Raymond L. Ethington

OXYGEN ISOTOPIC COMPOSITION OF BIOGENIC PHOSPHATE AND THE TEMPERATURE OF EARLY ORDOVICIAN SEAWATER

Abstract Stable isotopic values were measured on micrite, sparry calcite, dolomite, inarticulated brachiopods, and conodonts from the Lange Ranch section (central Texas) of the Lower Ordovician Tanyard Formation. The section spans the upper Cordylodus angulatus Zone through the lower Rossodus manitouensis Zone. An ∼2‰ negative δ13C shift from >0‰ to <−1.5‰VPDB through the section suggests the lower third of the Rossodus manitouensis Zone was sampled. Consistent with previous studies, the δ18O values of carbonates are low, ranging from −3.3‰ to −8.1‰VPDB. Phosphate δ18O values range from 15.4‰ to 17.1‰VSMOW. Paleotemperature estimates calculated from micrite δ18O values assuming an ice-free seawater δ18O value of −1‰VSMOW indicate Early Ordovician tropical seawater temperatures averaged 42°C, whereas δ18O values of co-occurring biogenic phosphate assuming the same seawater value yield paleotemperature estimates averaging 37°C. The phosphate values are interpreted as less affected by diagenesis than carbonate values and suggest Early Ordovician tropical paleotemperatures were not more than 10°C warmer or the oxygen isotopic composition of Early Ordovician hydrosphere was not more than 2‰ lower than present.

Fallen arches: Dispelling myths concerning Cambrian and Ordovician paleogeography of the Rocky Mountain region

High-resolution sedimentologic, biostratigraphic, and stable isotope data from numerous measured sections across Colorado reveal a complex architecture for lower Paleozoic strata in the central Cordilleran region. A lack of precise age control in previous studies had resulted in misidentification and miscorrelation of units between separate ranges. Corrections of these errors made possible by our improved data set indicate the following depositional history. The quartz-rich sandstone of the Sawatch Formation was deposited during onlap of the Precambrian erosion surface in the early Late Cambrian. The overlying Dotsero Formation, a regionally extensive carbonate- and shale-rich succession records blanket-like deposition with only minor facies changes across the state. An extremely widespread, meter-scale stromatolite bed, the Clinetop Bed, caps the Dotsero Formation in most areas. However, a latest Cambrian erosional episode removed 9–11 m of the upper Dotsero Formation, including the Clinetop Bed, from just east of the Homestake shear zone in the Sawatch Range eastward to the Mosquito Range. The overlying Manitou Formation differs in character, and thus in member stratigraphy, on the east vs. west sides of the state. These differences were previously interpreted as the result of deposition on either side of a basement high that existed within the Central Colorado Embayment or Colorado “Sag,” a region of major breaching across the Transcontinental Arch. This paleogeographic reconstruction is shown herein to be an artifact of miscorrelation. Biostratigraphic data show that the northwestern members of the Manitou Formation are older than the members exposed in the southeastern part of the state and that there is little or no overlap in age between the two areas. This circumstance is the result of (1) removal of older Manitou Formation strata in the southeast by an unconformity developed during the Rossodus manitouensis conodont Zone, and (2) erosion of younger Manitou strata in central and western Colorado along Middle Ordovician and Devonian unconformities. Deciphering these complex stratal geometries has led to invalidation of long-held views on western Laurentian paleogeography during the Cambrian and earliest Ordovician, specifically the existence of the Colorado Sag and a northeast-trending high within the sag that controlled depositional patterns on either side. The mid- Rossodus uplift and resultant unconformity eliminated any and all Upper Cambrian and Lower Ordovician deposits in southern Colorado and northern New Mexico, and thus their absence should not be misconstrued as evidence for earlier nondeposition in this region. Lithofacies distribution patterns and isopach maps provide no evidence that highlands of the Transcontinental Arch existed in Colorado prior to the mid- Rossodus age uplift event. In fact, regional reconstructions of earliest Paleozoic paleogeography along the entire length of the purported Transcontinental Arch should be reevaluated with similarly precise biostratigraphic data to reconsider all potential causes for missing strata and to eliminate topographic elements not supported by multiple stratigraphic techniques. This study illustrates how seriously paleogeographic reconstructions can be biased by the presumption that missing strata represent periods of nondeposition rather than subsequent episodes of erosion, particularly in thin cratonic successions where stratigraphic gaps are common and often inconspicuous.

