Abstract Exposures of Ordovician rocks of the Sauk megasequence in Missouri and northern Arkansas comprise Ibexian and lower Whiterockian carbonates with interspersed sandstones. Subjacent Cambrian strata are exposed in Missouri but confined to the subsurface in Arkansas. The Sauk-Tippecanoe boundary in this region is at the base of the St. Peter Sandstone. Ulrich and associates divided the Arkansas section into formations early in the 20th century, principally based on sparse collections of fossil invertebrates. In contrast, the distribution of invertebrate faunas and modern studies of conodonts will be emphasized throughout this chapter. Early workers considered many of the stratigraphic units to be separated by unconformities, but modern analysis calls into question the unconformable nature of some of their boundaries. The physical similarity of the several dolomites and sandstones, complex facies relations, and lack of continuous exposures make identification of individual formations difficult in isolated outcrops. The oldest formation that crops out in the region is the Jefferson City Dolomite, which may be present in outcrops along incised river valleys near the Missouri-Arkansas border. Rare fossil gastropods, bivalves, brachiopods, conodonts, and trilobites permit correlation of the Cotter through Powell Dolomites with Ibexian strata elsewhere in Laurentia. Conodonts in the Black Rock Limestone Member of the Smithville Formation and the upper part of the Powell Dolomite confirm regional relationships that have been suggested for these units; those of the Black Rock Limestone Member are consistent with deposition under more open marine conditions than existed when older and younger units were forming. Brachiopods and conodonts from the overlying Everton Formation assist in interpreting complex facies within that formation and its correlation to equivalent rocks elsewhere. The youngest cono-donts in the Everton Formation provide an age limit for the Sauk-Tippecanoe unconformity near the southern extremity of the great American carbonate bank. The correlation to coeval strata in the Ouachita Mountains of central Arkansas and in the Arbuckle Mountains of Oklahoma and to rocks penetrated in wells drilled in the Reelfoot rift basin has been improved greatly in recent years by integration of biostratigraphic data with lithologic information.

Abstract A revised lithostratigraphy for Lower Paleozoic strata in New Mexico and west Texas was developed through detailed sedimentological study of the Bliss and Hitt Canyon Formations within a refined temporal framework assembled from precise biostratigraphic (trilobite and conodont) and chemostratigraphic (carbon isotope) data. Member boundaries within the Hitt Canyon now correspond with mappable and essentially isochronous horizons that represent major depositional events that affected sedimentation in basins throughout Laurentian North America. This trip is designed to examine these and other important intervals, such as the extinction horizons at the base and top of the Skullrockian Stage, and to demonstrate the utility of associated faunas and isotopic excursions for correlation within and beyond the region.

Abstract Stratigraphic sections in the Monitor Range and Roberts Mountains provide both shelf and basin depositional records for two prominent stratigraphic intervals in the Ordovician System. The Whiterock Narrows section in the Monitor Range is proposed as a candidate global stratotype section for the base of the Middle Ordovician Series. The boundary interval in the upper Ninemile Formation and lower Antelope Valley Limestone, deposited on the carbonate platform, contains excellent biostratigraphic records of conodonts, used to define the boundary, trilobites and brachiopods. The boundary level can be correlated into a deep basin facies at the Red Canyon section in the Roberts Mountains because conodonts that define the boundary co-occur with graptolites in calcareous sandstones of the Lower Member of the Vinini Formation . The Vinini Creek section in the Roberts Mountains provides continuous exposure of the stratigraphic interval in the Upper Member of the Vinini Formation that includes the Late Ordovician mass extinction and was deposited in a deep basinal setting. The decline in diversity of the graptolite fauna coincides with the initiation of sea-level fall. However, conodont and chitinozoan faunas were unaffected. The Copenhagen Canyon section in the Hanson Creek Formation in the Monitor Range records the Late Ordovician events in a platform margin setting. There, faunas were affected by the sea-level fall much earlier than in the deep basinal setting, and the extinction of conodonts and chitinozoans occurred substantially later than that of graptolites .

