Burial Dolomitization of Lower Cambrian Platform Margin, Virginia

Roger J. Barnaby, J. Fred Read

Shady Dolomite platform carbonates (Lower Cambrian) record several episodes of dolomite replacement and cementation during deep burial. Rare relict nonluminescent dolomite cores (zone 1) were replaced and overgrown by dark orange-red cathodoluminescent (CL) dolomite (zone 2A), the dominant replacement phase. Zone 2A dolomite was corroded, fractured, brecciated, and overgrown by Mississippi-Valley-type or lead-zinc minerals. Dark orange-red CL saddle dolomite (zone 2B) precipitated after mineralization and was fractured, corroded, and overgrown by bright orange-red CL saddle dolomite (zone 3). Cementation by later dark orange-red CL saddle dolomite (zone 4) was followed by quartz, calcite, and dedolomite.

Zone 1 dolomite cannot be directly sampled. However, analysis of samples with relict zone 1 cores suggests it has similar ¹⁸O but slightly depleted ¹³C relative to zone 2A replacement dolomite, which is enriched in ¹³C (^dgr¹³C = 1.0 to 1.6^pmil PDB) relative to marine calcite (^dgr¹³C = 0.2 to 0.8^pmil) and other dolomites. ^dgr¹⁸O values from zone 2A replacement dolomite and zone 2B saddle dolomite overlap (^dgr¹⁸O = -10.3 to -7.4^pmil PDB) and are depleted relative to marine calcite (^dgr¹⁸O = -7.5 to -6.1^pmil), reflecting elevated precipitation temperatures. Zones 3 and 4 dolomites are further depleted (^dgr¹⁸O = -13.8 to -11.3 ^pmil). Fluid inclusions indicate hot overpressured bri es (125°-225°C, 20-33 wt. % NaCl); sulfur isotope geothermometry of sphalerite and galena (T ^approx 200°C) also indicates burial temperatures.

Only late-stage dolomite (zones 3 and 4) and calcite cement have elevated ⁸⁷Sr/⁸⁶Sr values (0.7103-0.7111) relative to marine carbonate and earlier dolomites (0.7090-0.7097), recording influx of radiogenic strontium-enriched fluids during late Paleozoic thrusting.

Although the replacement dolomites are buried, it is unclear whether they replaced limestone or an unstable near-surface precursor dolomite. If a precursor dolomite were present, it was completely overprinted during burial diagenesis, preserving no trace of its initial composition.

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