James F. Miller

High-resolution sequence stratigraphy of lower Paleozoic sheet sandstones in central North America: The role of special conditions of cratonic interiors in development of stratal architecture

Well-known difficulties in applying sequence stratigraphic concepts to deposits that accumulated across slowly subsiding cratonic interior regions have limited our ability to interpret the history of continental-scale tectonism, oceanographic dynamics of epeiric seas, and eustasy. We used a multi-disciplinary approach to construct a high-resolution stratigraphic framework for lower Paleozoic strata in the cratonic interior of North America. Within this framework, these strata proved readily amenable to modern sequence stratigraphic techniques that were formulated based on successions along passive margins and in foreland basins, settings markedly different from the cratonic interior. Parasequences, parasequence stacking patterns, systems tracts, maximum flooding intervals, and sequence-bounding unconformities can be confidently recognized in the cratonic interior using mostly standard criteria for identification. The similarity of cratonic interior and foreland basin successions in size, geometry, constituent facies, and local stacking patterns of nearshore parasequences is especially striking. This similarity indicates that the fundamental processes that establish shoreface morphology and determine the stratal expression of retreat and progradation were likewise generally the same, despite marked differences in tectonism, physiography, and bathymetry between the two settings. Our results do not support the widespread perception that Paleozoic cratonic interior successions are so anomalous in stratal geometries, and constitute such a poor record of time, that they are poorly suited for modern sequence stratigraphic analyses. The particular arrangement of stratal elements in the cratonic interior succession we studied is no more anomalous or enigmatic than the variability in architecture that sets all sedimentary successions apart from one another. Thus, Paleozoic strata of the cratonic interior are most appropriately considered as a package that belongs in a continuum of variable stratigraphic packages reflecting variable controls such as subsidence and shelf physiography. Special conditions of exceptionally slow subsidence rate, shallow bathymetry, and nearly flat regional shelf gradient are manifest mostly by the presence of individual systems tracts of relatively long duration that extend for much greater distances across depositional strike than those that characterize successions deposited in more dynamic tectonic and physiographic settings. These results suggest that if other cratonic interior successions are as anomalous as reported, a low sediment supply may have played a primary role in development of their apparently condensed stratal architecture. The results also lead us to suggest that a nonvegetated lower Paleozoic landscape played a relatively insignificant role in the development of what are commonly perceived to be enigmatic stratigraphic features of sheet sandstones, particularly their widespread yet thin geometry, and a scarcity of shale and siltstone.

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.

LOWER ORDOVICIAN (TREMADOCIAN) LINGULATE BRACHIOPODS FROM THE HOUSE AND FILLMORE FORMATIONS, IBEX AREA, WESTERN UTAH, USA

Abstract Seven genera and eight species of lingulate brachiopods were recovered from the House Limestone and lower Fillmore Formation, Ibex area, Utah, USA. These strata are assigned to the upper Skullrockian Stage and lower Stairsian Stage of the Ibexian Series (Iapetognathus Conodont Zone to Low Diversity Interval) and are correlated with the Tremadocian Series of the Acado–Baltic Faunal Province. The fauna includes two new linguloid species, Spinilingula prisca and Wahwahlingula sevierensis, one new siphonotretoid species, Schizambon obtusus, and two new acrotretoid species, Eurytreta fillmorensis and Ottenbyella ibexiana. The last species is the first record of the genus in North America and suggests a correlation of the basal Fillmore Formation with the Ceratopyge Limestone in Sweden. A Siphonobolus? covered by long hollow spines may be one of the oldest siphonotretides with such ornament. This fauna and those described previously from older Utah strata document the biodiversification of the Cambrian–Ordovician lingulate brachiopods and demonstrate their potential for regional and intercontinental correlation.

