THE LOBO FORMATION OF SOUTHERN NEW MEXICO: A LARAMIDE SYNTECTONIC DEPOSIT

The Lobo Formation (Paleocene-Eocene?) consists of intermontane deposits of lacustrine, fluvial and alluvial-fan origin. The Lobo near Capitol Dome in the Florida Mtns of SW New Mexico is ~116 m thick and consists of a thin basal pebble and cobble conglomerate composed of locally derived carbonate clasts, overlain by an interval of very fine to fine-grained sandstone interbedded with siltstone, an interval of reddish-brown siltstone and pebbly sandstone, and an upper cobble and boulder conglomerate about 20m thick. Basement clasts are absent from the basal conglomerate, but common in the upper conglomerate. The Lobo “supersol” is a well-developed paleosol carbonate that is present along a karstic paleosurface at the base of the Lobo just west of Capitol Dome. δ¹⁸O values of this carbonate range narrowly around -12‰ and suggest an elevation 775±250 m higher than the modern elevation (~1570 m). The age of the Lobo Formation remains unknown, but preliminary paleomagnetic analysis of this section indicates a thick reversed interval bracketed by two thin normal intervals; the data suggest these rocks were deposited after chron 27n (~61 Ma) and that the section has undergone a post-depositional clockwise rotation of 20-30°. These rocks were deposited in a low-accommodation setting following extensive Laramide faulting. In the nearby Victorio Mtns, the Lobo is a generally upward-fining succession that is ~325 m thick and composed of alluvial-fan and fluvial deposits with higher accommodation than those in the Florida Mtns. Sediment accumulation rates slowed in the latter half of Lobo deposition in the Victorio Mtns as indicated by paleosols in the middle and upper part of the section; depositional style and paleosols indicate deposition in arid conditions. Conglomerate clasts at the base of the section were derived from diverse sources, including a Proterozoic basement, Paleozoic carbonate and siliciclastic strata, Jurassic basalt flows and Lower Cretaceous strata of the Bisbee Basin. Previous interpretations of the Lobo in the Victorio Mtns suggest the section occupies a different intermontane basin than the section in the Floridas, which is consistent with the stratigraphic and petrographic differences between the sections.