A LARGE LATE QUATERNARY PALEOLAKE NEAR MONAHANS, TEXAS, INFERRED BY LARGE-SCALE BIOGENIC STRUCTURES

Herein we describe and interpret a shallow, spatially extensive, calcarenite/sandy limestone deposit near Monahans, TX. Core data and 3D modeling indicate the deposit is relatively flat, laterally continuous over > 80 km², and ranges in thickness from ~ 1 to 10 m. Outcrop observations indicate several subunits with a diversity of primary and biogenic sedimentary structures that reveal a complex history of subaqueous to subaerial processes. The base of the deposit (Unit 1; U1) is a bioturbated sandy calcarenite. The carbonate in U1 is primarily micrite with a peloidal microtexture. Root traces occur throughout U1. Simple, subvertical shafts (Camborygma isp.) crosscut the U1 upper contact and penetrate up to 55 cm. Burrow fill closely resembles the lithology of the overlying U2. U2 is characterized as a bioturbated, gravely limestone. U2 is mostly massive, but faint horizontal bedding occurs in zones between subvertical shafts. Root traces occur throughout U2 and are primarily horizontal to sub-horizontal. U3 is characterized as a finely bedded to laminated, sandy limestone with uncommon macroscopic biogenic structures. The uppermost unit (U4) is characterized as an undulatory, gravely limestone with an overall architecture composed of meter-scale domal structures. These undulatory structures are convex up and have an upward increase in amplitude. U4 is highly heterogeneous; some beds are nearly pure micrite and/or very fine sand, whereas others are composed of large limestone clasts within a mudstone matrix. Limestone clasts range from well-rounded to highly brecciated. In situ polygonal mud cracks can be observed on many bedding plane surfaces within the large domal structures. Bedding tends to onlap onto the mound structures, forming conglomeratic lenses. In some instances U4 is very finely laminated with a stramatolitic microfabric. The upper contact of U4 is wavy, defined by the architecture of the mounds. This unconformity surface represents a transition to a highly diachronous, modern sand sheet with Holocene pedogenic features. We interpret this deposit as a large paleolake, which is not uncommon in late Quaternary strata of NM and West TX, but has not been documented previously in this area (~ 150 km radius). OSL ages of 199 to 335 ka provide a minimum age constraint for this lake, at or near the lake margin.