deposits. Importance of this interpretation cannot be overemphasized with respect to the existence of a Mogollon caldera, for if correct, it confirms the continuity of pyroclastic eruptions, without appreciable time gaps from the last ignimbrite eruptions of the South Fork Member to the final eruptions of the Cooney Canyon Member, representing a total thickness of Cooney Tuff on the order of 1000 meters.

A major southeast trending fault near the south end of the Catwalk National Recreation Trail, on the east side of Whitewater Creek, places Whitewater Creek and Cooney Canyon ignimbrite members of Cooney Tuff down against ignimbrites and interlayered basaltic lava flows of the South Fork Member. This fault, which is cut off at Whitewater Creek by the frontal fault of the Mogollon Rang in this area, is interpreted as a possible, if not likely, segment of the structural wall of the Mogollon caldera, the proposed source of the ~34 MY old Cooney Tuff.