New Mexico Geological Society Annual Spring Meeting — Abstracts

The Mangas Basin in SW New Mexico, USA, is a Pliocene-Early Pleistocene extensional basin. During this time an axial fluvial system flowed SE through the basin and into a closed lake (Lake Buckhorn) near the present towns of Buckhorn and Cliff, NM. The sediments deposited in the paleolake are part of the Gila Formation, also known as the Gila Conglomerate. The Gila Formation is basin fill sediment deposited in the drainages of the Gila and Mimbres rivers, overlying Cenozoic volcanic rocks. The Gila river and its tributaries have exposed all or part of the Gila Formation, in the Mangas Basin. It is approximately 200m thick and is no older than Late Miocene. The majority of the sediment is Pliocene age based on vertebrate fauna and zirconfission-track dates of vitric tuffs. The younger age is poorly constrained at Lower Pleistocene due to the presence of Pleistocene fauna. The focus of this research is on the lacustrine sediments and using their sedimentology to determine the size, chemistry, depth, and the scale of the shrinking and expanding of the lake.

The lake was approximately 250 km² , extending from Cliff to north of Buckhorn, NM. The lacustrine facies are green mudstones, siltstones, sandstones, limestones, diatomites, and Magadi-type cherts. These are rooted and bioturbated indicating a shallow depositional environment. The lacustrine facies are interbedded with alluvial fan facies, mainly conglomerates on sandstones of distal to proximal fan environments. Periodically the lake was concentrated in silica and sodium and often alkaline. The lake might of had two scales of shrinking an expanding cycles. The small scales cycles possibly represent a time periods of thousands of years and the large scale millions of years.