Michael C. Pope1, Daniel M. Hunter2, Jessica B. Steffan1, Bryn E. Clark2
(1) Washington State University, Pullman, WA
ABSTRACT: 2nd-Order Sequence Stratigraphy of Late Middle to Late Ordovician Montoya Group, southern New Mexico and West Texas
The late Middle to Late Ordovician Montoya Group (130-160 m thick) of southern New Mexico and west Texas formed a gently dipping ramp on the southern Laurentia passive margin. A detailed sequence stratigraphic study of the Montoya Group based on measured sections, hand-held gamma-ray logs and conventional wireline logs indicates a single 2nd-order (10-15 m.y. duration) supersequence composed of 3 to 5 regionally correlative 3rd-order (2-5 m.y. duration) sequences. Smaller-scale, 4th- and 5th-order parasequence sets and parasequences are discernible at outcrops but do not appear to correlate regionally.
The basal Montoya supersequence unconformity, a composite karst surface representing a 10-30 m.y. hiatus, varies from a sharp planar surface to irregular karst sinkholes (from a few meters to a few 10's of meters deep) filled with blocks of the underlying El Paso Group in a sandstone matrix. The 2nd-order LST is not present in the study area. Updip the lower part of the 2nd-order TST consists of carbonate-cemented, burrowed, coarse granule sandstone (Cable Canyon Sandstone) that passes basinward and upward into-burrow mottled skeletal packstone with abundant hardground surfaces (Upham Formation). The skeletal packstone is abruptly overlain by cherty, laminated calcisiltite of the lower Aleman Formation. A 2nd-order maximum flooding surface is not discernible in these deepwater facies, but it is a maximum flooding "zone" evident on the gamma-ray logs almost immediately above the packstone/calcisiltite contact. The 2nd-order HST is composed of subtidal cherty dolostone (upper Aleman Formation) that passes upward into peritidal carbonate (Cutter Formation) capped by a regional disconformity.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado