Geology topics

Karri R. Sicard

Publications and source records attributed to Karri R. Sicard.

6 recordsLinked to original sources

USGS2020

Bedrock geologic map of the Mount Ascutney 7.5- x 15-minute quadrangle, Windsor County, Vermont, and Sullivan County, New Hampshire

The bedrock geology of the Mount Ascutney 7.5- x 15-minute quadrangle consists of highly deformed and metamorphosed Mesoproterozoic through Devonian metasedimentary and meta-igneous rocks intruded by rocks of the Mesozoic White Mountain Igneous Suite. In the west, Mesoproterozoic gneisses of the Mount Holly Complex are the oldest rocks and form the northeastern flank of the Chester dome. The allochthonous Cambrian through Ordovician rocks include the Moretown and Cram Hill Formations and the North River Igneous Suite; these rocks structurally overlie the Chester dome along the Keyes Mountain thrust fault. Silurian and Devonian metasedimentary and metavolcanic rocks of the Connecticut Valley trough (CVT) unconformably overlie the pre-Silurian rocks. The easternmost extent of the CVT in New Hampshire is exposed in the Meriden antiform. Ordovician to Silurian and Devonian metasedimentary rocks of the Bronson Hill anticlinorium structurally overlie the CVT along the Monroe thrust fault. The oldest part of the Bronson Hill anticlinorium, called the Bronson Hill arc, consists of Ordovician metamorphosed volcanic, plutonic, and sedimentary rocks of the Ammonoosuc Volcanics, the Partridge Formation, and the Oliverian Plutonic Suite. The rocks of the Bronson Hill arc may be partly correlative with the pre-Silurian rocks above the Chester dome and are exposed in two fault-bounded structural belts (Cornish City and Claremont belts) and in the Sugar River dome. Collectively, these belts form the regional Orfordville anticlinorium, Hardscrabble synclinorium, and the western part of the broader Bronson Hill anticlinorium in western New Hampshire. Silurian to Devonian metasedimentary rocks of the Clough Quartzite, and Fitch and Littleton Formations unconformably overlie the rocks of the Bronson Hill arc. Devonian granitic and pegmatitic dikes and sills of the New Hampshire Plutonic Suite intruded previously deformed rocks. Post-tectonic Cretaceous plutonic and volcanic rocks of the Ascutney Mountain Intrusive Complex underlie Mount Ascutney. Because of the historically significant scientific research, and its prominence in the landscape, Mount Ascutney is commonly regarded as Vermont’s most famous volcano. The bedrock geology was mapped to study the tectonic history of the area and to provide a framework for ongoing characterization of the bedrock of Vermont and New Hampshire. This Scientific Investigations Map of the Mount Ascutney 7.5- x 15-minute quadrangle consists of sheets 1 and 2 as well as a geographic information system (GIS) database that includes bedrock geologic units, faults, outcrops, structural geologic information, geochemistry, and photographs. Sheet 1 of the report includes a bedrock geologic map, a correlation of map units, and a description of map units. Sheet 2 includes a discussion of the geology, references, three cross sections from the geologic map on sheet 1, igneous rock geochemistry results of the main map units from the Mount Ascutney stock, a tectonic map showing major structural features, and a structural domain map showing the orientation and distribution of brittle features.

New Hampshire, Vermont↗

USGS2020

U-Pb geochronology of igneous and detrital zircon samples from the Tok River area, eastern Alaska Range, and Talkeetna Mountains, Alaska

This Alaska Division of Geological & Geophysical Surveys (DGGS) Raw Data File presents U-Pb zircon geochronology results from selected igneous, meta-igneous, and metasedimentary rocks collected during the Tok River and Wrangellia geologic mapping projects in the eastern Alaska Range and the northwestern Talkeetna Mountains, Alaska. The purpose of these analyses is to better constrain the age of select geologic units encountered during the mapping projects.

Alaska↗

USGS2020

A transect through Vermont's most famous volcano - Mount Ascutney

The Cretaceous Ascutney Mountain igneous complex affords a classic exposure of the White Mountain Igneous Suite. Often called Vermont’s most famous volcano, Mount Ascutney (elev. 3,144 feet, 958 m) stands as a prominent monadnock in the Connecticut River Valley. The mountain often serves as an inspirational landmark, as it does when viewed from locations throughout the valley including the Saint-Gaudens National Historic Site (Walsh, 2017). The Ascutney Mountain igneous complex (Ratcliffe and others, 2011) consists of several mafic to felsic nested plutons including gabbro-diorite exposed at Little Ascutney to the west, and the Ascutney Mountain stock composed of syenite, granite, and related volcanic rocks underlying the main summit to the east (Fig. 1) (Schneiderman, 1989, 1991). Foland and Faul (1977) and Foland and others (1985) dated the gabbro-diorite complex at 125.5 to 122.2 Ma by K-Ar on biotite and by whole rock Rb/Sr, and dated the syenite-granite complex at 123.2 to 121.4 Ma by K-Ar on biotite. During the field trip we will visit the host rocks south of the mountain and the main rocks types of the Ascutney Mountain stock exposed near the summit and along the Mount Ascutney toll road. Mount Ascutney is the classic location where Daly (1903) discussed the evidence for piecemeal stoping as a pluton emplacement mechanism. This theory was later modified to favor cauldron subsidence, or ring-fracture stoping, as an alternative mode of emplacement (Chapman and Chapman, 1940). Our new mapping (Walsh and others, in press), which supersedes an earlier provisional study (Walsh and others, 1996a, b), supports the cauldron subsidence model, and shows that the main Ascutney Mountain stock is a funnel shaped composite pluton in agreement with geophysical data (Daniels, 1990). This field guide will primarily highlight the results of the new geologic mapping. This field guide is modified from a field trip presented in 2017 (Walsh, 2017). Additional stops have been added to examine the host rocks in the region south of the Ascutney Mountain stock. Two hikes are planned as part of this trip. Other NEIGC field trip guides to Mount Ascutney include Stoiber (1954) and Schneiderman (1988).

