Paper No. 8
Presentation Time: 9:00 AM-6:30 PM
LATE DEVONIAN INCISED-VALLEY SEQUENCE (IGNACIO AND ELBERT FORMATIONS), SOUTHWESTERN COLORADO, U.S.A
Detailed stratigraphic studies show that the Ignacio Fm. is Devonian age and a lateral facies equivalent of the Famennian Elbert Fm. in southwestern Colorado. The age of the Ignacio Fm. has been problematic; arguments for Cambrian age are based on poorly preserved Obolus brachiopods but there is no evidence for unconformities or petrographic differences with the overlying Elbert Fm. Precambrian basement in the region is locally overlain by an unnamed sedimentary unit consisting of boulder conglomerate and sandstone of unknown (Late Precambrian-Devonian) age (Evans, 2007). The Ignacio Fm. (0-35 m thick), McCracken Sandstone Member of the Elbert Fm. (0-24 m thick) and the Upper Member of the Elbert Fm. (0-19 m thick) can be found either: (1) uncomformably overlying Precambrian basement, (2) unconformably overlying the unnamed sedimentary unit, (3) in a conformable succession, or (4) conformable, but locally missing one or more of the Late Devonian units. Paleotopography is the cause of this stratigraphic complexity, with a maximum relief of about 65 m on the Precambrian basement surface, and the Late Devonian units onlap the basement surface from west to east. The local presence or absence of the sedimentary units cannot be ascribed to faulting (e.g., previous authors) because of consistent lateral facies relationships, paleocurrents, and isopachs. The Ignacio Fm. is interpreted as an east-west transition from fluvial to bayhead delta to estuarine depositional environments. The fluvial Ignacio Fm. consists of cross-bedded sandstones with minor pebble conglomerates and red shale. The estuarine Ignacio Fm. consists of a variety of heterolithic sandstones, siltstones, and shales with flaser, lenticular, and wavy bedding, planar lamination, and sandstones with festoon and herringbone cross bedding. The McCracken Sandstone Member is interpreted as a transgressive strandplain/barrier sequence. The unit changes upward from inclined planar laminated, festoon cross-bedded, and wave rippled sandstone to hummocky and swaley stratified sandstones, representing a transition from overwash and upper shoreface to lower shoreface environments. The Upper Member is interpreted as a mixed clastic-carbonate peritidal sequence with tidal rhythmites, mudcracks, hopper crystals, and stromatolites.