Abstract

Limestones form most readily in warm, sunlit, shallow water between an offshore limit of cooler, darker, deeper water and a nearshore limit of detrital influx. In the basic northern-shelf ("Kansas") cyclothem, which resulted from a single glacial eustatic inundation and withdrawal of the sea over the Mid-Continent, these conditions were fulfilled over most of the shelf during intermediate stages of sea level, either during deepening, producing a transgressive limestone, or during shallowing, producing a regressive limestone. At low stands of sea level, detrital clastics overwhelmed most of the shelf to form a sandy nearshore to terrestrial shale; if limestones formed at all, they were restricted to impure carbonate flags along the shelf margin. At high stands of sea level, most of the shelf along outcrop was covered by deeper, darker and often anoxic water, which produced widespread, offshore, nonsandy, dark gray to black shale with a pelagic fauna, including abundant conodonts. Limestones during this phase formed only along distant cratonic shorelines, and are now eroded away.

Geographic extent of the cyclic limestones was controlled largely by the amount of sea-level rise, which determined how much of the shelf was covered. Thus, large rises covered most of the shelf and resulted in the widespread major cycles with well-developed deep-water as well as shallow-water facies, whereas small rises covered only the lower parts of the shelf and resulted in minor cycles, which typically have poorly developed deeper water facies because of the shallowness of the high stand. Thickness of limestones was controlled primarily by how long the sea bottom stayed within the shallow carbonate-producing zone. In the Pennsylvanian, this was probably controlled more by the relatively rapid rates of transgression and regression and their relations to local bottom slope, than by other factors.

Differences between transgressive and regressive limestones relate to both depositional and diagenetic factors. Transgressive limestones were deposited in deepening, darkening water, while detrital influx was trapped in ponded estuaries along the retreating shoreline. Because of this, they are thin, massive and pure. Some are absent, perhaps due to rapid enough transgression that too little carbonate production became established before the water became too deep to form a significant thickness, and here deep-water offshore shales overlie terrestrial to nearshore shales. In contrast, regressive limestones were deposited in shallowing water, where carbonate production intensified with time while detrital influx from the encroaching shoreline periodically was washed in by storms. Because of this, they are thick, and shale-parted. Following withdrawal of the sea, regressive limestones were penetrated by oxygen-rich meteoric water, which generally oxidized organic matter, leached unstable grains, and deposited sparry calcite in both intergranular and moldic void space, preserving much of the depositional and diagenetic fabric prior to compaction. Hence, regressive limestones tend to be lighter colored, more spar-rich, and also locally more porous. This influx of meteoric water rarely penetrated the offshore shale to affect the transgressive limestones, which became buried in decreasingly oxygenated water and underwent compaction, grain crushing and grain-contact welding before late cementation in greatly reduced void space. Thus, transgressive limestones tend to be dark and dense as well as thin, massive and pure.

Currently recognized members of all cycles on the northern shelf, from the Mid-Desmoinesian Verdigris up through the Missourian into the lowermost Virgilian, can be interpreted as some phase of deposition in the basic eustatic cyclothem. Some stratigraphic problems are brought up to date with new information, particularly in the thick shale sequence across the Desmoinesian-Missourian boundary. Here, a new marine cycle has been detected above the type Lenapah and it includes units previously classified as Lenapah in Iowa and Missouri. Many stratigraphic problems that remain unresolved are undergoing current investigation.