Early Ordovician conodonts from the Dumugol Formation in the Baegunsan Syncline, eastern Yeongweol and Samcheog areas, Kangweon-Do, Korea

Samples from the Dumugol Formation were collected from eight measured sections and two cores in the so-called Baegunsan Syncline, located between eastern Yeongweol and Samcheog areas, Kangweon-Do, South Korea. A total of 7,813 identifiable conodonts were recovered from 146 samples. They are classified into 24 multielement species referable to 16 genera, and 37 form species belonging to 17 genera; six species are newly described: Distacodus dumugolensis, Parapanderodus acontiformis, Scolopodus longibasis, Scolopodus n. sp. A, Triangulodus dumugolensis , and Oistodus (?) sp. The Dumugol Formation is divided biostratigraphically into four conodont zones, viz, Chosonodina herfurthi-Rossodus manitouensis, Glyptoconus quadraplicatus, Paracordylodus gracilis , and Triangulodus dumugolensis zones in ascending order. These zones are correlated with those of the early Ordovician conodont faunas of North America, northern Europe, China, Australia, and Iran. The Dumugol Formation ranges from late Tremadocian to early Arenigian in age. The boundary between these two series lies between 70 and 100 m above the base of the Dumugol Formation in each of the sections.

Stratotype of Ordovician Whiterock Series

Here the authors reaffirm and document fossil ranges at the stratotype section for the base of the Whiterock Series in Whiterock Canyon, northern Monitor Range, Nevada, and designate a reference section at Meiklejohn Peak, Bare Mountain quadrangle, Nevada. In both sections, stratigraphic ranges of conodonts indicate that the boundary between the Whiterock and the underlying Ibex Series lies between Oepikodus evae below and Tripodus laevis above. This level is correlated with the inception of the Isograptus victoriae graptolite lineage. Coeval brachiopod and trilobite faunas are summarized and their ranges graphically illustrated. The base of the Whiterock Series seems to be coincident with the start of the Taconic Orogeny. During the Whiterock other tectonically controlled events preceded the Tippecanoe Sequence in North America.

Paleoceanographic events and faunal crises recorded in the Upper Cambrian and Lower Ordovician of west Texas and southern New Mexico

A revised lithostratigraphy for Lower Paleozoic strata in New Mexico and west Texas was developed through detailed sedimentological study of the Bliss and Hitt Canyon Formations within a refi ned temporal framework assembled from precise biostratigraphic (trilobite and conodont) and chemostratigraphic (carbon isotope) data. Member boundaries within the Hitt Canyon now correspond with mappable and essentially isochronous horizons that represent major depositional events that affected sedimentation in basins throughout Laurentian North America. This trip is designed to examine these and other important intervals, such as the extinction horizons at the base and top of the Skullrockian Stage, and to demonstrate the utility of associated faunas and isotopic excursions for correlation within and beyond the region.

Geological implications of Late Cambrian trilobites from the Collier Shale, Jessieville area, Arkansas

Late Cambrian shelf trilobites characteristic of the Elvinia and Taenicephalus Zones (Franconian Stage) have been recovered from seven localities in newly recognized outcrops of the Collier Shale near Jessieville, Arkansas. Deposition of the Collier Shale spanned the Cambrian-Ordovician boundary; Late Cambrian trilobites occur in the lower part and Early Ordovician conodonts occur in the upper part of the formation. The trilobites are well known from many shallow-water shelf localities in North America, and their presence indicates that the Benton uplift is not an exotic terrane but an original part of North America. The thin, dark limestones containing the trilobites were deposited in a deep-water, outer-shelf or continental-slope location, and they establish an approximate position for the Late Cambrian shelf edge along the southern margin of North America.

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.

