--> Abstract: Diagenesis and Porosity Evolution Associated with Meter Scale Cycle Disconformities and Sequence Bounding Unconformities, by J. F. Read and A. D. Hobury; #91012 (1992).

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ABSTRACT: Diagenesis and Porosity Evolution Associated with Meter Scale Cycle Disconformities and Sequence Bounding Unconformities

READ, J. FRED, Virginia Tech, Blacksburg, VA, and ANDREW D. HORBURY, BP Exploration Company, Uxbridge, Great Britain

High-frequency, low-amplitude sea level fluctuations (Cambrian, Ordovician, and Triassic) produce regional tidal flat caps on cycles, and dolomitization if the climate is arid. There may be fibrous cementation, limited leaching, and some brecciation during development of the cycle capping disconformities under arid climate. Sparry calcite cementation and considerable leaching under humid climate

may occur. High-frequency moderate (few tens of meters) fluctuations in sea level (Carboniferous, Ordovician, and Jurassic) rarely produce regional tidal flats on cycles. Most cycles show small-scale karstic surfaces or caliche directly over shallow subtidal facies. Cementation is best developed toward the cycle top. Given sufficient rainfall, cement overgrowths in the deeper phreatic zone (although volumetrically minor) show a partial to complete record of high-frequency sea level fluctuations that affected the sequence. In drier climates, there is little sparry cementation associated with the sea level fluctuations. Dolomites may form associated with evaporatic flats. High-frequency, large fluctuations in sea level (Pleistocene and Pennsylvanian-Permian) inhibit regional tidal flats on cycles. Cycle tops commonly are disconformable and show sinkholes, caves, or caliche deposits. There is large-scale vertical migration of marine, mixed, meteoric phreatic and meteoric vadose zones. Under humid conditions, moldic and cavernous porosity may be extensively developed both downward and over wide areas, especially in aragonitic sediments. Under humid climate, associated sparry cements include vadose (not easily identified) and phreatic types, interlayered with internal sediments and perhaps dolomite; dry climates inhibit karsting, leaching, and sparry cementation. Sea level falls from 1 to over 10 m.y., or tectonic uplift, are the common cause of regional calcite cementation under wet climates. Aquifers evolve from diffuse flow to conduit flow, as karstic caves and cementat on channelizes the flow. Calcite cements in the diffuse flow aquifers show regional geochemical and isotopic trends related to fluid flow. Regional karsting occurs updip, along with subsurface leaching, porosity enhancement, and stabilization. Downdip, carbonates undergo little early cementation or leaching, and metastable mineralogies later may be carried into the subsurface where they could undergo burial leaching or microporosity formation. Paleokarstic cave fills, breccias, and cavern roofs may provide permeable reservoir facies enhanced by burial-induced fracturing.

 

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)