David H. Malone

Calcite twinning strain constraints on the emplacement rate and kinematic pattern of the upper plate of the Heart Mountain Detachment

Two models that address the emplacement rate of the upper plate of the Heart Mountain Detachment (HMD) have been advanced. These are the catastrophic tectonic denudation model and the slow, incremental continuous allochthon model. We compared the two models by analyzing upper and lower plate rocks for a detachment-related overprint of the older, layer-parallel Laramide–Sevier calcite twinning strain fabric. Lower plate Pilgrim Limestone samples, even those collected a few meters below the detachment, show no evidence of a detachment-related twinning strain overprint (negative expected values, NEVs, avg. 8.3%) of the Laramide–Sevier layer-parallel fabric. The allochthonous upper plate carbonate rocks preserve the layer-parallel Laramide–Sevier shortening strain, but the shortening axis (e1) for each block occurs in different, non-EW and both subhorizontal and non-subhorizontal orientations. For the six upper plate blocks that preserve chaotic shortening axis orientations, no evidence of any detachment-related twinning strain overprint (NEVs avg. 7.5%) was found. In the absence of any HMD-related twinning overprint, the upper plate allochthon motion must have been rapid enough to be accommodated by fracturing of upper plate rocks without additionally twinning any calcite. We conclude that the emplacement of the upper plate of the HMD was not accompanied by an overprint of the Laramide–Sevier twinning fabric and that models that require catastrophic, rather than slow, incremental emplacement rates are best supported by these data.

Dynamics of the emplacement of the Heart Mountain allochthon at White Mountain: Constraints from calcite twinning strains, anisotropy of magnetic susceptibility, and thermodynamic calculations

White Mountain is centrally located in the bedding-plane portion of the Eocene Heart Mountain detachment and contains the only upper plate Mississippian Madison Group rocks that have been metamorphosed into marble. The marble rests upon the thickest (1 m) part of a carbonate ultracataclasite that marks the detachment. Thermodynamic and mechanical calculations based on possible frictional melting of calcite and other minerals, geochemical data, the characteristics of the carbonate ultracataclasite, and the geometrical characteristics of White Mountain suggest a possible initial upper plate emplacement rate of 126–340 m/sec and that the duration of the emplacement event was less than 4 min, too brief a time to develop an emplacement-related calcite twinning strain overprint in upper or lower plate carbonates. While the detachment-related carbonate ultracataclasite did not form by melting, it does preserve a magnetic fabric where K max is parallel to the detachment slip direction and records a westward and down paleopole (287° and 27°), where magnetite is the carrier mineral. The Eocene (49.6 Ma) paleopole for this latitude in North America was southerly and upward (0° and 45°). This brief and catastrophic detachment event produced a signifi cant amount of CO 2 by fl ash heating. This report is the fi rst to quantify the emplacement rate of the upper plate of the Heart Mountain detachment based on physical and geochemical parameters.

Very large debris-avalanche deposit within the Eocene volcanic succession of the northeastern Absaroka Range, Wyoming

A very large debris-avalanche deposit found within the Eocene volcanic succession in the northeastern Absaroka Range, Wyoming, is a result of the failure and collapse of an ancient volcanic edifice in the vicinity of Sunlight Peak, a principal early middle Eocene center of volcanic-plutonic activity. The debris-avalanche deposit is a sheetlike body that consists of blocks, individually as large as several square kilometres, of well-stratified vent medial facies lava flows, breccias, and sandstones within a thin, heterogeneous, and complexly deformed matrix of boulder- to sand-sized volcaniclastic material. The avalanche was directed to the southeast along a broad paleovalley, and remnants have been found >40 km from the source area. The deposit initially covered an area of at least 450 km2 and had a minimum volume of 100 km3, making it one of the largest subaerial avalanches ever recognized.

Provenance of Quartz Arenites of the Early Paleozoic Midcontinent Region, USA

Quartz arenites characterize much of the early Paleozoic sedimentary history of the midcontinent region. Despite numerous studies, the century-long debate on how these arenites formed is still unresolved, primarily because of the compositional and textural purity of the deposits. In this study, we present an extensive data set of detrital zircon geochronology from the early Paleozoic supermature arenites of the midcontinent region, and we offer new constraints about their origin. Our results coupled with compiled provenance information from older basins and orogens may indicate that the Cambrian and Ordovician arenites represent sediment reworking primarily of two different older basins. The Cambro-Ordovician sediment was transported to the midcontinent region by two early Paleozoic river systems that sourced from the paleo-east (Huron basin) and paleo-northeast (midcontinent rift region).

Maximum depositional age of the Neoproterozoic Jacobsville Sandstone, Michigan: Implications for the evolution of the Midcontinent Rift

A crucial constraint on the evolution of the ca. 1100 Ma Midcontinent Rift (MCR) in North America comes from the Jacobsville Sandstone, Bayfield Group, and other equivalent sedimentary rocks (JBE) that overlie the volcanics and sediments deposited in the MCR basin near Lake Superior. The MCR began extending ca. 1120 Ma and failed—ceased extending—at ca. 1096 Ma, although volcanism continued for ∼10 m.y. The JBE’s age is poorly constrained, with proposed ages ranging from ca. 1100 to ca. 542 Ma (i.e., youngest Precambrian). It has been proposed that the JBE were deposited shortly prior to or during the time when the MCR failed due to regional compression occurring ca. 1060 Ma as part of the Grenville orogeny (1300–980 Ma). However, the JBE are not conformable with the youngest rift-filling strata and differ compositionally from them. We present an analysis of 2050 new detrital-zircon ages showing that the JBE are younger than 959 ± 19 Ma. Thus, the JBE and the compression recorded in them that inverted the basin postdate the Grenville orogeny and are unrelated to the rift’s failure. The JBE may be significantly, perhaps ∼200–300 m.y., younger than the maximum age from zircons and may have been deposited shortly after a Snowball Earth event.

