New Mexico Geological Society Annual Spring Meeting — Abstracts

In the central Peloncillo Mountains, Lower Cretaceous Bisbee Group and Laramide sedimentary rocks crop out in two blocks bounded by high angle, NW-SE-trending faults. Stratigraphic and provenance analysis of correlative units of the two blocks indicates an initial period of extension during which normal faults controlled Bisbee Group deposition, a second period of local tectonic activity during onset of Laramide volcanism, and a third period of structural inversion during which reverse movement of pre-existing normal faults controlled uplift patterns and coeval Laramide deposition.

Basal deposits of the McGhee Peak Formation, the lowermost unit in the Bisbee Group, record transgression during Aptian time. In the southern block, boulder conglomerates composed of Paleozoic clasts and interpreted as sediment-gravity flow deposits rest unconformably on Permian strata. Cherty biomicrite with oysters overlies the conglomerate; abrupt lateral lithofacies changes suggest deposition in fan-delta complexes. This basal succession is absent from the northern block, where the McGhee Peak Formation rests directly on Pennsylvanian-Permian strata.

Aptian-Albian deposits of the upper part of the McGhee Peak Formation and overlying Carbonate Hill and Still Ridge formations, all in the Bisbee Group, record a complete transgressive-regressive cycle of deposition. Cobble conglomerate and cross bedded sandstone directly overlie Paleozoic rocks in the northern block. These strata, interpreted as braid-delta deposits are overlain in both blocks by fine-grained tidal-flat deposits and shallow marine sandy biomicrite. The interval of transgressive strata in the southern block is at least twice as thick as the equivalent section on the northern block. Overlying regressive deposits consist of tidal-flats deposits with paleosols development. Quartz is the dominant mineral constituent on the sandstones, but chert, feldspars, plutonic rock fragments. and intrabasinal grains are present and increase in abundance upsection. The Burro uplift to the north has been considered as the source for this detritus.

Coastal-plain and shoreface facies of the Johnny Bull Formation, the upper most formation of the Bisbee Group, overlies the Still Ridge Formation in both blocks. In the northern block, coarse-grained quartz arenite rests on a paleosol-rich interval, but in the southern block the quartz arenite is fine-grained and interbedded with overbank deposits. Textural and compositional maturity of these arenites indicate a topographically subdued source area or one more distant than the Burro uplift.

The Laramide Bobcat Hill Formation, present only in the northern block, records at least two episodes of tectonic activity. The basal part crops out locally and consists of conglomerate composed exclusively of quartz arenite clasts resting disconformably on the Still Ridge Formation. It grades upsection to conglomerate with limestone, schist, granitic and volcanic clast. The conglomerate records uplift and erosion of Mesozoic and Paleozoic strata and basement rocks to the south. Uplift and erosion of both the northern and southern blocks probably took place at this time.

The Bobcat Hill Formation consists predominantly of intermediate volcanic, conglomerate and volcaniclastic deposits. Conglomerate near the southern block consists dominantly of sedimentary clasts, whereas in other areas of the northern block, sedimentary and volcanic clasts are subequal in abundance. The Andesite of Steins Pass overlies the Bobcat Hill Formation with slight angular unconformity on the northern block, but on the southern block it overlies Paleozoic units with angular discordance. These observations suggest that uplift, folding and erosion of Paleozoic and Bisbee Group strata took place largely in the Eocene, during deposition of the middle and upper parts of the Bobcat Hill Formation.