High-resolution sequence stratigraphy and detrital zircon provenance of the Ordovician Ancell Group in the Iowa and Illinois Basins: insight into the evolution of midcontinental intracratonic basins of North America
Date of Degree
PhD (Doctor of Philosophy)
William C. McClelland
Michael A. Pope
The Middle Ordovician Ancell Group, including the St. Peter Sandstone, Glenwood Shale and Starved Rock Formation, records intracontinental basin development during eustatic sea level changes in Iowa and Illinois. The St. Peter Sandstone overlies the Prairie du Chien Group across an erosional unconformity that marks a major sequence boundary, whereas upper contact of the St. Peter Sandstone with the Glenwood Shale also is a second sequence boundary. Data from 80 wells, selected well logs, and 20 cores were integrated to refine the high-resolution sequence stratigraphy of the Ancell Group. Two main sequences bounded by three sequence boundaries are interpreted to represent 3rd order sequences. Distinctive shallowing-upward parasequences bounded by flooding surfaces in many cores record higher frequency relative sea level fluctuations in the Ancell Group, but these cannot presently be correlated regionally. Facies variations define an aggradational transgressive systems tract TST), a prograding highstand systems tract (HST) and down stepping falling stage system tract (FSST) in both the St. Peter Sandstone and the Glenwood Shale-Starved Rock Formation units. The St. Peter Sandstone thickens towards the northeast and thins to the northwest and southwest in Iowa. In contrast, the St. Peter Sandstone in Illinois thickens to the south likely recording a prolonged FSST incised valley or channel fill. Detrital zircon geochronology of 13 samples from the St. Peter Sandstone and Starved Rock Formation define common peaks at 1100-1500 Ma and 2500-2700 Ma with minor components at 1670-1750 Ma and 3000-3600 Ma. The detrital zircon signature is dominated by Archean, and Grenville (1000-1300 Ma) ages. The detrital zircon geochronology indicates that the Ancell Group was sourced directly from the Archean Superior Province to the north and Grenville Province to the northeast, although recycling of Archean grains from the Paleoproterozoic Huron Basin cannot be ruled out. The near complete lack of 1800-1900 Ma ages argues against derivation of detritus from the Trans-Hudson or Penokean Orogens. The Transcontinental Arch northwest of the Iowa Basin acted as a barrier to sediment transport from the Trans-Hudson Orogen. Basement rocks of the Penokean Orogen are inferred to have been covered by water or younger sediments southeast of the Iowa Basin. CIA analyses of Ordovician shale samples from around the Transcontinental Arch indicate that the climate condition during Middle Ordovician time was warm and humid. This is consistent with a paleoclimate interpretation where mechanical erosion and chemical weathering yielded first cycle mature quartz arenites (Witzke, 1980).
The Middle Ordovician Ancell Group was deposited in the intracratonic Iowa Basin during a period of major transgression and high relative sea level. Stratigraphy, sediment provenance and paleoclimate conditions of the Ancell Group were established through detailed examination and chemical analysis of samples from bedrock exposures as well as core and wireline logs from wells in Iowa and Illinois. The main units of the Ancell Group, the St. Peter Sandstone, Glenwood Shale and Starved Rock Formation, were deposited as two 3rd-order sequences over periods of 10’s of millions of years. The sediment was variably sourced from local exposures of the Archean Superior Province to the north or more distal exposures of the Grenville Orogen to the northeast. Changes in sediment source is interpreted to reflect sea level fluctuations. Paleoclimatic conditions determined through chemical analysis of widespread shale samples indicates North America experienced warm humid conditions during Ordovician time. These conditions allowed widespread weathering of basement rocks and production of the relatively pure quartz arenite of the St. Peter Sandstone.
publicabstract, Geochronology, Paleoclimate, Petroleum Basin Analysis, Provenance, Detrital Zircon, Sequence Stratigraphy, Tectonics
Copyright 2016 Diar Mohammed Ibrahim