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 The postrifted margin of Laurentia in eastern Canada had a rugged paleomorphology, with major salients and recesses formed during the long-lasting (Ediacaran to late Early Cambrian) breakup of Rodinia. After short-lived carbonate production during the Early Cambrian, the great American carbonate bank (GACB) was firmly established in the earliest Middle Cambrian as the last rift-related event (Hawke Bay event, late Early Cambrian), and was followed by mostly passive thermal subsidence of the continental crust of Laurentia. Middle to Upper Cambrian carbonates are well preserved in the Port au Port Group in western Newfoundland (St. Lawrence promontory). Scattered outcrops of upper Middle to Upper Cambrian sedimentary rocks are found in southern and eastern Quebec (Quebec reentrant), although most of the preserved Upper Cambrian facies in the reentrant consist of nearshore to fluvial clastics unconformably overlying the Grenvillian basement. The Cambrian shallow-marine carbonates are dominated by high-energy facies with significant thrombolite reefs at the platform margin. The succession consists of large-scale transgressive-regressive cycles known as Cambrian grand cycles. Some anomalies in stacking patterns are suggestive of local tectonic events that were hypothesized based on the nature (facies and age) of carbonate clasts that accumulated on the continental slope. The Cambrian–Ordovician transition occurred at a time of a major sea level lowstand that resulted in a significant unconformity in southern Quebec and Ontario. In western Newfoundland, this sea level fall is recorded in the regressive facies of the last Cambrian grand cycle but did not culminate in subaerial exposure. The duration of the depositional hiatus at the Cambrian–Ordovician transition increases toward the west from an early Skullrockian gap in the Philipsburg thrust slice in southeastern Quebec; the hiatus covered the entire Skullrockian in eastern Ontario. A major sea level rise at or near the base of the Ordovician resulted in sedimentation on an extensive peritidal, mud-dominated, low-energy carbonate platform. This platform is known as the St. George Group (western Newfoundland), the Beekmantown Group (southwestern Quebec and Ontario), the School House Hill Group (southeastern Quebec), and the Romaine Formation (Anticosti Island). The carbonate facies are characterized by large- and small-scale depositional cycles. Two third-order cycles are well documented inwestern Newfoundland. The presence of such cycles is also proposed farther south, although their precise character still has to be documented. Multiple fifth-order meter-scale peritidal-dominated cycles have been documented in the Lower Ordovician carbonates. A diachronous change in depositional style occurred along the margin of Laurentia near the base of the Middle Ordovician. Facies patterns became controlled by faulting and accumulation rates increased significantly. These changes occurred first in the late Ibexian in southeastern Quebec and in the early Whiterockian elsewhere. At most localities, this transition is also expressed in a significant subaerial unconformity that is recognized along the entire eastern (paleosouthern) margin of Laurentia. This subaerial event is interpreted as resulting from lithosphere upwarping in front of the migrating Taconic orogenic wedge. The west-directed migration of the tectonic peripheral bulge resulted in the final destruction of the GACB as sedimentation resumed in a tectonically active foreland basin.

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.