New Mexico Geological Society Annual Spring Meeting — Abstracts

Detailed mapping (1:12000) and stratigraphic studies have shown that two distinct clastic synorogenic "Laramide" units-the Ringbone Fonnation and the Skunk Ranch Formation (tentatively named)-and the intermediate volcanic Hidalgo Formation were deposited and subsequently deformed within a complex tectonic/structural regime that existed in southwestern New Mexico during latest Cretaceous-Eocene time.

The late Campanian-Maastrichtian Ringbone Formation (Lucas et al, 1990; Basabilvazo 1991) unconformably overlies Lower Cretaceous rocks and consists of at least 1600 meters of clastic detritus in the axis of the Ringbone basin. Three members were mapped within the Ringbone including 1) a lower conglomerate unit deposited by a braided fluvial system, 2) a middle feldspathic sandstone-shale unit representing fluvial, lacustrine, and deltaic environments, and 3) an upper conglomerate-sublitharenite sandstone unit that records cyclic alluvial fan deposition. A laterally continuous olistostrome deposit consisting predominantly of Lower Cretaceous Hell-to-Finish clasts was noted within lacustrine deposits of the middle member.

The Skunk Ranch Formation has tentatively been assigned a Paleocene to Eocene age based on fresh-water ostrocode fossils (Lawton et al, 1990) although a latest Cretaceous age is possible. The Skunk Ranch is exposed only in the southern part of the study area where it lies variously on the Lower Cretaceous U-Bar and Mojado formations and the Ringbone Formation. Similarly to the Ringbone, the Skunk Ranch was mapped as three members: 1) a lower conglomerate -red shale member; 2) a middle sandstone-shale-limestone member of fluvial-lacustine origin; and 3) an upper cobble conglomerate that probably represents deposition by a braided fluvial system. A 60-m-thick purplish porphyritic basaltic andesite flow was also mapped in the upper pan of the middle member. Total exposed thickness is approximately 500 meters.

The Hidalgo Formation represents a stratovolcano complex and consists of at least 1700 meters of volcanic breccias and subordinate andesite flows but contains an increasing abundance of sedimentary beds in the upper part. The top of the unit is cut off by a thrust. The Hidalgo Formation has yielded zircon fission-track ages of 69.6± 3.2 to 57.9± 2.7 Ma (Marvin et aI, 1978; Lasky,1947).

The Hidalgo Formation interfingers with the upper member of the Ringbone in the northern part of the field area and we interpret the basaltic andesite flow within the Skunk Ranch to the south to be a "last gasp" Hidalgo volcanic event after a hiatus or a younger but similar flow not directly associated with the Hidalgo stratovolcano. Therefore we adopt the following stratigraphic sequence in ascending order: Ringbone Formation (late Campanian-Maastrichtian); Hidalgo Formation (Maastrichtian-Paleocene); and Skunk Ranch Formation (Paleocene-Eocene).

The Laramide stratigraphy and defonnation suggests a two-stage Laramide event in the Little Hatchet Mountains. Stage one began during Late Cretaceous time with relatively simple basement-cored block uplift(s?) along high-angle, northwest-trending reverse faults. The olistostrome deposits with Lower Cretaceous clasts in the middle member of the Ringbone Formation indicate topographically high (structurally high) Lower Cretaceous strata to the south or southeast which would coincide with uplift along the Granite Pass fault at the southern end of the Little Hatchets and/or uplift along the subsidiary Copper Dick fault in the northern part of the range. However the Granite Pass/ Copper Dick uplift was probably not a prominent feature until deposition of the upper member of the Ringbone began. Deposition of the Ringbone Formation occurred in a geographically restricted complementary basin adjacent to the uplifts.

Stage two began during the Paleocene with the inception of wrenching along the high-angle fault at Granite Pass. The Skunk Ranch Formation was deposited in an associated wrench basin. A wrench-related flower structure then developed which produced east-northeast yielding telescoped thrusts that folded and cut all three Laramide units. Skunk Ranch deposition was probably coeval with this latter stage of deformation as suggested by abundant soft-sediment deformation within middle member lacustrine strata.