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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 67 (1983)

Issue: 3. (March)

First Page: 499

Last Page: 500

Title: Recognition of Original Mineralogy in Micrites: ABSTRACT

Author(s): Zakaria Lasemi, Philip A. Sandberg

Article Type: Meeting abstract


Detailed SEM study of selected micrites (< 4 µm) and microspars (4 to 12 µm) from all Phanerozoic systems and various geographic localities suggest that textural properties of micrites and microspars are mineralogically controlled. Those micrites and microspars with apparent aragonite-dominated lime mud precursors (ADP) have neomorphic calcite crystals which show pitted surfaces or relic aragonite inclusions in polished, etched sections. The presence of relics in all crystal sizes in ADP micrites and microspars indicates an absence of secondary dissolution-precipitation or aggrading neomorphism. That is, formation of all neomorphic crystal sizes occurred in a single diagenetic event. Micritic limestones with apparent calcite-dominated precursors (CDP), howeve , are characterized by finely crystalline (< 4 µm) textures, lack any inclusions, and have unpitted crystal surfaces.

Strontium content of micrites and microspars studied are bimodally distributed. A similar distribution was recognized by Veizer in 1977 and Veizer and Demovic in 1973 and 1974, who suggested it was the result of original mineralogy. Preliminary results on the micrites and microspars studied show Sr distribution generally well correlated with textural properties. ADP and CDP micrites and microspars possess Sr values which fall, with few exceptions, within the high-Sr and low-Sr groups, respectively, of Veizer and others. Several ADP samples which fall within the low-Sr group are molluscan-rich. Thus, likely aragonite contribution to precursor muds was more probably low-Sr. Low-Sr ADP could also be the result of open-system diagenetic alteration. In such cases, low-Sr ADP micrites and m crospars are associated with high Mn content. We have found several low-Sr ADP samples that are high in Mn. High-Sr ADP micrites and microspars are, therefore, interpreted as being originally composed of high-Sr aragonite mineralogy. The Sr content of CDP

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micrites studied thus far is low and suggestive of high-Mg calcite mud precursors for those micrites.

Oxygen isotopic composition of both CDP and ADP samples are rather broad ranges suggesting varying contributions of original mineralogies for both groups. There is, however, an overall negative trend toward lighter ^dgr18O isotopic values with increasing age, indicative of either progressively higher temperatures with age or lower 18O16O ratios in ocean water.

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