Radiaxial-Fibrous Calcites of Shallow Subsurface Diagenetic Origin

S. J. Mazzullo, W. D. Bischoff, Harald Lobitzer

Radiaxial-fibrous calcites (RFC) in marine carbonates are generally considered syndepositional cements. In Upper Triassic and basal Liassic reef and platform limestones in Austria (Steinplatte complex), however, isopachous RFC is demonstrably a postdepositional diagenetic component that precipitated in shallow-burial phreatic environments during a time of periodic meteoric exposure. Isopachous RFC occurs solely within solution cavities and is interlayered with internal red sediment; discontinuities due to leaching separate sequential generations of RFC in the rocks. Accordingly, one possibility is that the RFC was originally low-magnesium calcite that precipitated in the meteoric phreatic zone during lowstands. Such calcites contain relatively low magnesium concentrations (average 0.87 mole % MgCO₃) and are ¹⁸O depleted (average -5.81 ^pmil PDB). However, most other RFC cements in the sequence average slightly higher magnesium comparable to crinoidal calcites (1.13 mole % MgCO₃), are less depleted in ¹⁸O (average -1.88 ^pmil PDB), and are partly dolomitized. Additionally, all the RFC cements are enriched in ¹³C to values similar to that of Triassic and Jurassic seawater (+2.86 ^pmil PDB) and are nonluminescent. Trace element studies indicate alteration of the rocks in partly closed, rock-dominated diagenetic systems. By these facts, we favor a precursor high-magnesium calcite mineralogy for the RFC cements, which possibly precipitated during highstands when meteoric pore waters were replaced by marine fluids. Thus, the geochemical trends observed are likely due to variations in the degree of meteoric alteration of high-magnesium calcite RFC rather than to differences in original mineralogy.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.