Vermont↗

USGS2019

U-Pb and Lu-Hf isotope, age, and trace-element data from zircons at four sites in the western Alaska Range and Talkeetna Mountains, Alaska

This Division of Geological & Geophysical Surveys (DGGS) Raw Data File presents U-Pb geochronology and Lu-Hf isotopic compositions, age-dating results, and additional trace-elemental composition of zircons from four granitoids sampled during investigations by DGGS geologists in the western Alaska Range and the Talkeetna Mountains. The purpose of the Lu-Hf and U-Pb isotopic study was to better understand the Jurassic through early Tertiary magmatic and tectonic evolution of the Alaska Range and the significance of that magmatism for the formation of metallic mineral deposits. The U-Pb isotopic data also serve to further refine the crystallization age of plutons, which were previously ambiguous in 40Ar/39Ar geochronology studies.

Alaska↗

USGS2016

Reaction softening by dissolution–precipitation creep in a retrograde greenschist facies ductile shear zone, New Hampshire, USA

We describe strain localization by a mixed process of reaction and microstructural softening in a lower greenschist facies ductile fault zone that transposes and replaces middle to upper amphibolite facies fabrics and mineral assemblages in the host schist of the Littleton Formation near Claremont, New Hampshire. Here, Na-poor muscovite and chlorite progressively replace first staurolite, then garnet, and finally biotite porphyroblasts as the core of the fault zone is approached. Across the transect, higher grade fabric-forming Na-rich muscovite is also progressively replaced by fabric-forming Na-poor muscovite. The mineralogy of the new phyllonitic fault-rock produced is dominated by Na-poor muscovite and chlorite together with late albite porphyroblasts. The replacement of the amphibolite facies porphyroblasts by muscovite and chlorite is pseudomorphic in some samples and shows that the chemical metastability of the porphyroblasts is sufficient to drive replacement. In contrast, element mapping shows that fabric-forming Na-rich muscovite is selectively replaced at high-strain microstructural sites, indicating that strain energy played an important role in activating the dissolution of the compositionally metastable muscovite. The replacement of strong, high-grade porphyroblasts by weaker Na-poor muscovite and chlorite constitutes reaction softening. The crystallization of parallel and contiguous mica in the retrograde foliation at the expense of the earlier and locally crenulated Na-rich muscovite-defined foliation destroys not only the metastable high-grade mineralogy, but also its stronger geometry. This process constitutes both reaction and microstructural softening. The deformation mechanism here was thus one of dissolution–precipitation creep, activated at considerably lower stresses than might be predicted in quartzofeldspathic rocks at the same lower greenschist facies conditions.

New Hampshire↗

USGS2012

A transect through the base of the Bronson Hill Terrane in western New Hampshire

This trip will present the preliminary results of ongoing bedrock mapping in the North Hartland and Claremont North 7.5-minute quadrangles in western New Hampshire. The trip will travel from the Lebanon pluton to just north of the Sugar River pluton (Fig. 1) with the aim of examining the lower structural levels of the Bronson Hill anticlinorium (BHA), and the nature of the boundary with the rocks of the Connecticut Valley trough (CVT). Spear and others (2002, 2003, 2008) proposed that western New Hampshire was characterized by five major faults bounding five structural levels including, from lowest to highest, the “chicken yard line”, Western New Hampshire Boundary Thrust, Skitchewaug nappe, Fall Mountain nappe, and Chesham Pond nappe. Lyons and others (1996, 1997) showed the lowest level cored by the Cornish nappe and floored by the Monroe fault. Thompson and others (1968) explained the geometry of units by folding without major thrust faults, and described the second level as the Skitchewaug nappe. This trip will focus on the two lowest levels which we have revised to call the Monroe and Skitchewaug Mountain thrust sheets. Despite decades of geologic mapping in the northeastern United States at various scales, little 1:24,000-scale (or larger scale) modern bedrock mapping has been published for the state of New Hampshire. In fact, of the New England states, New Hampshire contains the fewest published, modern bedrock geologic maps. Conversely, adjacent Vermont has a relatively high percentage of modern bedrock maps due to focused efforts to create a new state-wide bedrock geologic map over the last few decades. The new Vermont map (Ratcliffe and others, 2011) has identified considerable gaps in our knowledge of the bedrock geology in adjacent New Hampshire where published maps are, in places, more than 50 years old and at scales ranging from 1:62,500 to 1:250,000. Fundamental questions remain concerning the geology across the Connecticut River, especially in regards to the stratigraphy of the BHA and CVT, and the distribution, or even existence, of faults ranging in age from Devonian to Mesozoic (e.g., Spear and others, 2008; McWilliams and others, 2010; Walsh and others, 2010). Questions to ponder on this trip include, but are not limited to: 1) Is the Bronson Hill anticlinorium allochthonous? 2) What is the crust beneath the Bronson Hill anticlinorium? 3) Is there a “Big Staurolite nappe” as proposed by Spear and others (2002, 2003, 2008)? 4) What is the role of Taconic, Acadian, and Alleghanian orogenesis in the tectonic development of the region? Modern 1:24,000-scale mapping is the first step towards answering these questions. Mapping will be supplemented by modern geochronology and geochemistry as this project develops. We plan to share some of our provisional results during this field trip.

New Hampshire↗