Anita G. Harris

Evaluation of Southern Eastern Overthrust Belt Beneath Blue Ridge-Piedmont Thrust

Seismic reflection surveys in the southern part of the Eastern overthrust reveal that crystalline rocks of the Blue Ridge and Piedmont were thrust westward burying a large segment of sedimentary rocks of the Valley and Ridge. Surface and subsurface data suggest that the buried segment includes Cambrian and Lower Ordovician shallow-shelf clastic and carbonate rocks overlain by Middle Ordovician foredeep black shale. Because eastward-increasing regional thermal patterns existed prior to thrusting, westward movement of thrust sheets disrupted and telescoped that pattern by placing more thermally mature eastern rocks over less mature western rocks. The original thermal pattern can be reconstructed by restoring the thrust sheets to their prethrust position. The reconstruction mphasizes that rocks less metamorphosed than greenschist facies are contained in only two thrust sheets: all of the Saltville and part of the Pulaski. Because subgreenschist rocks of the Saltville and Pulaski thrust sheets appear to be confined to the subsurface of the Blue Ridge and westward, it is most likely that rocks with commercial gas potential will not be present in the subsurface of the Piedmont east of the Brevard fault. Nevertheless, the concealed area of the southern Eastern overthrust with possible gas potential is about as wide as its exposed part. Thus the combined surface and subsurface area for future exploration in the southern Appalachians is about doubled in size.

Lithostratigraphic, conodont, and other faunal links between lower Paleozoic strata in northern and central Alaska and northeastern Russia

Lower Paleozoic platform carbonate strata in northern Alaska (parts of the Arctic Alaska, York, and Seward terranes; herein called the North Alaska carbonate platform) and central Alaska (Farewell terrane) share distinctive lithologic and faunal features, and may have formed on a single continental fragment situated between Siberia and Laurentia. Sedimentary successions in northern and central Alaska overlie Late Proterozoic metamorphosed basement; contain Late Proterozoic ooid-rich dolostones, Middle Cambrian outer shelf deposits, and Ordovician, Silurian, and Devonian shallow-water platform facies, and include fossils of both Siberian and Laurentian biotic provinces. The presence in the Alaskan terranes of Siberian forms not seen in wellstudied cratonal margin sequences of western Laurentia implies that the Alaskan rocks were not attached to Laurentia during the early Paleozoic. The Siberian cratonal succession includes Archean basement, Ordovician shallow-water siliciclastic rocks, and Upper Silurian-Devonian evaporites, none of which have counterparts in the Alaskan successions, and contains only a few of the Laurentian conodonts that occur in Alaska. Thus we conclude that the lower Paleozoic platform successions of northern and central Alaska were not part of the Siberian craton during their deposition, but may have formed on a crustal fragment rifted away from Siberia during the Late Proterozoic. The Alaskan strata have more similarities to coeval rocks in some peri-Siberian terranes of northeastern Russia (Kotelny, Chukotka, and Omulevka). Lithologic ties between northern Alaska, the Farewell terrane, and the peri-Siberian terranes diminish after the Middle Devonian, but Siberian affinities in northern and central Alaskan biotas persist into the late Paleozoic.

Paleozoic and Mesozoic stratigraphy of the Peshawar basin, Pakistan: Correlations and implications

The most complete Paleozoic sequence described from Pakistan is exposed in bedrock inliers and in ranges fringing the eastern Peshawar basin. Interbedded quartzite and argillite of the Precambrian and Cambrian Tanawal Formation is overlain unconformably by the Cambrian(?) Ambar Formation. The Misri Banda Quartzite unconformably overlies the Ambar and contains Ordovician Cruziana ichnofossils. New conodont discoveries restrict the ages of overlying formations as follows: Panjpir Formation, Llandoverian to Pridolian; Nowshera Formation, Lochkovian to Frasnian; and Jafar Kandao Formation, Kinderhookian to Westphalian. The Karapa Greenschist, consisting of metamorphosed lava flows, separates the Jafar Kandao from Upper Triassic (Carnian) marbles of the Kashala Formation. The Upper Triassic and Jurassic(?) Nikanai Ghar Formation forms the top of the section. Correlatives to the Peshawar basin stratigraphy are present locally in the Sherwan synclinorium of Hazara and in the Khyber Pass region. The sequence contrasts markedly with the Paleozoic and Mesozoic section exposed south of the Khairabad thrust in the Attock-Cherat Range. This thrust and its northeastern continuation in Hazara north of Abbottabad thus form the boundary in Pakistan between the Lesser Himalayan and Tethyan Himalayan sections, a function performed by the Main Central thrust (MCT) in the central Himalaya of India and Nepal. The newly dated Carboniferous to Triassic horizons provide the first firm age constraints on the protoliths of the high-grade Swat metasediments. The dating of the metasediments has, in turn, provided age constraints on pre-Himalayan tectonism and associated intrusions. Two major tectonic episodes during the Late(?) Cambrian and Carboniferous produced positive areas north of the Peshawar basin that provided coarse detritus to the Misri Banda Quartzite and Jafar Kandao Formation.

