Marine hardground in the Pennsylvanian Atoka Bank Carbonates, Eddy County, New Mexico

The term hardground refers to a surface of synsedimentary lithification associated with non-deposition or slow sedimentation. This paper introduces a marine hardground with characteristic microscopic properties in the Pennsylvanian Atoka bank carbonates, which forms a stratigraphic trap for natural gas. The algal bank complex includes several elongated biostromes up to 21 m thick, oriented normally to the paleo-shelf margin. Several lithofacies have been identified in the bank carbonates and surrounding sediments: crinoidal limestone (grainstone/packstone), nodular shaly limestone (wackestone/mudstone), bank margin (grainstone/boundstone), algal bank (wackestone), brecciated limestone, basal bioclastic micritic pile (wackestone), limy shale, and black shale. Phylloid alga, Archaelithopyllum, is the main fossil component in the bank complex and indicates deposition in a shallow marine environment at depths less than 30 m. The other fossils found with phylloid algae such as bryozoans, brachiopods, and echinoderms (sessile benthic suspension feeders) and small benthic foraminifers point out an open shelf environment away from clastic input. The hardground in the carbonates is characterized by an irregularly eroded surface capping well-lithified peloidal grainstone including scattered phylloidal algal plates and overlain by phylloid algal plates and large brachiopod fragments which are extending subparallel to the bedding surface. When the bank complex during the growth approaches to wave base, that increases agitation in the environment and erosion at the upper surface of the banks. Furthermore, agitation leads to the lithification of the peloidal sediments which decreases from the hardground surface into the sediments. Micro-borings penetrate downward into the lithified sediments. The poorly lithified bioclastic sediments with marine internal lime silts in the sheltered pores just above the hardground surface gradually pass upward into algal wackestone and reflect deepening conditions in the environment.