Studies of conodonts from the North American Lower Ordovician have concentrated on taxonomy and biostratigraphy. These objectives will continue to have high priority in the near future; however, sequences of conodonts now known in the Lower Ordovician of central and western United States allow preliminary assessment of the geographic distributions of the faunas. By earliest Ordovician time, conodonts had differentiated into a population that inhabited the shallow seas of the craton and another population that was adapted to the deeper conditions of the marginal basins. The conodonts of younger Ibexian rocks were segregated into biofacies whose regional distributions suggest concentric belts around the central craton. Present knowledge of occurrences of these conodonts permits recognition of associations of genera and species that are characteristic of deposits that accumulated on (1) intertidal and shallow subtidal carbonate banks and flats, (2) shelf areas in the open ocean, and slope-rise regions in marginal basins. The possibility of further paleobiogeographic subdivision of shelf faunas is suggested by the evidence at hand, but greater geographic control will be needed to verify it. The factors that controlled the distribution of these conodonts along a probable bathymetric gradient are not known.

Abstract The problem of the origin of dolomite was initially addressed in 1791, when the French naturalist Déodat de Dolomieu first described dolomite rock. As sedimentary geologists are well aware, the literature on dolomite and dolomitization has become voluminous. The interest in dolomite derives not only from natural scientific curiosity, but also from the fact that many extensive sheet-like or other bodies of dolomite have proved to be of considerable economic importance as hydrocarbon reservoirs or hosts for ore deposits. This volume demonstrates the existence of a considerable range of opinion regarding the mechanism(s) of dolomitization. However, the diversity of dolomite occurrences described here must be considered to be the product of real differences between distinct dolomite types. Clearly, because of the variety of dolomite types that exist in nature, a single process of dolomitization does not exist and there is no one unique model to explain all dolomite. Several reviews of dolomite and dolomitization have been published in English in this century including works by Steidtmann (1911), Van Tuyl (1916), Fairbridge (1957), Ingerson (1962), Sonnenfeld (1964), Friedman and Sanders (1967), and Zenger (1972b). The latter two works discuss the several Holocene occurrences of marine marginal mostly supratidal, penecontemporaneous dolomite that had been discovered in the late 1950’s and early 1960’s in such areas as the Coorong of South Australia (Alderman and Skinner, 1957; von der Borch and others, 1964), the Bahamas (Shinn and others 1965), and Florida Keys (Shinn and Ginsburg, 1964), the Persian Gulf (Curtis and others, 1963; Illing

Abstract The hydrology and geochemistry of subsurface waters from a Holocene area of dolomitization, the coastal sabkhas of Abu Dhabi, have been investigated. The origins of the waters (lagoon, open marine, continental and mixed) were defined by their K: Br ratios and stable isotope values. Downward movement of Hood recharge waters was established by decreasing tritium contents with depth under the sabkha. Further, evaporation from the capillary zone imprinted a distinctive δD ratio upon waters from the intermediate sabkha. Measurements established the hydrologie framework under the sabkha. Two aquicludes separate the near surface sediments from the underlying aquifers. The presence of these aquicludes enabled us to measure vertical hydrauuc gradients. The gradient is locally directed downward for a short time after supratidal flooding, but for the most part is directed upward, especially in the area of Holocene dolomitization. Together, the data were synthesized into a hydrologie model for the intermediate sabkha, the area where dolomitization occurs. A single hydrologie cycle was defmed by three sequential stages; flood recharge, capillary evaporation and “evaporative pumping.” The processes involved in the evolution of a dolomitizing solution and the driving forces required to move the solutions through the sediments being dolomitized are inherent in the model.

Abstract Protodolomite is found in the sediments beneath hammocks of the tidal flats of west-central Andros Island, Bahamas. The protodolomite is disordered, contains 38-44 mole percent MgCO 3 , occurs as 1 μm equant rhombs, and fills pores between and grows around aragonite needles. The tidal flats are composed of Holocene lime sediments which overlie Pleistocene limestones, forming a wedge 0-4 m thick. The tidal flat wedge has prograded westwards several km into a slightly hypersaline shallow water bank which resembles an epeiric sea in aerial dimensions. Elevated ridges (hammocks), are common features on the tidal flats. They are underlain by fresh groundwater lenses which are surrounded by groundwater of more normal marine sahnity. Our analyses of several chemical parameters of these groundwaters indicate that all waters are supersaturated with respect to dolomite and that there is no special potential for precipitation of dolomite in the freshwater or adjacent mixing zones. Mg +2 loss from the system by precipitation, if it occurs, is in quantities too small to detect. We took sediment cores through hammocks and other environments, and determined the mineralogy of the sediments by X-ray diffraction. Though protodolomite is more common in sub-hammock sediments, its distribution is not reasonably explained solely in terms of mixing zone precipitation.

