Pre-Mississippian Stratigraphic Architecture of the Porcupine Shear Zone, Yukon and Alaska, and Significance in the Evolution of Northern Laurentia

The origin and displacement history of terranes emplaced along the northern margin of North America remain contentious. One of these terranes is the North Slope subterrane of the Arctic Alaska-Chukotka microplate, which is separated from the northwestern margin of Laurentia (Yukon block) by the Porcupine Shear Zone of Alaska and Yukon. Here, we present new field observations, geological mapping, detrital zircon U-Pb geochronology, and sedimentary/igneous geochemistry to elucidate the stratigraphic architecture of deformed pre-Mississippian rocks exposed within the Porcupine Shear Zone, which we distinguish herein as the newly defined Ch’oodeenjìk succession. The oldest rocks in the Ch’oodeenjìk succession consist of siliciclastic strata of the Lahchah and Sunaghun formations (new names), which yield detrital zircon U-Pb age populations of ca. 1050-1250, 1350-1450, 1600-1650, and 2500-2800 Ma (n =800). This succession is overlain by chert-bearing dolostone and limestone of the Caribou Bar formation (new name) that contains vase-shaped microfossils and yields carbonate carbon (δ13Ccarb) and strontium (87Sr/86Sr) isotopic data that range from ca. -3‰ to +3‰ and 0.70636 to 0.70714, respectively. These data suggest that Lahchah, Sunaghun, and Caribou Bar formations are late Tonian in age. These Neoproterozoic rocks are intruded by Late Devonian (Frasnian-Famennian) felsic plutons and mafic dikes, one of which yielded a sensitive high-resolution ion microprobe-reverse geometry (SHRIMP-RG) U-Pb age of 380 ± 4 Ma. Neoproterozoic strata of the Ch’oodeenjìk succession are also unconformably overlain by Upper Devonian-Carboniferous (?) siliciclastic rocks of the Darcy Creek formation (new name), which yields detrital zircon populations of ca. 365–385, 420-470 and 625-835 Ma, in addition to Proterozoic age populations similar to the underlying Tonian strata. Together, these new stratigraphic, geochronological, geochemical, and micropaleontological data indicate that pre-Mississippian rocks exposed within the Porcupine Shear Zone most likely represent a peri-Laurentian crustal fragment that differs from the adjacent Yukon block and North Slope subterrane; thus, the Porcupine Shear Zone represents a fundamental tectonic boundary separating autochthonous Laurentia from various accreted peri-Laurentian crustal fragments.

Paleoecology and taxonomy of Schoenaster carterensis, a new encrinasterid ophiuroid species from the Upper Mississippian (Chesterian) Slade Formation of northeastern Kentucky, USA

Schoenaster carterensis new species, is an asteroid-like ophiuroid (Echinodermata) from Upper Mississippian (Chesterian) shallow-water carbonates in the Ramey Creek Member of the Slade Formation in northeastern Kentucky. First described in the 1860s from Lower and Middle Mississippian rocks, Schoenaster Meek and Worthen, 1860 is not a well-known fossil genus, but the 39 specimens in this collection permitted further definition of the genus and extended its range by ca. 17 Ma into Late Mississippian (Chesterian) time. The number of specimens also permitted differentiation of growth stages based on average arm length and showed that arm length, disk perimeter, and disk area are interrelated in statistically significant ways. Although replaced by chert, the specimens are nearly intact due to rapid burial as rare constituents in habitat communities distributed among four once-contiguous habitats, including shoal, shoal margin, transitional, and basinal. Most of the ophiuroids were concentrated on firm grounds or hardgrounds in shoal and transitional environments, concentrations that probably reflect substratum stability and the ability to support the ophiuroid's generalist feeding strategy. Many fossil ophiuroid species are known from only a few specimens, severely limiting interpretations about their detailed taxonomy, individual variation, and ecology. In contrast, the greater number of specimens and extensive knowledge of geologic occurrence in this study permitted detailed interpretations regarding the taxonomic, intraspecific, and ecologic attributes of this species, which might be useful in the study of other fossil ophiuroids.UUID: http://www.zoobank.org/ffd945d8-63ac-4c38-a2d3-8647558dbbf0

The major pre-Mississippian unconformity in Rock Canyon, central Wasatch Range, Utah

In central Utah, the major pre-Mississippian unconformity is fairly well understood at most of the localities where it is recognized. However, the unconformity is more enigmatic in Rock Canyon of the central Wasatch Range. At this locality, dolomitization of most pre-Mississippian rocks obscures stratigraphic identification of Devonian and older units. The absence of any identifiable angular relationship further complicates resolution. Because of this, both identification of the stratigraphic level of the unconformity and, consequently, its magnitude remain controversial. Large-size dolomite samples taken in Rock Canyon at closely spaced intervals for the 3.6-m directly below definite Upper Devonian rocks yield microfossils, including conodonts, in the uppermost 1.6-m of that interval that indicate no unconformity exists between the Cambrian Maxfield Limestone and the Upper Devonian-Lower Mississippian Fitchville Dolomite at the horizon previously identified as unconformable. Rather, an unknown thickness of dolomitized Upper Devonian Pinyon Peak Formation and probable older rock (possibly Bluebell Dolomite and Victoria Formation) occurs between the top of definite Maxfield and base of the Fitchville. The identification of the unconformity horizon remains unknown. Our preliminary work outlines a promising procedure for future understanding of the magnitude and stratigraphic level of the unconformity.

The major pre-Mississippian unconformity in Rock Canyon, central Wasatch Range, Utah

In central Utah, the major pre-Mississippian unconformity is fairly well understood at most of the localities where it is recognized. However, the unconformity is more enigmatic in Rock Canyon of the central Wasatch Range. At this locality, dolomitization of most pre-Mississippian rocks obscures stratigraphic identification of Devonian and older units. The absence of any identifiable angular relationship further complicates resolution. Because of this, both identification of the stratigraphic level of the unconformity and, consequently, its magnitude remain controversial. Large-size dolomite samples taken in Rock Canyon at closely spaced intervals for the 3.6-m directly below definite Upper Devonian rocks yield microfossils, including conodonts, in the uppermost 1.6-m of that interval that indicate no unconformity exists between the Cambrian Maxfield Limestone and the Upper Devonian-Lower Mississippian Fitchville Dolomite at the horizon previously identified as unconformable. Rather, an unknown thickness of dolomitized Upper Devonian Pinyon Peak Formation and probable older rock (possibly Bluebell Dolomite and Victoria Formation) occurs between the top of definite Maxfield and base of the Fitchville. The identification of the unconformity horizon remains unknown. Our preliminary work outlines a promising procedure for future understanding of the magnitude and stratigraphic level of the unconformity.