Constraints on the Emplacement Age of the Heart Mountain Slide, Northwestern Wyoming

The Heart Mountain slide is the largest terrestrial landslide deposit as yet recognized on Earth. The slide covers an area of at least 3400 km2, and the upper-plate rocks include 2–4 km of Paleozoic carbonate and Eocene volcanic rocks thrust out over 45 km of Eocene landscape. The precise age and duration of sliding is critical to emplacement models as well the slide’s effect on regional Eocene river systems. To address the timing issues, we sampled zircons from the basal fluidized layer 2 km from the slide’s breakaway fault (Silver Gate, MT) and 40-km downslope, nearer the slide’s toe (White Mountain, WY). Within this basal layer, we have identified mineral content and features consistent with a partially solidified magma. We interpret these observations to be consistent with the slide catastrophically dismembering an active magma body that mixed with the basal fault layer. The results yield remarkably similar U/Pb zircon crystallization ages at the proximal and distal locations: 48.78 ± 0.51 Ma at Silver Gate (n = 48) and 48.88 ± 0.22 Ma at White Mountain (n = 22). These zircon ages from the basal layer are tightly bracketed using various radiometric ages of Eocene Absaroka volcanic units involved in the movement phase of the slide and those deposited after emplacement, including detrital U/Pb zircon ages from the dissected Crandall Conglomerate river system. Our interpretation of the data is that the slide was catastrophically emplaced at 48.87 ± 0.20 Ma.

A Faculty Survey on Field Trips in Undergraduate Structural-Geology Courses

A survey designed to determine the various activities, logistics, and pedagogical approaches that faculty employ on course-related field trips was sent to faculty who teach undergraduate structural-geology courses in the North-Central Section of the Geological Society of America. This paper describes the results and discusses the significance of the survey. Seventy-five colleges and universities were sent a questionnaire, and thirty faculty members responded. Eighty-six percent of respondents regularly run a required field trip as part of their structural-geology courses. Only sedimentology/stratigraphy field trips (100%) are more commonly required. Approximately 90% of structural-geology field trips require at least two days in the field, and more than 70% require travel of at least three hours to reach their destination. The most common destination for trips in the area surveyed is Baraboo, Wisconsin (48%). Department funds pay the transportation costs for most (66%) trips. About 55% of leaders give pre...

Vertical injectites of detachment carbonate ultracataclasite at White Mountain, Heart Mountain detachment, Wyoming

Carbonate ultracataclasite (CUC) is found as a veneer along the bedding plane portion of the Eocene Heart Mountain detachment (Wyoming, United States). At White Mountain, where the CUC is thickest, we report the discovery of six dikes, as much as 1 m wide, that were intruded vertically ∼120 m from the detachment into the overlying Madison and Bighorn Formations. The horizontal basal detachment and injectite material is texturally, petrographically, geochemically, and isotopically identical, containing sparse, sand- to pebble-sized andesitic clasts, various quartz-calcite melt spherules, and armored lapilli in a matrix (95%) of fine-grained calcite. The relationship of hanging wall displacement to fault gouge generation, and the presence of vertical, 120 m, fault gouge injectites, makes the Heart Mountain slide anomalous to all fault systems, especially as this fault system had only one episode of motion.

Porosity controls on secondary recovery at the Loudon field, south-central Illinois

AbstractWaterflooding has been used as an effective means to enhance oil recovery in mature oil fields for decades. The success of waterflooding is a function of geology, facies changes, and fluid dynamics, specifically, formation porosity and permeability. Within the Loudon oil field (Illinois), waterflooding has been used to increase production, but the degree of success has been variable. We have used 3D facies modeling was evaluate the variables controlling the success or failure of waterflooding. Three leases within the Loudon field exhibiting varying degrees of waterflood success were investigated. The K. Stubblefield lease, with the highest mean porosity of 13.5%, responded most favorably to waterflooding, with an increase of more than 750  bbl/month. Thick, high-porosity zones are well connected within the lease area, contributing to greater communication among the injection wells and the producing wells. The Rhodes-Williams lease, with porosity of 11.5%, had an increase in production of 350  bbl/...

Detrital Zircon Geochronology of the Bighorn Dolomite, Wyoming, USA: Evidence for Trans-Hudson Dust Deposition on the Western Laurentian Carbonate Platform

This study uses detrital zircon U-Pb geochronology from shallow-water carbonates of the Bighorn Dolomite in Wyoming, USA, to provide robust evidence for long-distance eolian sediment transport during the Ordovician. The Bighorn Dolomite was deposited in a shallow-water carbonate platform that developed approximately 10° south of the Ordovician paleoequator on the western edge of Laurentia. The ages and textures of detrital zircons from the Bighorn indicate that the grains were transported by winds through saltation and suspension from the paleo east where rocks of the Paleoproterozoic Trans-Hudson orogenic belt were exposed in present-day Manitoba and Saskatchewan. Our interpretation of long-distance eolian transport is consistent with the paleogeography of Laurentia and expected prevailing wind directions and draws on modern analogs where Saharan sediment is transported by trade winds for distances of more than 3000 km.