Abstract: An Environmental Approach to Limestone Diagenesis

By

Eugene A. Shinn

Shallow-marine lime sediments may convert to rock in one of three major diagenetic environments: (A) marine, i.e., on the sea floor; (B) freshwater zone; and (C) deeper subsurface. A lime sediment may undergo only one of these diagenetic environments or all three. Petrographic imprints that may identify these environments include:

A. Marine environment-(1) fibrous cement; (2) polygonal cement sutures; (3) superimposed borings; (4) geopetal sediment; (5) botryoidal or spherical cements; (6) mud-textured cement in various combinations with 1 to 5 of the foregoing; (7) fine-grained dolomite (i.e., supratidal marine).

B. Freshwater environment-(1) meniscus cement; (2) laminated crusts and other evidences of subaerial exposure; (3) leached fossils; (4) associated freshwater limestones; (5) light O¹⁸ composition; (6) blocky calcite.

C. Deeper subsurface environment-(1) compactional features; (2) blocky calcite; (3) pressure solution (i.e., stylolites); (4) leached grains and moldic porosity; (5) saccharoidal dolomite.

Some of these features, such as blocky calcite, leached grains, and dolomite, are present in more than one environment; exposure of some rocks to all three environments complicates diagenetic interpretation.

Knowledge of early diagenetic environments and their indelible imprint is based on investigations of seafloor cementation in the Persian Gulf, the Bahamas, and Bermuda, and more recently has been acquired through drilling on the Belize barrier reef and through studies of south Florida Pleistocene limestones exposed to fresh water. Recognition of petrographic features associated with deeper subsurface diagenesis is based on interpretive studies of subsurface and outcropping Cretaceous limestones in Texas, Louisiana, and Mexico, and on experimental compaction studies conducted at the Fisher Island Laboratory. Future research may modify these interpretations, but the immediacy of energy problems necessitates this attempt to form criteria which may explain, evaluate, and help predict those diagenetic environments controlling the presence, discovery, and extraction of oil and gas from ancient limestones.

Note: Mr. Shinn's presentation will include the USGS color film "Geology of Belize Barrier Reef."

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