Late Paleozoic orogeny in Alaska's Farewell terrane

Evidence is presented for a previously unrecognized late Paleozoic orogeny in two parts of Alaska’s Farewell terrane, an event that has not entered into published scenarios for the assembly of Alaska. The Farewell terrane was long regarded as a piece of the early Paleozoic passive margin of western Canada, but is now thought, instead, to have lain between the Siberian and Laurentian (North American) cratons during the early Paleozoic. Evidence for a late Paleozoic orogeny comes from two belts located 100–200 km apart. In the northern belt, metamorphic rocks dated at 284–285 Ma (three 40 Ar/ 39 Ar white-mica plateau ages) provide the main evidence for orogeny. The metamorphic rocks are interpreted as part of the hinterland of a late Paleozoic mountain belt, which we name the Browns Fork orogen. In the southern belt, thick accumulations of PennsylvanianPermian conglomerate and sandstone provide the main evidence for orogeny. These strata are interpreted as the eroded and deformed remnants of a late Paleozoic foreland basin, which we name the Dall Basin. We suggest that the Browns Fork orogen and Dall Basin comprise a matched pair formed during collision between the Farewell terrane and rocks to the west. The colliding object is largely buried beneath Late Cretaceous flysch to the west of the Farewell terrane, but may have included parts of the so-called Innoko terrane. The late Paleozoic convergent plate boundary represented by the Browns Fork orogen likely connected with other zones of plate convergence now located in Russia, elsewhere in Alaska, and in western Canada. Published by Elsevier B.V.

New paleontologic evidence constraining the age and paleotectonic setting of the Talladega slate belt, southern Appalachians

The Talladega slate belt in the southern Appalachian orogen of Alabama and Georgia is a thick sequence of lower greenschist-facies metaclastic, metacarbonate, and metavolcanic rocks thrust above miogeoclinal rocks of the foreland and overthrust by higher grade metamorphic rocks of the eastern Blue Ridge terrane(s). The age assignments and tectonic affinities of this sequence have been highly controversial. Recent fossil discoveries in key stratigraphic units, however, combined with confirmed fossil occurrences in the Jemison Chert, have established a firm correlation with the Appalachian foreland, thus stratigraphically linking the Talladega belt with Laurentia. The lower predominantly clastic sequence, tectonically bounded at its base by the frontal Blue Ridge thrust system, grades upward into a 3.5-km-thick marble sequence. The basal carbonate unit (Jumbo Dolomite) contains Early Cambrian archaeocyathids, and these fossils, in addition to the stratigraphic position and carbonate lithofacies, establish correlation of this unit with the Lower Cambrian Shady Dolomite. The uppermost unit in the marble sequence (Gantts Quarry Formation) contains Early Ordovician (middle to late Canadian; = early to middle Arenigian) conodonts that confirm correlation of this unit with the Newala Limestone and Kingsport and Mascot Formations of the Appalachian foreland. The carbonate platform sequence is unconformably overlain by a thick clastic sequence that accumulated in a deep successor basin; the Lay Dam Formation is a marine fanlike deposit at the base of this sequence. Conodont molds from the top of the Lay Dam and fossils from the stratigraphically higher Jemison Chert indicate a Silurian to Early Devonian age for the Lay Dam Formation. Paleontologic data and the stratigraphic and structural setting indicate that the Talladega slate belt is the most distally preserved and relatively complete fragment of the Appalachian miogeocline; thus, the tectonic evolution of the Talladega belt is crucial to understanding the western margin of Iapetus. Linkage of the Talladega slate belt rocks with those of the western Blue Ridge to the northeast suggests that the latter once contained a thick Cambrian to Devonian cover sequence which subsequently has been mostly removed.

Calibration of a conodont apatite-based Ordovician 87Sr/86Sr 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 diffi cult 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 signifi cant 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 Sr/

Conodont Thermal Maturation Patterns in Paleozoic and Triassic Rocks, Northern Alaska—Geologic and Exploration Implications