LINGULATE BRACHIOPODS FROM THE CAMBRIAN-ORDOVICIAN BOUNDARY BEDS OF UTAH

Abstract Seven genera and eight species of lingulate brachiopods are described from the Cambrian-Ordovician boundary beds (Cambrooistodus minutus Conodont Subzone to Rossodus manitouensis Conodont Zone) at the Lawson Cove and Lava Dam North sections, Ibex area, Utah, USA. The fauna includes one new linguloid genus, Wahwahlingula, and four new species, Lingulella? incurvata, Zhanatella utahensis, Conotreta millardensis, and Quadrisonia? lavadamensis. Lingulate brachiopods from this interval are very poorly known from Laurentia, but the recorded fauna is very similar to that described from coeval beds at Malyi Karatau, Kazakhstan, and both areas contain Eurytreta cf. bisecta (Matthew, 1901); E. sublata Popov, 1988; ZhanatellaKoneva, 1986; SchizambonWalcott, 1889; and Wahwahlingula. Eurytreta cf. bisecta is also known from the Lower Ordovician of Avalonian Canada, Britain, and Scandinavia.

The Great American Carbonate Bank in the Miogeocline of Western Central Utah: Tectonic Influences on Sedimentation.

Cambrian and Ordovician strata in Millard and Juab counties, western central Utah, are a thick (17,500 ft [5334 m]) succession that was deposited on a tropical miogeoclinal platform that experienced rapid thermal subsidence after a Neoproterozoic sea-floor spreading ridge formed along the western margin of Laurentia. In this area, which includes the Cricket Mountains, Drum Mountains, Fish Springs Range, House Range, Confusion Range, and Wah Wah Mountains, the Cambrian to Middle Ordovician Sauk megasequence is approximately 15,875 ft (4839 m) thick, and the Upper Ordovician part of the Tippecanoe megasequence is approximately 1525 ft thick (465 m). Basal deposits of the Sauk megasequence are the transgressive Lower Cambrian Prospect Mountain Quartzite, and the top of the Sauk megasequence is the upper Whiterockian Watson Ranch Quartzite. Strata between these sandstones are mostly limestone with several shaly intervals. The Sauk megasequence is divided into four parts, Sauk I to IV, in this area, and these parts have been divided into smaller sequences. The Ordovician part of the Tippecanoe megasequence is mostly dolomite and quartzite. Major influences on the depositional history of these strata include rapid generation of accommodation space caused by thermal subsidence following continental rifting, in-situ generation of tropical carbonates that generally kept pace with accommodation, eustatic fluctuations, influx of siliciclastics during sea level lowstands, and vertical tectonic adjustments of regional tectonic elements inherited from Neoproterozoic rifting: the Wah Wah arch, House Range embayment, Tooele arch, and Ibex Basin. The resulting strata comprise one of the best known Middle Cambrian–Middle Ordovician stratigraphic successions in North America and include the reference sections of the Upper Cambrian Millardan Series and the Cambrian–Ordovician Ibexian Series. Stratigraphers established a Global boundary Stratotype Section and Point (GSSP) for the base of the Middle Cambrian Drumian Stage in the Drum Mountains and proposed another GSSP for the base of the uppermost Cambrian stage in the Wah Wah Mountains. Middle Cambrian–Middle Ordovician strata are very fossiliferous, and some intervals have incredibly abundant fossils, such as the numerous complete specimens of the Middle Cambrian trilobite Elrathia kingii in the central House Range. Trilobites, conodonts, brachiopods, and other fossil groups have been used for biozonation and correlation, and these strata comprise a North American standard for uppermost Cambrian–Middle Ordovician trilobite and conodont zonations. Upper Ordovician dolomites and quartzites are less fossiliferous. These Cambrian and Ordovician strata are the lower half of a Lower Cambrian–Lower Triassic succession that is approximately 34,000 ft (10,300 m) thick and was thrust onto the Jurassic Navajo Sandstone in the southern Wah Wah Mountains during the Sevier orogeny. These strata are exposed in block-faulted mountain ranges resulting from basin and range extension during the late Tertiary.

