Abstract Decimeter-scale sampling of the Cambrian and the lowermost Ordovician (Sauk megasequence) rocks of the Llano uplift, Texas, has produced a finely resolved biostratigraphic framework based primarily on trilobites and conodonts. Systematically collected trilobites of the Llano Uplift allow recognition of 13 biozones that extend from the Bolaspidella Biozone (Cambrian System, Marjuman Stage) through the Symphysurina Zone (Ordovician System, upper Skullrockian Stage). Systematic collection of conodonts has produced specimens assignable to 13 zones that range from the Proconodontus tenuiserratus Zone (Cambrian System, Sunwaptan Stage) through the Rossodus manitouensis Zone (Ordovician System, upper Skullrockian Stage). The base of the Ordovician System in the Llano uplift, as elsewhere, has been identified by the lowest occurrence of the conodont Iapetognathus fluctivagus and is closely approximated by the lowest occurrences of the cosmopolitan trilobite Juyjuyaspis and the Laurentian trilobite Symphysurina “bulbosa.” Although the overlying Ordovician strata of the Ellenburger Group have not been systematically sampled, scattered trilobite collections do establish the approximate positions of the base of the Stairsian Stage (based on Paraplethopeltis ) and the base of the Jeffersonian Stage (based on Rananasus and Jeffersonia ) in the Tanyard and Honeycut Formations, respectively.

Abstract 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 (∼465m). 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. Stratigra-phers 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 Collenia 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.

Abstract More than 15,000 ft (>4572 m) of Sauk megasequence strata are exposed in mountain ranges across the eastern Great Basin of western Utah (Figure 1 ; see Miller et al., 2012 ). The upper parts, Sauk II to Sauk IV, are particularly well exposed in the western desert of Utah, in and around the Ibex area (Figure 2 ). Division of the Sauk megasequence into four parts follows Miller et al. (2012) . This area was also the location of several wildcat wells that were drilled in the late 1970s and early 1980s (Figure 1 ). From 1991 to 2003, Evans et al. have collected gamma-ray profiles from measured stratigraphic sections in the Confusion, Fish Springs, and House Ranges, as well as the Wah Wah Mountains and sections in eastern Nevada. Profiles from the Ibex area can be correlated confidently with some subsurface gamma-ray logs. As a consequence, key stratigraphic markers exposed in measured sections can be identified in well logs, and conversely, well logs can be used to help delineate the distribution of facies and large-scale structures.

Abstract 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 micro-bialite 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 de-positional conditions.

Abstract Middle Cambrian–Lower Ordovician (Sauk megasequence) rocks of southern Missouri have attracted interest because they host the large Mississippi Valley-type ore deposits of that region. A Proterozoic igneous and metamorphic complex nonconformably underlies this megasequence and Mississippian and Pennsylvanian strata unconformably overlay these units. Strata of the Sauk megasequence in southern Missouri include, in ascending order, the Cambrian Lamotte Sandstone, Bonneterre Dolomite, Davis Formation, Derby-Doerun Dolomite, Potosi Dolomite, and Eminence Dolomite, which are overlain by the Ordovician Gasconade Dolomite, Roubidoux Formation, Jefferson City Dolomite, and Cotter Dolomite. Cambrian depositional facies in southern Missouri are small- to large-scale, unconformity-bounded, transgressive-regressive sequences, and are characterized by distinct facies in linear belts that developed on and adjacent to basement highs. These include intrashelf-basin facies distal from basement highs, platform-edge facies in narrow belts adjacent to highs, and back-reef facies proximal to and within the St. Francois Mountains. A distinct rift-graben facies is characteristic of deposition of these units in the Reelfoot rift. An epeiric sea that extended over the region controlled Lower Ordovician deposition. Lower Ordovician strata are com-posed of five third-order sequences, punctuatedbyregional and sub regional unconformities. The strata are characterized by facies that have regional lateral continuity.

