Origin of Dolomite, Red River Formation, Richland County, Montana: ABSTRACT

Abstract

Distribution of limestone, dolomite, and anhydrite as determined from compensated neutron-density logs and core studies of the Ordovician Red River Formation in Richland County indicates that: (1) virtually all deposition, including the B and zone anhydrites, occurred subtidally (contrary to previous interpretations invoking localized tidal flats on paleo-highs); (2) various rock units such as the anhydrites and B and laminated zones have remarkable uniformity and lateral continuity over many tens of miles; and (3) dolomitization occurred in subtidal stromatolites, by primary precipitation, or by gravitational seepage of Mg-rich brines into normal marine subtidal carbonates. The lateral persistence of the A and B zone dolomites indicates they formed mainly by primar precipitation and/or dolomitization of subtidal stromatolites whereas the localized replacement dolomites of the and D zones formed by gravitational seepage of dense brines. End_Page 595------------------------------ The zone dolomite of the Red River is the major oil-producing zone in Richland County. This dolomite occurs immediately beneath the C zone anhydrite as concentrated lenses of: (1) tight cryptocrystalline anhydritic dolomite, (2) porous fine to very fine-grained dolomite, and (3) relatively tight partly dolomitized limestone. The dolomitized lenses are typically one to two km in diameter and up to 50 m thick. They apparently formed beneath holes in the anhydrite through which dense Mg-rich brines (formed during precipitation of the subtidal anhydrite) seeped. These holes formed almost randomly, probably by hydraulic fracturing as interstitial waters from compacting sediments beneath the anhydrite escaped upward, but minor faulting may have created more linear holes locally. /P> The D zone dolomite along the eastern edge of Richland County also formed by gravitational seepage, but the absence of a D zone anhydrite allowed for relatively laterally persistent dolomitization. Distribution of D zone dolomite was controlled mainly by paleo-topography of the basin floor; the brines filled in lows whereas dolomitization was minor or nonexistent over paleo-highs. End_of_Article - Last_Page 596------------