Shallow Subsurface Dolomitization of Subtidally Deposited Carbonate Sediments in the Hanson Creek Formation (Ordovician—Silurian) of Central Nevada
John B. Dunham, Eric R. Olson, 1980. "Shallow Subsurface Dolomitization of Subtidally Deposited Carbonate Sediments in the Hanson Creek Formation (Ordovician—Silurian) of Central Nevada", Concepts and Models of Dolomitization, Donald H. Zenger, John B. Dunham, Raymond L. Ethington
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Stratigraphie sequences of the Hanson Creek Formation are grouped into two categories based on differing paleogeographic setting and diagenetic history. Sequences in northern and eastern Eureka County record deposition in shallow subtidal to peritidal environments. Replacement dolomitization was the main diagenetic process affecting the original calcium carbonate skeletal grains, ooids, and mud of these sections. Lack of associated evaporite minerals or their traces precludes sabkha diagenesis. The relation of the dolomite crystals to other diagenetic features and textures indicates that dolomite replacement took place after deposition, but before significant compaction could produce strained carbonate grains or overly close packing in grain-supported fabrics. In addition, dolomitization took place before the formation of stylolites.
In contrast, stratigraphie sequences in southwestern Eureka County consist of laminated shaly limestone of moderately deep-water deposition, succeeded by a shoaling-upward sequence capped by oolitic grainstone. Stabilization of original carbonate sediment to low-Mg calcite was the main diagenetic process affecting these rocks. The observation of dolomite in the shallow water northern and eastern sections, in contrast to limestone in the deeper water southwestern sections, suggests that dolomitization of the carbonate platform was an early diagenetic process related to the Ordovician-Silurian paleogeography of the region. In addition, the absence of significant quantities of dolomite in the deeper-water sections of the southwest suggests that dolomitization of the carbonate platform was not influenced by solutions that were derived from burial compaction of the Vinini Formation (Ord.-Dev.), a laterally equivalent basinal shale that accumulated to the west of the Paleozoic carbonate platform. The textural evidence from the rocks and the geographic distribution of the dolomite indicates that dolomitization preceded deep burial and that the solutions which led to dolomitization were derived from a northeasterly source.
Sr+2 concentrations of Hanson Creek dolomites are much lower than values reported for Holocene dolomites. The low values are interpreted as the result of formation from solutions of Sr+2/Ca+2 ratio lower than that of seawater. Oxygen isotopie ratios (δ18O) range from −0.8 to −6.7 ‰ PDB. These distinctly light values are interpreted as representing the influence of solutions depleted in 180 relative to normal seawater. Trace element and isotopie data lead to the interpretation that the major dolomitization event which affected Hanson Creek sections took place in the shallow subsurface as a result of the mixing of meteoric derived groundwater and marine pore-water. Intrusion of freshwater into subtidally deposited sediment took place as a result of vertical and lateral extension of freshwater lenses that developed beneath subaerially exposed tracts of the inner carbonate platform. Dolomite to limestone transitions mark the maximum extent of freshwater lenses in the subsurface. Undolomitized deeper water carbonates of southwestern Eureka County, remote from areas of freshwater recharge, were positioned beyond the greatest lateral extent of freshwater lenses and were not subjected to the early influence of meteoric-derived ground water. The study indicates that although the dolomite is diagenetic, and not related to the depositional environment of the original carbonate sediment, the distribution of Hanson Creek limestone and dolomite is paleogeographically controlled. Thus the mapping of diagenetic dolomite distribution may communicate paleogeographic information if the dolomite is in fact early diagenetic, that is, formed through the operation of surface-related processes; such dolomite would be eogenetic in the sense of Nichols and Silberling (this volume), as opposed to mesogenetic, such as described by Mattes and Mountjoy (this volume).
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Concepts and Models of Dolomitization
Special Publication 28 has its roots in the 22nd Annual Research Symposium of SEPM entitled Concepts and Models of Dolomitization – Their Intricacies and Significance held on April 3,1979 in Houston, Texas as part of the joint annual meetings of AAPG and SEPM. The purpose of that symposium was to express the state-of-the-art of the study of the elusive process(es) of dolomitization. Most of the contributions in this volume are concerned with apparent early, nearsurface dolomitization, either by hypersaline brines, by the marine-meteoric mixing model or some variant thereof, or by both mechanisms where more than one phase or kind of dolomite exists, or where the origin of a particular dolomite is uncertain. Other models and aspects of dolomitization are treated here as well.