Abstract The Sauk Sequence comprises more than 5 km of mixed carbonate and siliciclastic strata on the Paleozoic miogeocline of the eastern Great Basin. Rapid, post-rifting subsidence was the single most important factor for providing accommodation for accumulation of sediments. Despite the enormous thickness of strata and the tendency for unconformities to die out toward the margin of the continent, bounding surfaces of the Sauk Sequence and several sequence boundaries within this interval are preserved in mountain ranges of western Utah. The base and top of the Sauk Sequence are thick sandstones. The development of microkarst or truncation surfaces associated with major facies disclocations and deposition of major influxes of siliciclastics are the hallmarks of sequence boundaries and correlative conformities in this setting. The style of sequence boundary development was mostly a function of magnitude and duration of sea-level fall but was also influenced by tectonic features such as the House Range Embayment.

Abstract The numerous extinctions that affected shallow marine faunas on the tropical shelves surrounding Laurentia in the Cambrian and Early Ordovician have been the focus of many detailed biostratigraphic, evolutionary, and paleoecologic studies. Data from carbonate platform and off-platform strata have been used to propose process-response models that invoke sea level change as a forcing mechanism for extinctions and/or radiations within the Cambrian and Early Ordovician. Some regressive features observed near horizons of faunal change within the Cambrian-Ordovician boundary interval on various continents have been used to propose a series of "eustatic events" (Nicholl et al., 1992). These include the "Lange Ranch Eustatic Event" and "Black Mountain Eustatic Event" of Miller (1984, 1992) and the Acerocare Regressive Event and Peltocare Regressive Event of Erdtmann (1986). There is much debate about the nature of these proposed events (Ludvigsen et al. 1986; Taylor et al. 1992; Landing 1993) based, at least in part, on the ambiguous nature of the sedimentological data and insufficient precision of correlation.

Conodonts are divided into three groups with different histologies: protoconodonts (most primitive), paraconodonts, and euconodonts (most advanced). The first is poorly known, but paraconodonts included a Westergaardodina and a coniform evolutionary lineage, and each was the ancestor of one or more euconodont lineages. Early euconodonts are thus polyphyletic and included the Proconodontus and Tendonitis Lineages, which appeared in the middle Late Cambrian, and the Fryxellodontus and Chosonodina Lineages, which appeared in the Early Ordovician. Major changes in conodont evolution, biofacies adaptation, and development of provincialism coincided with sea-level fluctuations near the end of the Cambrian (here named the Lange Ranch Eustatic Event, or LREE) and similar fluctuations recorded at the Lower/Upper Tremadoc boundary (here named the Black Mountain Eustatic Event, or BMEE). Protoconodonts and paraconodonts were probably pelagic and cosmopolitan. Genera of the Proconodontus Lineage were probably also pelagic. Some genera of the latter lineage are found only in low- to mid-paleolatitude areas; others were cosmopolitan, including Cordylodus . Genera of the Teridontus and Fryxellodontus Lineages may have been nektobenthic. Some were adapted to warm, high-salinity environments that existed during the LREE, but younger genera probably were adapted to normal salinity and were more widely distributed. No apparent provincialism existed until the appearance of euconodonts, after which two broad faunal realms are distinguishable. The warm faunal realm included shallow seas in low to middle paleolatitudes; the cold faunal realm included high-paleolatitude seas and open-ocean areas. Early euconodonts of the Proconodontus Lineage appeared and quickly became dominant in the warm faunal realm during the latest Cambrian. Much of the preexisting protocondont-paraconodont fauna was displaced from the warm faunal realm but continued to dominate the cold faunal realm through the Early Tremadoc. Major faunal changes occurred in the warm faunal realm as a result of the LREE, and after this event conodonts in this ream consisted for the most part of genera from the Teridontus Lineage. During the BMEE a different euconodont fauna of uncertain ancestry became adapted to the cold faunal realm, after which most of the previously dominant primitive fauna became extinct. Cosmopolitan Cordylodus lived in both faunal realms during much of the Tremadoc, but after it became extinct prior to the Arenig, provincialism was extreme because few species were adapted to both faunal realms. Oneotodus tenuis Müller is reclassified as the type species of a new genus, Phakelodus.