No abstract

This chapter deals with the geologic controls on the distribution of arsenic in rocks.Specifically, it focuses on rare but geographically extensive paleohydrologic events that produced widespread arsenic enrichments in the earth's crust in the form of elevated contents of arsenic-rich (arsenian) pyrite. We summarize evidence documenting the existence of ancient large-scale hydrothermal fluid migration events in the central and eastern United States and discuss impacts on the arsenic content of aquifer rocks through which the fluid migrated. There are two specific geologic settings discussed. One is the midcontinent region, and the other is the Appalachian region (with a focus on coal in the Appalachian Basin). Evidence for a hydrothermal fluid flow event in the midcontinent comes largely from studies on the genesis of large zinclead deposits (the so-called Mississippi Valley-type or MVT ores) of the region that are hosted in Paleozoic carbonate rocks. These studies demonstrate that ore-fluids for MVT deposits (brines derived from adjacent sedimentary basins) were driven towards the craton by gravity flow during a late stage of the Ouachita orogeny about 270 million years ago. These warm brines migrated laterally for hundreds of kilometers. Arsenian pyrite was deposited along these fluid pathways. The same aquifers that were pathways for ore fluid migration are exploited today as drinking water aquifers, raising the possibility that given conditions favorable for release of arsenic from arsenian pyrite, drinking water arsenic concentrations might become elevated. Further east, Paleozoic sediments of the Appalachian basin were aquifers for westward migrating fluids during a late phase of the Appalachian orogeny (also about 270 million years ago). Coal beds were particularly favorable sites for deposition of arsenian pyrite. In marked contrast to the midcontinent flow event, the fluids involved in the Appalachian region were likely derived from deep-seated metamorphic processes and were not as saline as those in the midcontinent. We argue that in the southernmost Appalachians of Alabama, the metamorphic fluids impacting coal were related in time and composition to 128Chapter 5 metamorphic fluids that formed arsenic-bearing gold deposits in the metamorphic rocks to the east. Pennsylvanian bituminous coals of Alabama locally contain abundant arsenian pyrite, which also is enriched to a lesser extent in copper, selenium, molybdenum, mercury and antimony. Mining of Alabama coal has resulted in elevated arsenic content in stream sediments, and in non-potable (saline) waters produced during coalbed methane recovery, but arsenic enrichment in drinking water supplies is apparently localized to individual water wells. Invasion of metamorphic fluids into coal-bearing Pennsylvanian rocks was widespread throughout the Appalachian basin, and arsenian pyrite occurrences are also widespread but sporadic. Large fluid flow systems are not restricted to the examples described in detail in this chapter, and additional areas impacted b...

Specimens of the Late Mississippian productoid genera Inflatia and Keokukia from northeastern Oklahoma and northwestern Arkansas, collected from the Boone and “Moorefield” Formations, Hindsville Limestone, and Fayetteville Shale, display morphologic similarities and differences that delineate species and determine their biostratigraphic ranges. Generic assignments are based primarily on internal characters. Systematic descriptions include seven species of Inflatia Muir-Wood and Cooper: Productus inflatus McChesney (the type species), P. cherokeensis Drake, P. clydensis Girty (figured herein for the first time and for which a lectotype is designated), four new species of Inflatia (I. cooperi, I. gracilis, I. pusilla, and I.? succincta), and one species of Keokukia (the type species for the genus, K. sulcata Carter). Also proposed and described is a new genus, Adairia, with its type species Productus (Marginifera) adairensis Drake. All these species of Inflatia, Keokukia, and Adairia have biostratigraphically restricted ranges within the Meramecian and Chesterian sequence in the Ozark region.

The Lower Carboniferous Fort Payne and Ullin Formations in the Illinois Basin form the transgressive and highstand systems tracts that were deposited in a carbonate ramp setting. During deposition of the Ullin Limestone, biotic communities dominated by fenestrate bryozoans and echinoderms (primarily crinoids) proliferated, possibly in response to global tectonic, biological, and oceanographic events that affected bathymetry and nutrient supply. The Fort Payne Formation consists of a dark grey-brown, siliceous and argillaceous lime mudstone in the lower part (transgressive systems tract) and a very fine-grained wackestone to packstone with rare mud mounds in the upper part (early highstand), and was deposited in an outer ramp to basinal environment. During deposition of the lower Ullin Limestone (mostly early highstand), hryozoan-crinoidal build-ups accreted both laterally and vertically into several relatively large carbonate banks, which were partly surrounded by siliceous Fort Payne sea. Bryozoans (primarily fenestrates) were especially prevalent during the late stage of bank development and formed mud-free bioherms up to 120 m thick. In places, carbonate mud mounds also formed during the early stage of bank deposition. Bioherm development declined during deposition of the upper Ullin Limestone (late highstand), and a broad, storm-dominated carbonate ramp was established that became the site for widespread deposition of bryozoan-crinoidal sandwaves. Gradual shallowing led to ooid formation at the end of Ullin deposition. This sequence was terminated by a relative rise in sea level that resulted in deposition of the transgressive facies of the lower part of the overlying Salem Limestone. The depositional style and the nature of skeletal material of the Fort Payne and Ullin Formations are similar to those of cool-water carbonates. A deep-water setting along with upwelling of cool, nutrient-rich oceanic waters may have been responsible for the proliferation of bryozoans and crinoids at this time. The deep-water setting and abundant nutrient supply also may have restricted the formation of ooids and proliferation of shallow-water calcareous organisms.