--> Abstract: Giant Hams from the Laborcita Formation, Sacramento Mountains, New Mexico, by Kevin M. Moore, Ian T. Gordon, Brenda L. Kirkland, and Darrel W. Schmitz; #90124 (2011)

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AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA

Giant Hams from the Laborcita Formation, Sacramento Mountains, New Mexico

Kevin M. Moore1; Ian T. Gordon1; Brenda L. Kirkland1; Darrel W. Schmitz1

(1) Department of Geosciences, Mississippi State University, Mississippi State, MS.

Microbial deposits known as “giant hams” are found within a 50 m section of restricted/near shore siliciclastic sediments of the Lower Permian Laborcita Formation, Sacramento Mountains, New Mexico. The calcareous, ham-shaped deposits are 20 to 50 cm wide, and range from 50 to 90 cm long. This study documented their microbial origin using petrography and scanning electron microscopy. The ham-shaped deposits began as large (20 cm) microbial oncoids. Cross sections of the hams show three distinct growth zones. The first two growth zones are clearly concentric oncoidal growth zones, but the third growth zone developed on only the upper portion of the deposit. This in situ growth transformed the oncoid to a mini-bioherm and resulted in a shank-like shape. The interpretation of the inner layers of the hams as oncoids is based on irregular, concentric, biogenic micritic, 1-6 mm thick laminae surrounding a nucleus. Microbial origin is based on irregular laminations, fenestral fabrics, peloids, and fossil bacteria seen in SEM. The third growth zone is dominated by carbonate, but also contains quartz grains and clay minerals. These enigmatic carbonate deposits are significant because they formed in a dominantly siliciclastic, ecologically stressful setting. Improved understanding of the role of microbes in calcium carbonate precipitation in stressed settings is potentially applicable to medical, industrial, and academic fields.