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Abstract: Sponge-Algal Mud Mounds of the Lower Cretaceous Cupido Formation and their Sedimentologic Significance, Northeastern Mexico

MURILLO-MUNETON, GUSTAVO and STEVEN L. DOROBEK, Department of Geology & Geophysics, Texas A&M University.

Previously unreported sponge-algal mud mounds have been found in outcrops of the Lower Cretaceous Cupido (Barremian-Lower Aptian) Formation, NE Mexico. These carbonate buildups appear first near the contact with the underlying Taraises Formation and are distributed throughout the remainder of the lower Cupido Formation.

Individual mud mounds have lenticular to domal morphologies; smaller mounds commonly amalgamated or coalesced into larger mound complexes. Both the length and height of individual mud mounds range from a few meters to several tens of meters. Mud mound core lithofacies dominantly consist of massively bedded, sponge/algal boundstones. In situ siliceous sponges include hexactinellid and lithistid types that have been completely replaced by calcite. Algal crusts and peloidal cements locally formed on the calcitized sponge skeletons. Other skeletal components include serpulid worm tubes, bryozoans, sponge spicules, and encrusting foraminifera. Mound flank facies consist of well-bedded wackestone and packstone containing subangular to subrounded lithoclasts, tuberoids (fragments of sponges and algal crusts), peloids, benthic foraminifera (miliolids, Textularia sp., Lenticulina sp.), ostracodes, and mollusc fragments. Intermound facies consist of medium- to thick-bedded mudstone and wackestone with similar grain types as mound-flank facies, but with mud-supported fabrics. Some mud mounds also contain complex thrombolitic fabrics, local pockets of grainy sediment, and rare pelecypods, small colonial corals and patches of marine cement. The Cupido mud mounds, thus, have remarkable lithologic and faunal similarities with Upper Jurassic sponge-algal mud mounds of Europe.

The lithologies, biota, and sedimentary structures of the Cupido mud mounds suggest moderately deep-water (sub-wave base) depositional conditions, probably in outer ramp environments. The Cupido mud mounds are a volumetrically important facies component within the Lower Cretaceous strata of NE Mexico and they may provide an important outcrop analog for the evolution of Early Cretaceous platforms around the Gulf of Mexico passive margin.