Jennifer A. Anziano

Advisor: John Craddock
Senior Honors Thesis: 2002

Calcite Twin and Anisotropy of Magnetic Susceptibility Analyses of Ultramafic Lamprophyre Dikes: A Study of Strain Orientations and Flow Fabrics

Jennifer A. Anziano Oriented samples were taken from two perpendicularly oriented ultramafic lamprophyre dikes in Little Presque Isle near Marquette, Michigan. The dikes intrude Archean gneisses, which are unconformably overlain by the Jacobsville Sandstone, and are presumed to be Keweenawan age. Magmatic calcite grains and groundmass are preset in a mineral assemblage of pholgopite and Fe-rich oxides. The two dikes strike east-west and north-south, with the north-south dike being the younger of the two as established by crosscutting relations. Eleven oriented samples (4 E-W, 7 N-S) were collected and 29 thin sections were analyzed using the calcite strain-gauge technique. Analysis of the E-W dike (2 samples, 4 thin sections, 89 grains, 26.97% NEVs) reveals a sub-horizontal dike-acute shortening strain fabric (N54ºE, 1º). Analysis of the N-S dike (7 samples, 7 thin sections, 10 grains, 22.43% NEVs) reveals a sub-horizontal and dike-parallel strain fabric (N7ºE, 6º). The NEVs from both dikes represent a possible secondary sub-vertical shortening strain event. In addition, analyses of dike-parallel calcite veins from both dikes reveal as NE-SW sub-horizontal shortening.

Alexander W. McKiernan

Advisor: John Craddock
Senior Honors Thesis: 2002

Stress-Strain Analysis in Precambrian Quartzites from Wisconsin: Evidence for Eastward Continuation of the ca. 1650 Ma Mazatzal and Central Plains Orogenies

Alexander W. McKiernan"Baraboo interval" quartzites (BIQ) deposited throughout Wisconsin between 1750-1630 Ma were analzyed using three independent strain gauge techniques, including the auto-correlation function (ACF) within current NIH Image software. The ACF has been used to quantify fabric intensities, and is here used as a proxy for bulk strain. Strain ellipse orientations (which parallel regional fold orientations) suggest approximately north-directed shortening along a roughly east-northeast trending orogenic margin. The strain gradient preserved in these rocks decreases to the north, consistent with a post-Penokean thrust margin located south of present-day Wisconsin. These analyses provide evidence of Early-Mid Proterozoic deformation in Wisconsin, contemporaneous with the Mazatzal-Central Plains (southwest) and Labrador (northeast) orogenies that partially define the southern margin of the North American craton at 1650 Ma. Until recently, when more accurate depositional dates were obtained for the BIQ, deformation in the region was usually attributed to the Penokean Orogeny (ca. 1850 Ma) along the east-west trending Niagara fault north of the BIQ. This research shows that although strain orientations are complementary with the Niagara fault, such a relationship would not be possible with the observed strain gradients.

Elizabeth A. Hajek

Advisor: Ray Rogers
Senior Honors Thesis: 2002

Comparative Sedimentology of Two Late Cretaceous Localities near New Ulm, Minnesota

Elizabeth A. Hajek Upper Cretaceous strata are preserved between Precambrian bedrock and Pleistocene glacial deposits in the Minnesota Valley Minerals (MVM) Courtland and Ridgley clay and gravel quarries near New Ulm, Minnesota. These exposures provide rare glimpses into the enigmatic eastern margin of the Western Interior Seaway. Field observations of outcrops were coupled with laboratory analyses (including X-ray Diffractometry, shale disaggregation, sandstone sieve analysis, and petrography) and were used to characterize the nature of the depositional environment for each locality.

Three distinct facies were identified in the MVM Courtland mine. The Lower Laminated Claystone (LLC) facies contains virtually no coarse-grained (>coarse silt) particles. Fine (mm- to cm- scale) color bands, scattered well-preserved leaves, and siderite concretions also characterize the LLC, which is interpreted to reflect deposition in a large, open intracratonic lake. A fine-grained, cross-bedded quartz arenite body comprises the Sandstone facies (SS) and caps the LLC in three of four measured sections. The SS facies includes a basal lag deposit with imprints of carbonaceous material and is interpreted to be a channel deposit. The Upper Massive Claystone facies (UMC) crops out above the SS facies in one of the four sections. Based on apparent soft-sediment deformation structures on the contact between the SS and UMC facies, the UMC is interpreted to be part of the same channel deposit as the SS facies. The juxtaposition of a high energy channel environment on an open lake environment is unconformable, and thus might reflect a channel avulsion event or a fall in sea level.

The MVM Ridgley mine is dominated by inclined heterolithic stratification (IHS) deposits characterized by gently dipping 5-10 cm-thick packages of alternating sandstone and claystone. Sandstone units are fine-grained and shows tabular cross-bed sets with tangential toes and carbonaceous drapes on foresets. Claystone units are massive. A dark, lignitic mudstone layer caps Cretaceous exposure at Ridgley. Ridgley deposits as a whole are likely the result of deposition in a tidally influenced meandering river system.

Detailed sedimentologic anlysis of limted Cretaceous nonmarine and marginal marine exposures in southern Minnesota afford a better understanding of the eastern margin of the Cretaceous Western Interior Seaway.