The Paleozoic through Jurassic stratigraphic sequence in the Brooks Range consists of platform to intraplatform basin deposits 1-5 km thick. This comparatively thin sequence of heterogeneous lithologies was tectonically disrupted and shortened at least 600 km to form a stack of allochthons that were transported relatively northward and emplaced during the earliest Cretaceous. Thermal patterns in Paleozoic and Triassic rocks, based on conodont color alteration indices (CAI) from about 600 localities, show: (1) a gradual increase in thermal level from the northern margin to about 3/4 of the distance southward across the range (from CAI 1 to 5.5 and higher); (2) a belt of mixed high values (CAI 4.5 to 7) along the south border of the range; (3) thermal levels in surface and ubsurface samples in the range related to tectonic burial and not to pre-thrust burial metamorphism; (4) the same CAI values in rocks above and below the Ellesmerian unconformity in the northeast Brooks Range; (5) an association of anomalously high CAI values with mineralized areas and plutonic rocks; (6) a few anomalously high CAI areas of unknown origin that deserve further study; (7) thermal potential of hydrocarbons (CAI = 4.5) only in the westernmost and northern margins of the Brooks Range; and (9) mineralization potential related to anomalously high CAI values in the southern Brooks Range. Triassic through Mississippian rocks in wells on the north flank of the Colville basin show conventional burial metamorphism patterns within each well and from well to well. All rocks indexed have thermal potential for hydrocarbons (CAI = 1-4.5). The geology of the Seaward Peninsula appears to be a southwestern continuation of that in the Brooks Range. Most of the western one-fifth of the Seaward Peninsula contains 4-5 km of unmetamorphosed platformal, dominantly shallow-water carbonate rocks of Early Ordovician through Devonian age having CAI values of 3-4. Eastward, these rocks are thrust onto a blueschist terrane of mafic volcanogenic and clastic deposits and mixed carbonate and clastic rocks of probable Ordovician through Silurian age. Adjacent to, and possibly infolded with, these rocks are metamorphosed shallow-water carbonates of Early Ordovician through Devonian age similar to the rocks of the westernmost Seward Peninsula. Conodonts from the metacarbonate rocks have CAI values of 5.5-7, indicating temperatures of 350-4 0°C that are consistent with blueschist metamorphism. End_of_Article - Last_Page 666------------

Conodont-Based Thermal Maturation of Paleozoic Rocks in Arizona

Color changes in conodonts are established thermal indices that have been related to the thermal window for hydrocarbon generation and preservation and that also have value for targeting mineralization potential. Conodont color-alteration values from 141 localities in Arizona appear to characterize the thermal maturation of the major Paleozoic outcrop areas in the state. Paleozoic rocks in the Colorado Plateau, in part of the belt of faulted plateau-like rocks bordering the plateau, and in a small area (from 31°20^prime to 31°44^primeN and from 110°00^prime to 110°40^primeW) in southeast Arizona have thermal potential for oil. Paleozoic rocks in the same areas as well as in the Pedregosa basin (southeasternmost Arizona) have thermal potential for gas. f these areas, southeast Arizona appears to have the best hydrocarbon potential because of the association of possible source beds and favorable thermal-maturation levels.

Stratigraphic contrasts and tectonic relationships between Carboniferous successions in the Trans‐Alaska Crustal Transect corridor and adjacent areas, northern Alaska

The Carboniferous succession along the Trans-Alaska Crustal Transect (TACT) corridor in the Atigun Gorge area of the central Brooks Range consists of the Kayak Shale (Kinderhookian) and the Lisburne Group (Kinderhookian through Chesterian). The Kayak Shale is at least 210 m thick; it is chiefly black, noncalcareous shale with several limestone beds of pelmatozoan-bryozoan packstone and formed in an open-marine setting. The Lisburne Group is a carbonate rock succession about 650 m thick and consists mainly of skeletal packstone, wackestone, and milestone which contain locally abundant calcispheres, ostracodes, algae, and sponge spicules; it accumulated largely in a shallow water platform environment with restricted circulation. This restriction was probably produced by a coeval belt of skeletal sand shoals recognized 70 km to the west in the Shainin Lake area. Significant and apparently abrupt shifts in the age and lithofacies of Carboniferous strata occur across the central and eastern Brooks Range. These shifts are most marked in a zone roughly coincident with what is interpreted by many workers to be the leading edge of the Endicott Mountains allochthon. Notable lithologie contrasts are also observed, however, between sections in the northern and southern parts of the Endicott Mountains allochthon. This suggests that considerable tectonic shortening has taken place within the allochthon, as well as between it and parautochthonous rocks to the northeast. The Carboniferous section near Mount Doonerak is more similar in age and lithofacies to coeval sections in the central Brooks Range that are considered allochthonous than to parautochthonous sections to the northeast.

Radiolarians and conodonts from pebbles in the Franciscan assemblage and the Great Valley sequence of the California Coast Ranges

Radiolarians from pebbles in chert-pebble conglomerates that occur in blocks in the Franciscan assemblage are similar to the radiolarians in lithologically similar conglomerate of the Toro Formation and correlative strata of the Great Valley sequence. The blocks may have been derived from the Toro or from a stratigraphic unit with the same source as the Toro. Most radiolarians are Middle and Late Triassic and in part may have been eroded from the Calaveras Formation of the Sierra Nevada. Radiolarians in Upper Cretaceous Franciscan conglomerates are Late Jurassic, which suggests that they were reworked from the Franciscan itself or from chert overlying the Coast Range ophiolite. Triassic conodonts found in Upper Jurassic and Lower Cretaceous conglomerates have low color-alteration indices indicative of low temperature, whereas those found in an Upper Cretaceous conglomerate have high indices indicative of biotite-zone metamorphism. These data thereby bracket the time of metamorphism of the source area. The color index of conodonts in a Franciscan melange suggests a maximum temperature of about 60 °C.