Abstract Many types of penecontemporaneous dolomites have been explained in the literature by involving the well-known sabkha model which is typically associated with an arid climate. The various carbonates now precipitating in the ephemeral lakes of the South Australian Coorong Lagoon, however, are the products of a more humid climatic and hydrologie regime. The distribution of carbonates in the Coorong region is largely controlled by the hydrology of the depositional environment. Characteristic suites of sedimentary structures are formed in specific parts of the Coorong system, including desiccation features and stromatolites. These suites can also be identified with a remarkable degree of confidence in the 1600 m.y. old Yalco Formation of the McArthur Group, Northern Territory, Australia. The following conclusions can be drawn from the comparison. (1) All penecontemporaneous dolomites are not necessarily formed in an arid sabkha-like environment; the Yalco Formation dolomites formed in a more humid environment analogous to that of the Coorong, in which distinct climatic and seasonal controls prevail. (2) The lack of evaporite minerals or evaporitic casts in an ancient dolomite sequence does not mean that concentrated brines were never present. Evaporite minerals may be precipitated during dry summer months, but are flushed out during winter by a reflux mechanism as occurs in the Coorong dolomite region. (3) By comparison with the Coorong system, ancient water table movements can be inferred from vertical sequences of sedimentary structures in the Yalco Formation, in conjunction with evidence from the underlying and overlying formations. The stratigraphy of the Lynott Formation, Yalco Formation, and Stretton Sandstone can be explained as a diachronous regressive sequence using the hydrological model developed in this paper.

Abstract Despite recent suggestions to consider dolomite a product involving freshwater, especially the reaction between freshwater and seawater, more recent work in the rock record and in sea-marginal ponds of the Red Sea indicates that dolomite is an evaporite mineral, and that most dolomites owe their origin to hypersaline brines. Although dolomite may form under a variety of depositional conditions, including freshwater, soils, caliche or deep sea, most dolomites in the rock record formed under conditions of hypersahnity. A close lateral, vertical, or temporal relationship commonly exists between dolomite and evaporite deposits, although this relationship is not universal nor everywhere demonstrable. In most situations in which evaporites fail to accompany dolomite, original evaporites have commonly been removed following their precipitation. However, the imprint of evaporite minerals and other evidence for hypersahnity may be preserved. Evidence for vanished evaporites has been inferred from mineralogic, sedimentary, and biotic characteristics, including pseudomorphs after sulfate nodules, pseudomorphs after halite, euhedral quartz crystals, quartzine and lutecine, saddle-shaped dolomite crystals, authigenic potassium feldspar, solution-co11apse breccias, abundance of oöids, half-moon oöids, presence of stromatolites, and paucity of fauna. In places evaporite minerals have been preserved as small inclusions. Research in modern sea-marginal ponds of the Red Sea shows that dolomite forms only where gypsum and /or anhydrite is also present. Among submerged algal mats where gypsum is absent, the carbonate minerals are aragonite or high-magnesian calcite; by contrast, where gypsum is abundant in deeper parts of ponds, or among submerged algal mats, dolomite is present. Likewise, in a pond-marginal sabkha, not only haute, gypsum, and anhydrite, but also dolomite, form a cement between constituent particles. The high salinities at which gypsum precipitates (up to 330 × 10 3 mg/l in the summer) and the observation that dolomite prefers sulfate association suggest that both minerals owe their origin to hypersaline brines. Fresh or brackish waters are unavailable in these ponds; the waters are hypersaline at all times. Dolomite, almost by definition, must be considered an evaporite mineral in these hypersaline ponds.

Abstract Experimental studies of the kinetics of reaction of calcium carbonate with magnesium-calcium chloride solutions indicate a solution-reprecipitation mechanism with a cation-ordered protodolomite as the initial reaction product. Nucleation of ordered dolomite is extremely difficult at low temperatures and is an important factor in the reaction. The kinetics of the reaction are strongly dependent on temperature and on the reactant (calcite or aragonite). Experimental dolomitization of aragonite at 100°C and atmospheric pressure has allowed the reaction to be studied under conditions approaching those of natural sedimentary environments. These studies indicate that other important kinetic factors include the ionic strength (salinity) and Mg ++ /Ca ++ ratio in the dolomitizing fluids and the presence of strongly hydrated ions. Certain amino acids and soluble proteins severely inhibit the reaction, but may be removed by oxidation. The results of these experiments may aid in the interpretation of the processes involved in sedimentary dolomitization.