--> ABSTRACT: Recognition of Subaerial Exposure through the Style of Echinoderm Preservation, by J.Anthony D. Dickson, Arthur H. Saller, and Hiroshi Oda; #90906(2001)

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J.Anthony.D. Dickson1, Arthur H. Saller2, Hiroshi Oda3

(1) Cambridge University, Cambridge, United Kingdom
(2) Unocal Corp, Sugar Land, TX
(3) Japan National Oil Company, Japan

ABSTRACT: Recognition of Subaerial Exposure through the Style of Echinoderm Preservation

The Reinecke reservoir on the southern side of the Horseshoe Atoll consists of crinoidal/foraminifera grainstones and packstones of the Cisco Formation (Pennsylvanian). The Cisco carbonates can be divided by cycle top surfaces that are immediately underlain by sediment containing soil-related products, such as alveolar septal fabric, peloids, rhizoliths and laminar calcrete. The combination of soil-related features is distinctive to each surface allowing correlation across the Reinecke structure.

Echinoderm ossicles occur throughout Cisco sediments. These ossicles are texturally variable and composed of either calcite or a mixture of calcite and dolomite. The textural variation is organized relative to cycle tops. Ossicles close to the exposure surfaces developed large dissolution holes that subsequently become filled by calcite cement. These dissolved echinoderms were found up to 1.5m below one cycle top and up to 0.5m below two others. Soil-related features extend 0.1 to 0.5m below these cycle tops. Echinoderm ossicles from the middle and lower parts of the cycles are characteristically composed of microrhombic dolomite crystals uniformly distributed through single calcite crystals.

Ossicles with dissolution cavities occur in Pleistocene sediments where meteoric dissolution was particularly intense. Rain falling on the exposed, soil covered Horseshoe Atoll would have been particularly aggressive to surficial sediment causing wholesale dissolution in echinoderms but on moving through the sediment would approach carbonate saturation creating uniformly distributed microdolomite in echinoderms. Echinoderm preservation can be used to recognize emergence surfaces and where erosion has stripped other features from cycle tops may be the only diagnostic feature.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado