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John E. Repetski

Publications and source records attributed to John E. Repetski.

At least 19 recordsLinked to original sources
USGS2026

Cambrian and Ordovician stratigraphy, conodont biostratigraphy, and microfacies analysis to support 1:24,000-scale geologic mapping of the southern Lake Champlain valley, New York and Vermont

Introduction Geologic mapping in the southern Lake Champlain valley of New York and Vermont (fig. 1) has required evaluation of stratigraphic nomenclature used since the early 1900s. The paleogeography of the Cambrian and Ordovician adjacent to the Adirondack Highlands, the high-relief mountains of Proterozoic igneous and metamorphic rocks of northern New York, has impacted the lithostratigraphic succession, facies changes, and several hiatuses that occur within the package of clastic and carbonate rocks that demonstrates overall deepening consistent from the southern to northern Appalachians. The stratigraphic nomenclature used for the geologic mapping of the Cambrian and Ordovician strata of the study area is derived from evaluation of names put forth by previous workers from areas of northern New York and western Vermont. Descriptions of type localities or type sections and other criteria set forth by the North American Stratigraphic Code (North American Commission on Stratigraphic Nomenclature [NACSN], 2021) were consulted. The North American Stratigraphic Code states that lithostratigraphic units, such as the fundamental unit, the formation, be defined by lithic character and be mappable (NACSN, 2021, Article 24). However, some early workers (Walcott, 1912; Kay, 1937; Fisher and Hanson, 1951; Oxley and Kay, 1959) have defined stratigraphic units based on fossil content, which may or may not be facies dependent, and hence may not be mappable as lithostratigraphic units. Although preservation of older names takes priority in stratigraphic nomenclature (NACSN, 2021, Article 7), some do not have adequately defined stratotypes or are located a distance away with different lithologies than that in the study area. Biostratigraphy can be a helpful tool for understanding correlations and facies changes. Herein, conodont biostratigraphy is utilized to date and correlate separate lithologic units. Further, microfacies analyses of units through inspection of petrographic thin sections provide paleogeographic information that helps in understanding the genesis of geologic units.

New York, Vermont↗
USGS2019

Lithologies, ages, and provenance of clasts in the Ordovician Fincastle Conglomerate, Botetourt County, Virginia, USA

The Fincastle Conglomerate is an Ordovician polymictic, poorly sorted, matrix- and clast-supported cobble to boulder-rich conglomerate located just north of Fincastle, Botetourt County, VA. At least nine other cobble and boulder conglomerates are located in a similar stratigraphic position from Virginia to Georgia west of the Blue Ridge structural front. All except the Fincastle are dominated (~80%) by carbonate clasts; Fincastle clasts are much more varied and siliceous and it is this clast diversity that provides increased value for provenance and related studies. We have used a multidisciplinary approach that involves conodont analysis, sandstone petrography, in-situ outcrop clast characterization, optical petrography, electron-beam petrography and chemical analysis, and X-ray diffraction to provide data on lithologies, ages, and provenance. The size, roundness, and lithology of 1,656 clasts (> 1 cm) were measured in the field. Although, the clast lithology varies among the studied localities, the average lithology is sandstone and siltstone 12 %, vein quartz 17 %, limestone 31 %, low-grade quartzite/metasandstone 31 %, chert 6 %, and others 3 %. Dolomite, igneous, or high-grademetamorphic rock clastswere not identified in field study or in detailed laboratory analysis.Dolomite rhombs and authigenic albite feldsparwere observed in some limestone clasts. Quantitative petrographic data for the Fincastle sandstone clasts indicate tectonic environments from passive margin to transitional continental uplift, but the conglomeratematrixmodes have considerably less feldspar and plot in the foreland basin tectonic environment region. Proto-, para-, and euconodonts were identified from clast and matrix, but are long-ranging fauna indicating middle Cambrian toMiddle or Late Ordovician ages; color alteration index (CAI) for euconodonts varied from 3 to 3.5. The occurrence of well-rounded clasts including limestone suggests a nearby, high-energy environment, and that transport was rapid enough to preserve limestone before deposition into a foreland basin. The lack of igneous or high-grade metamorphic rocks clasts suggests that the erosional level sampled by the Fincastle Conglomerate did not include the underlying Grenville basement of igneous or high-grade metamorphic rocks.

Virginia↗
USGS2016

Reevaluating the age of the Walden Creek Group and the kinematic evolution of the western Blue Ridge, southern Appalachians

An integrated synthesis of existing datasets (detailed geologic mapping, geochronologic, paleontologic, geophysical) with new paleontologic and geochemical investigations of rocks previously interpreted as part of the Neoproterozoic Walden Creek Group in southeastern Tennessee suggest a necessary reevaluation of the kinematics and structural architecture of the Blue Ridge Foothills. The western Blue Ridge of Tennessee, North Carolina, and Georgia is composed of numerous northwest-directed early and late Paleozoic thrust sheets, which record pronounced variation in stratigraphic/structural architecture and timing of metamorphism. The detailed spatial, temporal, and kinematic relationships of these rocks have remained controversial. Two fault blocks that are structurally isolated between the Great Smoky and Miller Cove-Greenbrier thrust sheets, here designated the Maggies Mill and Citico thrust sheets, contain Late Ordovician-Devonian conodonts and stable isotope chemostratigraphic signatures consistent with a mid-Paleozoic age. Geochemical and paleontological analyses of Walden Creek Group rocks northwest and southeast of these two thrust sheets, however, are more consistent with a Late Neoproterozoic (550–545 Ma) depositional age. Consequently, the structural juxtaposition of mid-Paleozoic rocks within a demonstrably Neoproterozoic-Cambrian succession between the Great Smoky and Miller Cove-Greenbrier thrust sheets suggests that a simple foreland-propagating thrust sequence model is not applicable in the Blue Ridge Foothills. We propose that these younger rocks were deposited landward of the Ocoee Supergroup, and were subsequently plucked from the Great Smoky fault footwall as a horse, and breached through the Great Smoky thrust sheet during Alleghanian emplacement of that structure.

North Carolina, Tennessee↗
USGS2016

Proposed Auxiliary Boundary Stratigraphic Section and Point (ASSP) for the base of the Ordovician System at Lawson Cove, Utah, USA

T he Global boundary Stratotype Section and Point (GSSP) for the base of the Ordovician System is at the First Appearance Datum (FAD) of the conodont Iapetognathus fluctivagus at Green Point in Newfoundland, Canada. Strata there are typical graptolitic facies that were deposited near the base of the continental slope.We propose establishing an Auxiliary boundary Stratotype Section and Point (ASSP) at the FAD of I. fluctivagus at the Lawson Cove section in the Ibex area of Millard County, Utah, USA. There, strata consist of typical shelly facies limestones that were deposited on a tropical carbonate platform and contain abundant conodonts, trilobites, brachiopods, and other fossil groups. Cambrian and Ordovician strata in this area are ~5300m thick, with the Lawson Cove section spanning 243m in three overlapping segments. Six other measured and studied sections in the area show stratigraphic relationships similar to those at Lawson Cove. Faunas have been used to divide these strata into 14 conodont and 7 trilobite zonal units. The widespread olenid trilobite Jujuyaspis occurs ~90cm above the proposed boundary at Lawson Cove; this genus is generally regarded as earliest Ordovician. Rhynchonelliform and linguliform brachiopods are common to abundant and are useful for correlation. The FAD of Iapetognathus fluctivagus and occurrences of Jujuyaspis and the Lower Ordovician planktonic graptolite Anisograptus matanensis all occur within a 2.4m interval of strata at a nearby section. Non-biological correlation tools include a detailed sequence stratigraphic classification and a detailed carbon-isotope profile. Especially useful for correlation is a positive 13C excursion peak ~15cm below the proposed boundary horizon. All of these correlation tools form an integrated framework that makes the Lawson Cove section especially useful as an ASSP for global correlation of strata with faunas typical of shallow, warm-water, shelly facies.

Utah↗
USGS2015

Cambrian–Ordovician of the central Appalachians:Correlations and event stratigraphy of carbonate platform andadjacent deep-water deposits

This trip seeks to illustrate the succession of Cambrian and Ordovician facies deposited within the Pennsylvania and Maryland portion of the Great American Carbonate Bank. From the Early Cambrian (Dyeran) through Late Ordovician (Turinan), the Laurentian paleocontinent was rimmed by an extensive carbonate platform. During this protracted period of time, a succession of carbonate rock, more than two miles thick, was deposited in Maryland and Pennsylvania. These strata are now exposed in the Nittany arch of central Pennsylvania; the Great Valley of Pennsylvania, Maryland, and Virginia; and the Conestoga and Frederick Valleys of eastern Pennsylvania and Maryland. This fi eld trip will visit key outcrops that illustrate the varied depositional styles and environmental settings that prevailed at different times within the Pennsylvania reentrant portion of the Great American Carbonate Bank. In particular, we will contrast the timing and pattern of sedimentation in off-shelf (Frederick Valley), outer-shelf (Great Valley), and inner-shelf (Nittany arch) deposits. The deposition was controlled primarily by eustasy through the Cambrian and Early Ordovician (within the Sauk megasequence), but was strongly infl uenced later by the onset of Taconic orogenesis during deposition of the Tippecanoe megasequence.

Pennsylvania, Maryland↗
USGS2015

Geologic assessment of undiscovered oil and gas resources of the U.S. portion of the Michigan Basin

In 2004, the U.S. Geological Survey (USGS) completed an assessment of the undiscovered oil and gas potential of the U.S. portion of the Michigan Basin. For this assessment, the Michigan Basin includes most of the State of Michigan, as well as parts of Illinois, Indiana, Minnesota, Ohio, and Wisconsin. The assessment was based on the geologic elements of each of the six total petroleum systems defined in the basin, including (1) hydrocarbon source rocks (source-rock maturation and hydrocarbon generation and migration), (2) reservoir rocks (sequence stratigraphy and petrophysical properties), and (3) hydrocarbon traps (trap formation and timing). Using this geologic framework, the USGS estimated mean technically recoverable undiscovered continuous and conventional resources that total 990 million barrels of oil, 11.4 trillion cubic feet of natural gas, and 219 million barrels of natural gas liquids. Digital Data Series 69-T (DDS-69-T) is cataloged in the Pubs Warehouse as Data Series 69-T (DS-69-T).

Illinois, Indiana, Michigan, Minnesota, Ohio, Wisc↗
USGS2015

Bedrock geologic map of the Spring Valley, West Plains, and parts of the Piedmont and Poplar Bluff 30'x60' quadrangles, Missouri, including the upper Current River and Eleven Point River drainage basins

This map covers the drainage basins of the upper Current River and the Eleven Point River in the Ozark Plateaus physiographic province of southeastern Missouri. The two surface drainage basins are contiguous in their headwaters regions, but are separated in their lower reaches by the lower Black River basin in the southeast corner of the map area. Numerous dye-trace studies demonstrate that in the contiguous headwaters areas, groundwater flows from the Eleven Point River basin into the Current River basin. Much of the groundwater discharge of the Eleven Point River basin emanates from Big Spring, located on the Current River. This geologic map and cross sections were produced to help fulfill a need to understand the geologic framework of the region in which this subsurface flow occurs. The map includes all of the Ozark National Scenic Riverways, a national park created by an Act of Congress in 1964 to protect 134 miles of the Current and Jacks Fork Rivers in south-central Missouri. Located within the park are numerous large springs, including Big Spring, the largest spring in Missouri and one of the ten largest springs in the world. Also within the map area is Greer Spring, which is the main source of the Eleven Point River. Greer Spring is the largest spring on National Forest land in the United States. During flood, flow from Greer Spring is almost as large, volumetrically, as that from Big Spring. The Wild and Scenic Rivers Act in 1968 established a 44-mile section of the Eleven Point River as the Eleven Point National Scenic River, which is entirely within the boundaries of this map. Potentially economic mineral resources are present in the subsurface in the map area. Exploration drill-hole data indicate that anomalously high concentrations of base-metal sulfides locally occur within the Cambrian Bonneterre Formation. The geologic setting of these anomalous concentrations is similar to that found in the Viburnum Trend, part of the largest lead-mining district in the world. The southernmost part of the Viburnum Trend extends into the northern part of the map area and is exploited by the Sweetwater Mine. Undeveloped and potentially economic occurrences of base metals are known also beneath Blair Creek, a tributary to the Current River in the north-central part of the map area.

Missouri↗
USGS2015

Life on the edge in eastern Alaska: Basal Ordovician(Tremadocian), platform-margin faunas of the Jones Ridge Formation

As the most fossiliferous and least deformed succession of unequivocally Laurentian lower Paleozoic strata in Alaska, the Jones Ridge Limestone has provided critical data for numerous stratigraphic studies (e. g. Palmer 1968; Harris et al. 1995; Dumoulin et al. 2002; Dumoulin and Harris 2012) focused on the Cambrian and Ordovician of northwestern North America/northeastern Laurentia (Figure 1). The Jones Ridge faunas are also significant in having provided the type material for some of the widespread and biostratigraphically useful latest Furongian and (perhaps) earliest Tremadocian species described by Kobayashi (1936) and Palmer (1968). Unfortunately, some of those taxa were based on very limited material for which, in the earlier study in particular, no detailed information regarding locality or stratigraphic horizon was provided. The limited amount of information and material available for study from Jones Ridge results largely from its remote location on the Yukon-Alaska boundary approximately 25km north of Eagle, Alaska, which renders it accessible only by helicopter. Parts of three field seasons (2010, 2011, and 2014) were invested in re-description and intensive sampling of the type section of the Jones Ridge Formation in order to produce an integrated and greatly refined set of biostratigraphic, chemostratigraphic, and sedimentological data. The new data support the interpretation offered by Palmer (1968) of the Jones Ridge strata as the product of deposition in outermost platform to upper slope environments offered by Palmer (1968) on the basis of taxonomic content of the faunas and close proximity of deep water units of equivalent age a very short distance to the southwest.

Alaska↗
USGS2015

Field trip guidebook for the post-meeting field trip: The Central Appalachians

The lower Paleozoic rocks to be examined on this trip through the central Appalachians represent an extreme range of depositional environments. The lithofacies we will examine range from pelagic radiolarian chert and interbedded mudstone that originated on the deep floor of the Iapetus Ocean, through mud cracked supratidal dolomitic laminites that formed during episodes of emergence of the long-lived Laurentian carbonate platform, to meandering fluvial conglomerate and interstratified overbank mudstone packages deposited in the latest stages of infilling of the Taconic foredeep. In many ways this field trip is about contrasts. The Upper Cambrian (Furongian) and Lower Ordovician deposits of the Sauk megasequence record deposition controlled primarily by eustatic sea level sea level fluctuations that influenced deposition along the passive, southern (Appalachian) margin of the paleocontinent of Laurentia. The only tectonic influence apparent in these passive margin deposits is the expected thickening of the carbonate stack toward the platform margin as compared to the thinner (and typically shallower) facies that formed farther in toward the paleoshoreline. Carbonates overwhelmingly dominate the passive margin succession. Clastic influx was minimal and consisted largely of eastward transport of clean cratonic sands across the platform from the adjacent inner detrital belt to the west during higher order (2nd and 3rd order) regressions. In contrast, Middle and Upper Ordovician deposits of the Tippecanoe megasequence record the strong influence of tectonics, specifically Iapetus closure. The first signal of this tectonic transformation was the arrival of arc-related ash beds that abound in the active margin carbonates. Subsequent intensification of Taconic orogenesis resulted in the foundering of the carbonate platform under the onslaught of fine siliciclastics arriving from offshore tectonic sources to the east, creating a deep marine flysch basin where graptolitic shale and sandstone turbidites accumulated. The foreland basin thus created would fill with progressively coarser and more shallow/proximal clastic facies through the Upper Ordovician, culminating in deposition of fluvial redbeds that cap the Taconic clastic wedge. Arguably the most controversial rocks within the Tippecanoe Sequence in this area are unusual, Lower Ordovician deep marine facies that are associated with the much younger flysch of the Martinsburg Formation in the Great Valley of eastern Pennsylvania. Long considered the erosional remnants of a Taconic-style thrust sheet, and referred to as the Hamburg Klippe, these deep marine deposits have recently been reinterpreted as olistostromal deposits that were introduced by gravity sliding into the flysch basin contemporaneous with Martinsburg deposition. Besides their constituent lithofacies, rocks of the Sauk and Tippecanoe megasequences also present a stark contrast in faunas. Cambrian and Lower Ordovician faunas predate the Great Ordovician Biodiversification Event (GOBE), a global event that saw unprecedented diversification within many major invertebrate groups (mollusks, corals, and bryozoans to name a few) that previously were only minor components of the marine fauna. Unfortunately, the much higher diversity of Middle and Upper Ordovician faunas wrought by the GOBE is somewhat muted in this region by the stresses introduced by conversion of the Appalachian shelf into a flysch basin. Another noteworthy difference between the Cambrian and Ordovician biota related to the paleogeographic setting of the rocks to be examined on this trip derives from their evolution in the shallow marine environments of Laurentia. Several shelf-wide extinctions decimated the shallow marine faunas of the Laurentian shelf through the late Cambrian producing stage-level biostratigraphic units known as biomeres. The biomere phenomenon is discussed in this guidebook and a few stops to examine Cambrian faunas and one biomere boundary extinction are included to provide contrast with stage boundary extinctions that occurred later, in the Ordovician, that lack the defining attributes of the biomere boundary extinctions. Again, it’s all about contrast.

Central Appalachians↗
USGS2014

Calibration of a conodont apatite-based Ordovician 87 Sr/ 86 Sr curve to biostratigraphy and geochronology: Implications for stratigraphic resolution

The Ordovician 87 Sr/ 86 Sr isotope seawater curve is well established and shows a decreasing trend until the mid-Katian. However, uncertainties in calibration of this curve to biostratigraphy and geochronology have made it difficult to determine how the rates of 87 Sr/ 86 Sr decrease may have varied, which has implications for both the stratigraphic resolution possible using Sr isotope stratigraphy and efforts to model the effects of Ordovician geologic events. We measured 87 Sr/ 86 Sr in conodont apatite in North American Ordovician sections that are well studied for conodont biostratigraphy, primarily in Nevada, Oklahoma, the Appalachian region, and Ohio Valley. Our results indicate that conodont apatite may provide an accurate medium for Sr isotope stratigraphy and strengthen previous reports that point toward a significant increase in the rate of fall in seawater 87 Sr/ 86 Sr during the Middle Ordovician Darriwilian Stage. Our 87 Sr/ 86 Sr results suggest that Sr isotope stratigraphy will be most useful as a high-resolution tool for global correlation in the mid-Darriwilian to mid-Sandbian, when the maximum rate of fall in 87 Sr/ 86 Sr is estimated at ∼5.0–10.0 × 10 –5 per m.y. Variable preservation of conodont elements limits the precision for individual stratigraphic horizons. Replicate conodont analyses from the same sample differ by an average of ∼4.0 × 10 –5 (the 2σ standard deviation is 6.2 × 10 –5 ), which in the best case scenario allows for subdivision of Ordovician time intervals characterized by the highest rates of fall in 87 Sr/ 86 Sr at a maximum resolution of ∼0.5–1.0 m.y. Links between the increased rate of fall in 87 Sr/ 86 Sr beginning in the mid-late Darriwilian ( Phragmodus polonicus to Pygodus serra conodont zones) and geologic events continue to be investigated, but the coincidence with a long-term rise in sea level (Sauk-Tippecanoe megasequence boundary) and tectonic events (Taconic orogeny) in North America provides a plausible explanation for the changing magnitude and 87 Sr/ 86 Sr of the riverine Sr flux to the oceans.

GSA Bulletin↗
USGS2014

The conodont Iapetognathus and its value for defining the base of the Ordovician System

Nicoll et al. (1999, Brigham Young University Geology Studies 44, 27–101) published the taxonomy of species of the ramiform conodont Iapetognathus Landing in Fortey et al. (1982, The Cambrian–Ordovician boundary: sections, fossil distributions, and correlations , National Museum of Wales, Geological Series No. 3, Cardiff, 95–129) and its ancestor Iapetonudus Nicoll et al., 1999. Cooper et al. (2001, Episodes 24 , 19–28) used the First Appearance Datum of Iapetognathus fluctivagus Nicoll et al., 1999 to mark the base of the Ordovician System at Green Point, Newfoundland. Terfelt et al. (2012, Lethaia 45 , 227–237) re-evaluated Iapetognathus at Green Point and made several taxonomic and stratigraphic conclusions, nearly all of which we refute herein.

GFF↗
USGS2014

Carbonate rocks of the Seward Peninsula, Alaska: Their correlation and paleogeographic significance

Paleozoic carbonate strata deposited in shallow platform to off-platform settings occur across the Seward Peninsula and range from unmetamorphosed Ordovician–Devonian(?) rocks of the York succession in the west to highly deformed and metamorphosed Cambrian–Devonian units of the Nome Complex in the east. Faunal and lithologic correlations indicate that early Paleozoic strata in the two areas formed as part of a single carbonate platform. The York succession makes up part of the York terrane and consists of Ordovician, lesser Silurian, and limited, possibly Devonian rocks. Shallow-water facies predominate, but subordinate graptolitic shale and calcareous turbidites accumulated in deeper water, intraplatform basin environments, chiefly during the Middle Ordovician. Lower Ordovician strata are mainly lime mudstone and peloid-intraclast grainstone deposited in a deepening upward regime; noncarbonate detritus is abundant in lower parts of the section. Upper Ordovician and Silurian rocks include carbonate mudstone, skeletal wackestone, and coral-stromatoporoid biostromes that are commonly dolomitic and accumulated in warm, shallow to very shallow settings with locally restricted circulation. The rest of the York terrane is mainly Ordovician and older, variously deformed and metamorphosed carbonate and siliciclastic rocks intruded by early Cambrian (and younger?) metagabbros. Older (Neoproterozoic–Cambrian) parts of these units are chiefly turbidites and may have been basement for the carbonate platform facies of the York succession; younger, shallow- and deep-water strata likely represent previously unrecognized parts of the York succession and its offshore equivalents. Intensely deformed and altered Mississippian carbonate strata crop out in a small area at the western edge of the terrane. Metacarbonate rocks form all or part of several units within the blueschist- and greenschist-facies Nome Complex. The Layered sequence includes mafic meta¬igneous rocks and associated calcareous metaturbidites of Ordovician age as well as shallow-water Silurian dolostones. Scattered metacarbonate rocks are chiefly Cambrian, Ordovician, Silurian, and Devonian dolostones that formed in shallow, warm-water settings with locally restricted circulation and marbles of less constrained Paleozoic age. Carbonate metaturbidites occur on the northeast and southeast coasts and yield mainly Silurian and lesser Ordovician and Devonian conodonts; the northern succession also includes debris flows with meter-scale clasts and an argillite interval with Late Ordovician graptolites and lenses of radiolarian chert. Mafic igneous rocks at least partly of Early Devonian age are common in the southern succession. Carbonate rocks on Seward Peninsula experienced a range of deformational and thermal histories equivalent to those documented in the Brooks Range. Conodont color alteration indices (CAIs) from Seward Peninsula, like those from the Brooks Range, define distinct thermal provinces that likely reflect structural burial. Penetratively deformed high-pressure metamorphic rocks of the Nome Complex (CAIs ≥5) correspond to rocks of the Schist belt in the southern Brooks Range; both record subduction during early stages of the Jurassic–Cretaceous Brooks Range orogeny. Weakly metamorphosed to unmetamorphosed strata of the York terrane (CAIs mainly 2–5), like Brooks Range rocks in the Central belt and structural allochthons to the north, experienced moderate to shallow burial during the main phase of the Brooks Range orogeny. The nature of the contact between the York terrane and the Nome Complex is uncertain; it may be a thrust fault, an extensional surface, or a thrust fault later reactivated as an extensional fault. Lithofacies and biofacies data indicate that, in spite of their divergent Mesozoic histories, rocks of the York terrane and protoliths of the Nome Complex formed as part of the same lower Paleozoic carbonate platform. Stratigraphies in both

GSA Special Papers↗
USGS2014

Stratigraphic framework of Cambrian and Ordovician rocks in the central Appalachian basin from Medina County, Ohio, through southwestern and south-central Pennsylvania to Hampshire County, West Virginia

This chapter is a re-release of U.S. Geological Survey Bulletin 1839-K, of the same title, by Ryder and others (1992; online version 2.0 revised and digitized by Robert D. Crangle, Jr., 2003). It consists of one file of the report text as it appeared in USGS Bulletin 1839-K and a second file containing the cross section, figures 1 and 2, and tables 1 and 2 on one oversized sheet; the second file was digitized in 2003 as version 2.0 and also includes the gamma-ray well log traces.

Appalachian basin↗
USGS2014

Thermal maturity patterns (conodont color alteration index and vitrinite reflectance) in Upper Ordovician and Devonian rocks of the Appalachian basin: A major revision of USGS Map I-917-E using new subsurface collections

Introduction The conodont color alteration index (CAI) introduced by Epstein and others (1977) and Harris and others (1978) is an important criterion for estimating the thermal maturity of Ordovician to Mississippian rocks in the Appalachian basin. Consequently, the CAI isograd maps of Harris and others (1978) are commonly used by geologists to characterize the thermal and burial history of the Appalachian basin and to better understand the origin and distribution of oil and gas resources in the basin. The main objectives of this report are to present revised CAI isograd maps for Ordovician and Devonian rocks in the Appalachian basin and to interpret the geologic and petroleum resource implications of these maps. The CAI isograd maps presented herein complement, and in some areas replace, the CAI-based isograd maps of Harris and others (1978) for the Appalachian basin. The CAI data presented in this report were derived almost entirely from subsurface samples, whereas the CAI data used by Harris and others (1978) were derived almost entirely from outcrop samples. Because of the different sampling methods, there is little geographic overlap of the two data sets. The new data set is mostly from the Allegheny Plateau structural province and most of the data set of Harris and others (1978) is from the Valley and Ridge structural province, east of the Allegheny structural front (fig. 1). Vitrinite reflectance, based on dispersed vitrinite in Devonian black shale, is another important parameter for estimating the thermal maturity in pre-Pennsylvanian-age rocks of the Appalachian basin (Streib, 1981; Cole and others, 1987; Gerlach and Cercone, 1993; Rimmer and others, 1993; Curtis and Faure, 1997). This chapter also presents a revised percent vitrinite reflectance (%R 0 ) isograd map based on dispersed vitrinite recovered from selected Devonian black shales. The Devonian black shales used for the vitrinite studies reported herein also were analyzed by RockEval pyrolysis and total organic carbon (TOC) content in weight percent. Although the RockEval and TOC data are included in this chapter (table 1), they are not shown on the maps. The revised CAI isograd and percent vitrinite reflectance isograd maps cover all or parts of Kentucky, New York, Ohio, Pennsylvania, Virginia, and West Virginia (fig. 1), and the following three stratigraphic intervals: Upper Ordovician carbonate rocks, Lower and Middle Devonian carbonate rocks, and Middle and Upper Devonian black shales. These stratigraphic intervals were chosen for the following reasons: (1) they represent target reservoirs for much of the oil and gas exploration in the Appalachian basin; (2) they are stratigraphically near probable source rocks for most of the oil and gas; (3) they include geologic formations that are nearly continuous across the basin; (4) they contain abundant carbonate grainstone-packstone intervals, which give a reasonable to good probability of recovery of conodont elements from small samples of drill cuttings; and (5) the Middle and Upper Devonian black shale contains large amounts of organic matter for RockEval, TOC, and dispersed vitrinite analyses. Thermal maturity patterns of the Upper Ordovician Trenton Limestone are of particular interest here, because they closely approximate the thermal maturity patterns in the overlying Upper Ordovician Utica Shale, which is the probable source rock for oil and gas in the Upper Cambrian Rose Run Sandstone (sandstone), Upper Cambrian and Lower Ordovician Knox Group (Dolomite), Lower and Middle Ordovician Beekmantown Group (dolomite or Dolomite), Upper Ordovician Trenton and Black River Limestones, and Lower Silurian Clinton/Medina sandstone (Cole and others, 1987; Jenden and others, 1993; Laughrey and Baldassare, 1998; Ryder and others, 1998; Ryder and Zagorski, 2003). The thermal maturity patterns of the Lower Devonian Helderberg Limestone (Group), Middle Devonian Onondaga Limestone, and Middle Devonian Marcellus Shale-Upper Devonian Rhine street Shale Member-Upper Devonian Ohio Shale are of interest, because they closely approximate the thermal maturity patterns in the Marcellus Shale, Upper Devonian Rhinestreet Shale Member, and Upper Devonian Huron Member of the Ohio Shale, which are the most important source rocks for oil and gas in the Appalachian basin (de Witt and Milici, 1989; Klemme and Ulmishek, 1991). The Marcellus, Rhinestreet, and Huron units are black-shale source rocks for oil and (or) gas in the Lower Devonian Oriskany Sandstone, the Upper Devonian sandstones, the Middle and Upper Devonian black shales, and the Upper Devonian-Lower Mississippian(?) Berea Sandstone (Patchen and others, 1992; Roen and Kepferle, 1993; Laughrey and Baldassare, 1998).

Appalachian basin↗
USGS2013

The origin of conodonts and of vertebrate mineralized skeletons

Conodonts are an extinct group of jawless vertebrates whose tooth-like elements are the earliest instance of a mineralized skeleton in the vertebrate lineage , inspiring the ‘inside-out’ hypothesis that teeth evolved independently of the vertebrate dermal skeleton and before the origin of jaws . However, these propositions have been based on evidence from derived euconodonts. Here we test hypotheses of a paraconodont ancestry of euconodonts using synchrotron radiation X-ray tomographic microscopy to characterize and compare the microstructure of morphologically similar euconodont and paraconodont elements. Paraconodonts exhibit a range of grades of structural differentiation, including tissues and a pattern of growth common to euconodont basal bodies. The different grades of structural differentiation exhibited by paraconodonts demonstrate the stepwise acquisition of euconodont characters, resolving debate over the relationship between these two groups. By implication, the putative homology of euconodont crown tissue and vertebrate enamel must be rejected as these tissues have evolved independently and convergently. Thus, the precise ontogenetic, structural and topological similarities between conodont elements and vertebrate odontodes appear to be a remarkable instance of convergence. The last common ancestor of conodonts and jawed vertebrates probably lacked mineralized skeletal tissues. The hypothesis that teeth evolved before jaws and the inside-out hypothesis of dental evolution must be rejected; teeth seem to have evolved through the extension of odontogenic competence from the external dermis to internal epithelium soon after the origin of jaws.

Nature↗
USGS2013

Laurentian origin for the North Slope of Alaska: Implications for the tectonic evolution of the Arctic

The composite Arctic Alaska–Chukotka terrane plays a central role in tectonic reconstructions of the Arctic. An exotic, non-Laurentian origin of Arctic Alaska–Chukotka has been proposed based on paleobiogeographic faunal affinities and various geochronological constraints from the southwestern portions of the terrane. Here, we report early Paleozoic trilobite and conodont taxa that support a Laurentian origin for the North Slope subterrane of Arctic Alaska, as well as new Neoproterozoic–Cambrian detrital zircon geochronological data, which are both consistent with a Laurentian origin and profoundly different from those derived from similar-aged strata in the southwestern subterranes of Arctic Alaska–Chukotka. The North Slope subterrane is accordingly interpreted as allochthonous with respect to northwestern Laurentia, but it most likely originated farther east along the Canadian Arctic or Atlantic margins. These data demonstrate that construction of the composite Arctic Alaska–Chukotka terrane resulted from juxtaposition of the exotic southwestern fragments of the terrane against the northern margin of Laurentia during protracted Devonian(?)–Carboniferous tectonism.

Lithosphere↗