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The purpose of this study was to define the lithofacies and areal extent of the Fort Atkinson Formation (Maquoketa Group, Upper Ordovician) in the subsurface in Indiana. Two distinct lithologic facies (Members) of the Fort Atkinson Formation can be distinguished in the subsurface. The upper Shoaling Member consists of coarse grained crinoid-bryzoan grainstone deposited in a high energy depositional environment. Beneath the Shoaling Member is the Transition Member of the Fort Atkinson. This Member consists of alternating beds of shale and limestone which were deposited in depositional environments that varied from high to low energy. The Fort Atkinson is an important stratigraphic marker used to define four depositional provinces within the Maquoketa Group in Indiana. The four depositional provinces include the Maquoketa Shelf, the Overlap Ramp. the Cincinnati Shelf, and Michigan Shelf. / Department of Geology

The carbonate storm-dominated sedimentary succession superbly exposed on Anticosti Island in Eastern Canada represents one of the most complete and well-preserved paleotropical stratigraphic records spanning the Ordovician-Silurian (O-S) boundary. We sampled the nearly continuous coastal outcrop exposed at low tide along the west coast of Anticosti Island for high-resolution δ13C and δ18O chemostratigraphy. These new isotopic curves comprise more than 500 data points spaced at ~0.6 m intervals; for a total of 320 m of strata across the O-S boundary. The δ13C curve displays two distinctive positive excursions in the Hirnantian Ellis Bay Formation; a small lower excursion (+2.5‰) and an upper larger excursion (+4.5‰). These two positive isotopic carbon excursions provide a distinctive chemostratigraphic signature for regional and global correlations with other Hirnantian sections. The continuing descending δ13C trend, at least 30 m above the currently interpreted O-S boundary, suggests a readjustment of that boundary on the Anticosti succession. The δ18O curve, similarly to the Quaternary δ18O marine curve, is tightly coupled with multi-order cyclic facies changes. Our tightly coupled lithological and oxygen isotopic data suggest that the Anticosti succession was influenced by glacio-eustatic fluctuations during the end-Ordovician. Furthermore, the estimated duration of these multi-order cycles supports an astronomical forcing. A primary isotopic signal record is also supported by the lack of significant covariance between δ13C and δ18O, by the excellent microfabric preservation of both macro and microfossils in petrographic, cathodoluminescence, and SEM microscopy, and by little or no diagenetic resetting as suggested by the trace element geochemistry, which is unusual in the deep geological time.

The transition from the end of the Ordovician to the beginning of the Silurian Period is characterized by the glaciation of the Gondwana paleocontinent, eustatic sea level change, a perturbation to the global carbon cycle and one of the ve major mass extinctions of the Phanerozoic Eon. Due to signi cant sea level fall, the Ordovician-Silurian (O-S) boundary is often marked by hiatus and exposure in the shallow marine geologic record (e.g., Copper et al. [2013]). Two locations that host stratigraphic succession close to the boudary are Anticosti Basin of Quebec (Canada, e.g., Desrochers et al. [2010]), and the carbonate mounds of the Siljan ring district (Dalarna County, Sweden, e.g., Ebbestad et al. [2015]). The exact timing and dynamics of the glaciation and mass extinction are yet to be understood. Similarly, the interplay between those events and the carbon cycle perturbation are still unclear. As a result, there is a serious need for radiometric age constraints in this crucial part of the Paleozoic Era. The acquisition of more radiometric dates, achieved in this study, aims to address the present dearth of absolute dates close to the boundary. The dates produced in this study represent the first modern geochronologic constraints on the O-S boundary, leveraging the development of the EARTHTIME initiative and the latest U-Pb dating techniques that have improved accuracy and allowed for dating of single zircon crystals at <=0.1% precision level. Here I present two new U-Pb zircon ages obtained via bentonite dating. The first bentonite, 443.61+-0.52 Ma (2, including analytical, tracer calibration and decay constant uncertainties) was collected from the base of the Lousy Cove Member, Ellis Bay Formation (Anticosti Island, Quebec, Canada). The second one, 443.28+-0.50 Ma (including analytical, tracer calibration and decay constant uncertainties) comes from a karstic void within the Boda Core Facies of the Boda Formation (Dalarna County, Sweden). U-Pb geochronology (chemical abrasion, isotope dilution, thermal ionization mass spectrometry: CA-ID-TIMS) on single zircons was used to obtain these ages. These results are the closest radiometric ages to the current O-S boundary (compared to any time constraints in the 2012 Geologic Time Scale) and allow to signifcantly reduce the uncertainty of the current age boundary (443.8+-1.5, Cohen et al. [2018]). Furthermore these absolute ages have been used to make models that explore drivers of Earth system change, such as an end-Ordovician global carbon cycle perturbation. / Graduate / 2022-07-07

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<p> 49 beds of ripple-drift cross-lamination were measured in the Ordovician Cloridorme Formation of Gaspe, Quebec. Most of the beds are Bouma C (cross-laminated), some are BC (parallel lamination pas sing up into cross-lamination) and a few are AC and ABC types (graded bedding passing up into parallel and cross-lamination). </p> <p> Six climbing patterns have been recognised in the ripple-drift beds, namely: concave -upward, straight, sigmoidal, convex-upward, sinuous, and disconnected-irregular. Angles of climb range from 1 to 44 degrees. Commonly the angle of climb steepens up through the coset to about 1/2 or 3/4 of the coset thickness and then gradually flattens out until the bed is plane. </p> <p> Measurements of wave length, amplitude, stoss -angle, lee-angle, angle of climb and ratio of lee- to stoss-lamina thicknesses show that the wave length continues to increase upward, and the lee/stoss thickness ratio decreases upward through the coset. The amplitude, and stoss- and lee-angles increase upward through that portion of the coset in which the angle of climb increases upward. Simultaneously with the increase in amplitude and angle of climb, the shape of the foreset laminae becomes progressively more and more sigmoidal. </p> <p> Down-current changes in lamination types were recorded in several beds. The most interesting change is from parallel lamination down-current into ripple-drift, continuing downcurrent back into parallel lamination. </p> / Thesis / Master of Science (MSc)

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