Stiptognathus new genus (Conodonta: Ibexian, Lower Ordovician), and the apparatus of Stiptognathus borealis (Repetski, 1982)

Abstract Collections from upper Ibexian (Tulean Stage) rocks of western United States, from the Canning Basin in western Australia, and from the Argentine Precordillera contain a seximembrate apparatus of multidenticulate conodonts whose elements have been included by authors in species of Prioniodus Pander, 1986, and Reutterodus Serpagli, 1974. The individual elements as well as the complete apparatus are not consistent with assignment of the species to either of these genera or to any other extant genus. A new generic name, Stiptognathus Ethington, Lehnert, and Repetski, is proposed with Reutterodus borealis Repetski, 1982, as type species. The apparatus consists of Pa, Pb, Sa-c, and M elements; the genus represents either the Prioniodontidae or the Periodontidae.

Ordovician of the Sauk Megasequence in the Ozark Region of Northern Arkansas and Parts of Missouri and Adjacent States

Exposures of Ordovician rocks of the Sauk megasequence in Missouri and northern Arkansas comprise Ibexian and lower Whiterockian carbonates with interspersed sandstones. Subjacent Cambrian strata are exposed in Missouri but confined to the subsurface in Arkansas. The Sauk-Tippecanoe boundary in this region is at the base of the St. Peter Sandstone. Ulrich and associates divided the Arkansas section into formations early in the 20th century, principally based on sparse collections of fossil invertebrates. In contrast, the distribution of invertebrate faunas and modern studies of conodonts will be emphasized throughout this chapter. Early workers considered many of the stratigraphic units to be separated by unconformities, but modern analysis calls into question the unconformable nature of some of their boundaries. The physical similarity of the several dolomites and sandstones, complex facies relations, and lack of continuous exposures make identification of individual formations difficult in isolated outcrops. The oldest formation that crops out in the region is the Jefferson City Dolomite, which may be present in outcrops along incised river valleys near the Missouri-Arkansas border. Rare fossil gastropods, bivalves, brachiopods, conodonts, and trilobites permit correlation of the Cotter through Powell Dolomites with Ibexian strata elsewhere in Laurentia. Conodonts in the Black Rock Limestone Member of the Smithville Formation and the upper part of the Powell Dolomite confirm regional relationships that have been suggested for these units; those of the Black Rock Limestone Member are consistent with deposition under more open marine conditions than existed when older and younger units were forming. Brachiopods and conodonts from the overlying Everton Formation assist in interpreting complex facies within that formation and its correlation to equivalent rocks elsewhere. The youngest conodonts in the Everton Formation provide an age limit for the Sauk-Tippecanoe unconformity near the southern extremity of the great American carbonate bank. The correlation to coeval strata in the Ouachita Mountains of central Arkansas and in the Arbuckle Mountains of Oklahoma and to rocks penetrated in wells drilled in the Reelfoot rift basin has been improved greatly in recent years by integration of biostratigraphic data with lithologic information.

Stratigraphic Implications of Lower Ordovician Conodonts from the Munising and Au Train Formations at Pictured Rocks National Lakeshore, Upper Peninsula of Michigan

Abstract Early Ordovician conodonts have been recovered from the upper Munising Formation and the lower part of the Au Train Formation at Pictured Rock National Lakeshore in the Upper Peninsula of Michigan. Two trilobites from the lower Au Train probably represent the Symphysurina Zone (S. woosteri Subzone) and confirm the Early Ordovician age of the Au Train. The presence of middle Late Cambrian (Franconian) trilobites in the underlying Munising Formation led to traditional assumption of a major hiatus between the Munising and the Au Train. An unconformity between these formations has been difficult to recognize in outcrops because upper Munising sandstones grade upward into weathered, reworked beach sands deposited during Ordovician transgression. The reworked sands traditionally have been assigned to the Munising, with the base of the Au Train placed at the lowest occurrence of carbonate rocks in the sequence. Conodonts from the uppermost Munising and lower Au Train are characteristic of the Rossodus manitouensis Zone (upper Skullrockian Stage, Millardan Series, Lower Ordovician). The conodont and trilobite data permit correlation of the rocks of this interval with sequence stratigraphic units in the Burnout Canyon Member of the House Limestone in the Ibex area of Utah and with equivalent strata in other parts of Laurentia.