Geology topics

P.R. Ames

Publications and source records attributed to P.R. Ames.

4 recordsLinked to original sources

USGS2009

Incised channel fills containing conifers indicate that seasonally dry vegetation dominated Pennsylvanian tropical lowlands

The idea that the Pennsylvanian tropical lowlands were temporally dominated by rainforest (i.e., the Coal Forest) is deeply ingrained in the literature. Here we challenge two centuries of research by suggesting that this concept is based on a taphonomic artifact, and that seasonally dry vegetation dominated instead. This controversial finding arises from the discovery of a new middle Pennsylvanian (Moscovian) fossil plant assemblage in southeast Illinois, United States. The assemblage, which contains xerophytic walchian conifers, occurs in channels incised into a calcic Vertisol below the Baker Coal. These plants grew on seasonally dry tropical lowlands inferred to have developed during a glacial phase. This xerophytic flora differs markedly from that of the typical clubmoss-dominated Coal Forest developed during deglaciation events. Although preserved only very rarely, we argue that such xerophytic floras were temporally as dominant, and perhaps more dominant, than the iconic Coal Forests, which are overrepresented in the fossil record due to taphonomic megabias. These findings require the iconography of Pennsylvanian tropical lowlands to be redrawn. ?? 2009 Geological Society of America.

Geology↗

USGS2009

Catastrophically buried middle Pennsylvanian sigillaria and calamitean sphenopsids from Indiana, USA: What kind of vegetation was this?

A catastrophically buried stand of calamitean sphenopsids and sigillarian lycopsids is reported from the Middle Pennsylvanian of southwestern Indiana, in the Illinois Basin. The plants were exposed in the highwall of a small surface mine and were rooted in a thin bed of coal (peat), thus representing a flooded and buried swamp surface. Coarse, floodborne silts and sands buried the forest to a depth of <3 m or more, before further incursions of water and sediment truncated the deposit. The rocks are part of the Staunton Formation. Taking up >250 linear meters of exposed highwall surface, the vegetation appears to have been a patchwork of calamitean thickets, with stems perhaps as tall as 3-5 m, within which scattered, but much larger, emergent Sigillaria trees grew, possibly reaching heights of 10-15 m. No ground cover was observed, nor were foliage or reproductive organs attributable to the dominant plants found. The growth of this vegetation in a peat-forming swamp indicates conditions of high water availability, likely in a humid, high-rainfall climate. This kind of plant assemblage, however, cannot be characterized as a rain forest, given that it consisted of medium-height thickets of horsetails with scattered, emergent, and polelike, giant lycopsids, thus lacking a closed upper canopy and possibly only partially shading the ground. Copyright ?? 2009, SEPM (Society for Sedimentary Geology).

Palaios↗

USGS2007

Ecological gradients within a Pennsylvanian mire forest

Pennsylvanian coals represent remains of the earliest peat-forming rain forests, but there is no current consensus on forest ecology. Localized studies of fossil forests suggest intermixture of taxa (heterogeneity), while, in contrast, coal ball and palynological analyses imply the existence of pronounced ecological gradients. Here, we report the discovery of a spectacular fossil forest preserved over ???1000 ha on top of the Pennsylvanian (Desmoinesian) Herrin (No. 6) Coal of Illinois, United States. The forest was abruptly drowned when fault movement dropped a segment of coastal mire below sea level. In the largest study of its kind to date, forest composition is statistically analyzed within a well-constrained paleogeographic context. Findings resolve apparent conflicts in models of Pennsylvanian mire ecology by confirming the existence of forest heterogeneity at the local scale, while additionally demonstrating the emergence of ecological gradients at landscape scale. ?? 2007 The Geological Society of America.

Geology↗

USGS2003

Coals of the Brazil Formation (Pennsylvanian) in Indiana: Observations of correlation inconsistencies and their implications

The coals of the upper part of the Mansfield, Brazil, and the lower part of the Staunton Formations (Atokan and Desmoinesian, Pennsylvanian) in Indiana (Illinois Basin) are characteristically thin and discontinuous. As a result, problems with correlation and identification of the seams have persisted for both researchers and industry. These discrepancies affect coal exploration, mine planning, and subsequently coal-fired utilities. This study presents exploration and operational examples demonstrating some of the correlation problems associated with the coals of the Brazil Formation, and the Upper Block and Lower Block, in particular, and the surrounding upper part of the Mansfield Formation and lower part of the Staunton Formations. Based on exploration boreholes, mine scale observations, and coal quality and petrographic data, this study suggests that (1) the coal mapped as the Upper Block Coal Member of Clay County may, in fact, be the same seam as the Lower Block Coal Member of Daviess County; and (2) the Lower Block coal of Clay County is not present south of the Switz City area of central Greene County, IN. ?? 2003 Elsevier Science B.V. All rights reserved.

International Journal of Coal Geology↗