Geology topics

Brian J. Witzke

Publications and source records attributed to Brian J. Witzke.

3 recordsLinked to original sources

USGS2006

A new Lagerstätte from the Middle Ordovician St. Peter formation in northeast Iowa, USA

A new fossil fauna has been discovered from a recently recognized shale unit within the middle Ordovician St. Peter Formation in northeast Iowa. It contains a variety of invertebrates and vertebrates, including soft body tissues, impressions, and 3-dimensionalpreservations. The exceptional preservation reveals a new Konservat-Lagerstätte, the Winneshiek Lagerstätte, and opens a unique window into the community that inhabited the margins of the Laurentian cratonic seaway during Middle Ordovician transgression. Among the fossils, several conodont assemblages, including the apparatus of enigmatic coleodontids, are preserved. Some conodont assemblages associated with soft body tissues are particularly noteworthy.

Geology↗

USGS1992

Preliminary results of the U.S. Geological Survey-Iowa Department of Natural Resources Geological Survey Bureau Manson Core Drilling Project

The U.S. Geological Survey and Iowa Department of Natural Resources Geological Survey Bureau completed, in 1992, a two-year core drilling program in the Manson Impact Structure, a 35-km-diameter, Cretaceous-Tertiary boundary-age feature located in north-central Iowa. A total of 12 cores sampled in excess of 1,200 m (4,000 ft) of crater rocks, supplementing the two previously drilled shallow cores from the structure. The cores penetrated the three major terranes in the Manson Impact Structure, the terrace terrane, crater moat, and central peak. Preliminary interpretations identified several important impact-related lithologies in these cores. The most widespread is sedimentary clast breccia, a postimpact polymictic breccia or mixtite that mantles at least part of all crater terranes. Two types of crystalline clast breccias were cored on the central peak, one with a melt matrix, the other with a matrix dominated by silt- to sand-sized grains. Large blocks of basement gneiss that formed the interior of the central peak were encountered in several cores. Within the terrace terrane, structurally preserved Cretaceous strata and an overturned ejecta flap of Proterozoic and Paleozoic sedimentary rocks were encountered. Only sedimentary clast breccia was encountered in the crater moat. The preliminary investigation of these cores has provided significant information on the geometry and history of the Manson Impact Structure, but it has also prompted many more questions.

Iowa↗

USGS1987

Possible Late Middle Ordovician organic carbon isotope excursion: Evidence from Ordovician oils and hydrocarbon source rocks, mid-continent and east-central United States

Oils generated by Middle Ordovician rocks are found throughout the Mid-Continent and east-central regions of the United States. Gas chromatographic characteristics of these oils include a relatively high abundance of n-alkanes with carbon numbers less than 20, a strong predominance of odd-numbered n-alkanes between C 10 and C 20 , and relatively small amounts of branched and cyclic alkanes. Saturated and aromatic hydrocarbon fractions of 43 Ordovician oils from the Anadarko, Ardmore, Forest City, Illinois, Michigan, Salina-Sedgwick, and Williston basins and the Iowa shelf demonstrate a wide range in carbon isotope composition (^dgr 13 C sat = -24.9 ^pmil to -33.9 ^pmil, (^dgr 13 C arom = -24.3 ^pmil to -33.7 ^pmil). Saturated and aromatic hydrocarbons extracted from late Middle Ordovician shales (17 core samples) show ranges in ^dgr 13 C similar to that of the oils. The wide ranges in ^dgr 13 C for oils and rock extracts reflect a major, positive excursion(s) (6-9 per ^pmil) in organic matter ^dgr 13 C in late Middle Ordovician rocks. This excursion has at least a regional significance in that it can be documented in sections 480 mi (770 km) apart in south-central Kansas and eastern Iowa. The distance may be as much as 930 mi (1,500 km) if the carbon isotope variations observed in Michigan basin Ordovician oils and in organic matter from late Middle Ordovician rocks in southwestern Ontario are related to the same carbon isotope excursion. Organic-matter ^dgr 13 C in core samples from south-central Kansas and eastern Iowa is not directly related to variations in quantity or quality of organic matter, or maceral compositi n. The positive excursion in organic matter ^dgr 13 C is a possible result of increased organic matter productivity and/or preservation. The parallel shifts in organic and carbonate ^dgr 13 C in core samples from 1 E. M. Greene well, Washington County, Iowa, imply changes in the isotope composition of the ocean-atmosphere carbon reservoir. Differences in the magnitude of the carbon isotope shifts between organic matter (8.8 ^pmil) and carbonate (4.2 ^pmil) in this core suggest a decrease, either locally or regionally, in available dissolved CO 2 , possibly a result of high organic-matter productivity and/or limited circulation in the late Middle Ordovician seas.

American Association of Petroleum Geologists Bulle↗