LATEST PENNSYLVANIAN TO EARLIEST PERMIAN CONODONTS FROM THE HORQUILLA FORMATION, BIG HATCHET MOUNTAINS, SOUTHWESTERN NEW MEXICO

The Horquilla Formation in the Big Hatchet Mountains comprises over 1000 meters of predominately cyclical shallow subtidal shelf carbonates that accumulated from Early Pennsylvanian (Bashkirian) through Early Permian (Asselian) time along the margin of the Pedregosa Basin.  The excellent exposures and apparent stratigraphic continuity of the Big Hatchet Horquilla section, although complicated by faulting, makes it the ideal place to resolve the pattern of cyclical carbonate deposition through time relative to fusulinid (Wilde) and conodont biostratigraphy.

Two sections of the Horquilla Formation that cross the Pennsylvanian/Permian boundary were sampled for conodonts, the New Well Peak section near the southeastern end of the Big Hatchet Mountains and the Borrego section near the center of the range.  Wilde previously sampled both sections for fusulinids and provided a summary of the fusulinid succession against which the conodont succession can be compared.  Conodont faunas are dominated by species of Streptognathodus, with smaller number of elements of Hindeodus and Sweetognathus (Permian samples) and rare Adetognathus and Ellisonia.  Most elements were recovered from packstones and grainstones in the upper parts of the carbonate cycles; few elements were obtained from carbonate mudstones and wackestones that form the lower parts of the cycles. 

Virgilian samples (fusulinid zones VC-2 and 3) are dominated by elements of Streptognathodus pawhuskaensis and related forms, including specimens similar to S. virgilicus in higher samples.  Fusulinids become more common higher in the section and readily delimit zone PW-1 (Bursumian; Newwellian of Wilde).  Conodonts from the lower part of PW-1 differ little from the underlying fauna, but in the middle part of the zone species characteristic of the latest Gzhelian (eg. S. bellus) appear.  Conodonts from the highest beds of PW-1 include species of the S. wabaunsensis group.  Conodonts are less common in basal Permian beds (PW-2) and the Pennsylvanian/Permian boundary is not easily identified using conodonts.  Primitive Sweetognathus and narrow species of Streptognathodus indicate an Asselian age for the portion of the Permian studied thus far.