New Mexico Geological Society Annual Spring Meeting — Abstracts

Cretaceous marine and nonmarine strata of late Albian-middle Cenomanian age exposed around the Cerro de Cristo Rey uplift in southern Doña Ana County, New Mexico comprise a section ~350 m thick and are assigned to the (ascending order) Finlay, Del Norte, Smeltertown, Muleros, Mesilla Valley, Mojado (= “Anapra”), Del Rio, Buda and Mancos (= “Boquillas”) formations. Macro- and microfossils from these strata indicate that the Finlay, Del Norte, Smeltertown, Muleros and Mesilla Valley formations are of late Albian age, whereas the Del Rio, Buda and Boquillas formations are of Cenomanian age. The base of the Cenomanian is most likely at a trangressive surface within the uppermost Mojado Formation. The late Albian (Manuaniceras powelli ammonite zone) to early Cenomanian (Neophlycticeras hyatti ammonite zone) sedimentary succession at Cerro de Cristo Rey consists of alternating fossiliferous limestone, shale with limestone and sandstone intercalations, and sandstone. Muddy limestone types are commonly wavy to nodular and represent deposits of an open marine shelf environment below wave base. Intercalated coquina beds rich in mollusc shells are interpreted as storm layers. Shale was deposited in an open shelf environment below or near wave base during periods of increased siliciclastic influx. Intercalated thin limestone and sandstone beds are suggested to be storm layers. The siliciclastic Mojado Formation is a regressive-transgressive succession formed in depositional environments ranging from lower shoreface to upper shoreface and even fluvial sediments, again overlain by shallow marine siliciclastics. Although the Washita Group section at Cerro de Cristo Rey is much thicker and displays some differences in facies, the succession shows similar transgressive and regressive trends when compared to the Washita Group of North Texas. Thus, we recognize eight unconformity-bounded depositional cycles in the Cretaceous section at Cerro de Cristo Rey, the upper Finlay Formation (youngest cycle of the Fredericksburg Group), lower Mancos Formation (base of Greenhorn cycle) and six Washita Group cycles: WA1 = Del Norte Formation, WA2 = Smeltertown Formation, WA3 = Muleros Formation, WA4 = Mesilla Valley Formation, WA 5 = most of Mojado Formation and WA6 = upper most Mojado and Del Rio and Buda formations.