Jujuyaspis borealis and associated trilobites and conodonts from the Lower Ordovician of Texas and Utah

Jujuyaspis borealis is reported from earliest Ordovician (North American usage) limestones in central Texas and western Utah, the first time this species has been recognized in the United States. Jujuyaspis is a widespread olenid trilobite that occurs near the base of the Tremadoc Series in a variety oflithologies in North and South America, Europe, and Asia. When international agreement is reached on the exact horizon at or near the base of the Tremadoc Series that is to be used as the Cambrian-Ordovician boundary, Jujuyaspis will likely prove to be a very useful taxon for recognition of the boundary interval.

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.

First Report of a Larval Shell Repair Scar on a Lingulate Brachiopod: Evidence of Durophagous Predation in the Cambrian Pelagic Realm?

Abstract A dorsal valve of an Upper Cambrian lingulate brachiopod exhibits a repair scar on the anterior lateral edge of its larval shell. This species is characterized by an abrupt change in ornamentation from larval to postlarval growth. Shell material secreted in the injured area after the damage occurred exhibits ornamentation that is characteristic of postlarval growth, although equivalent growth exhibits characteristics of the larval stage. A break in the edge of the shell is visible, and the growth lines of the larval and postlarval shell were distorted until the broken area was filled in. Damage to the surface of the shell is interpreted to have been caused by the same event. Modern lingulate brachiopod larvae are planktotrophic and are interpreted to have been so throughout their long geologic history. Therefore, an environmental cause of shell damage seems unlikely and the injuries are interpreted to have been caused by an unknown durophagous predator. This specimen offers evidence that lingulate brachiopod larvae were able to survive shell breakage and repair their shells.

The Sauk Megasequence in the Cratonic Interior of North America: Interplay between a Fully Developed Inner Detrital Belt and the Central Great American Carbonate Bank

The Sauk megasequence in the far inboard region of the cratonic interior of North America (Minnesota, Wisconsin, and Iowa) is divisible into two packages that fundamentally differ from one another in facies and stratigraphic attributes. A lower Sauk succession package, Marjuman–early Skullrockian in age, is characterized by deposits of the traditional inner detrital belt (IDB) that interfinger hundreds of kilometers seaward with the middle carbonate belt or cratonward margin of the central mid-continent great American carbonate bank (GACB). The IDB contains a typical suite of nearshore siliciclastic facies containing features that document the importance of both wave- and tide-dominated currents in the depositional system. The transitional area between the IDB and the GACB in the Cambrian and earliest Ordovician was a moat, characterized by relatively deep-water deposition, which served as a catchment for mud that was winnowed from landward parts of the shelf and then deposited near the stormwave base. Mixed carbonate and siliciclastic facies in the moat are characterized by condensation features and other attributes indicative of suppressed carbonate productivity and starvation of siliciclastic sand. These facies contrast with shallower water facies that commonly filled available accommodation space in both seaward (central part of the GACB) and landward (cratonic shoreline) directions, the former dominated by typical stacks of oolitic, ribbon-rock, and microbialite lithofacies, and the latter by stacks of nearshore siliciclastic sand-dominated parasequences. Our chronostratigraphic framework provides temporal constraints that support the long-postulated hypothesis that these two depositional systems expanded and contracted in reciprocating fashion: substantial landward migration and expansion of the GACB occurred when siliciclastic input was diminished during the most rapid rates of transgression (marked by maximum flooding intervals in the IDB). Retreat and diminishment in the extent of the GACB corresponded to falls in sea level that led to major progradations of nearshore siliciclastics of the IDB and terrigenous poisoning of the carbonate factory. An overlying upper Sauk succession package records the establishment of a fundamentally different depositional system in the far inboard regions of the cratonic interior beginning in the later Skullrockian. The Prairie du Chien Group and its equivalents represent a major landward migration and perhaps cratonwide distribution of the oolitic, ribbon-rock, and microbialite lithofacies that were previously restricted mostly to the GACB of Missouri and adjacent areas. This change was triggered by a pronounced continental-scale flooding event that led to onlap across much, or all, of the cratonic interior. The resultant burial of terrigenous source regions by carbonate strata is in part responsible for this fundamental change in depositional conditions.