Abstract “Precambrian rocks reach the surface in the Llano region of Central Texas in the highest part of a broad domal arch, the Llano uplift, which appears on a regional geologic map as an island of igneous and metamorphic rocks surrounded by Paleozoic and Cretaceus sedimentary rocks. The widest expanse of Precambrian rocks is about 65 mi ( 105 km), extending westward from the valley of the Colorado River through a subdued topographic basin drained by the Llano River. The broad, gentle basin carved into the Precambrian rocks is bordered by a discontinuous rim of flat-topped limestone hills which are the dissected edge of the Edwards Plateau. Within the basin and at its margins are erosional remnants and down-faulted blocks of Paleozoic rocks which form prominent hills, locally referred to as mountains.” Clabaugh and McGehee (1972, p. 9). The main Precambrian units present in the Llano region are listed in Table 1, and the distribution of some of these units is shown in Figure 1. This figure is derived from the Llano Sheet of the Geologic Atlas of Texas, scale 1:250,000 (Barnes, 1981). On that map (as well as Fig. 1), the Oatman Creek and Sixmile Granites (Table 1) are lumped as younger granitic intrusive rocky locally, in the southeast part of the Llano Sheet, the Packsaddle Schist is subdivided into four formations (McGehee, 1979). Detailed 7½-minute quadrangle maps include most of the area in which the Packsaddle Schist has been subdivided (Barnes and McGehee, 1976; 1977a, b, c).

ABSTRACT This study provides new stratigraphic data and identifications for fossil marine mammals from the Monterey Formation in the Capistrano syncline, Orange County, California, showing that there are two distinct marine mammal assemblages. Until now, marine mammals from the Monterey Formation of Orange County have been considered to represent a single assemblage that is 13.0–10.0 Ma in age. By combining data from diatoms with the geographic positions of sites, faunal analysis, and data from the literature, we can assign 59 sites to three main levels: the lower part (ca. 16–13 Ma), the middle part (ca. 13–10 Ma), and the upper part (ca. 10–8 Ma). We assigned 308 marine mammal specimens to 38 taxa, resulting in 97 occurrences (unique record of a taxon for a given site). Of the 38 taxa we identified within the study area, 15 taxa are restricted to the lower part of the Monterey Formation, 15 are restricted to the upper part of the Monterey Formation, eight were found in both, and none has yet been reported from the middle (possibly condensed) section. Six of the eight taxa that occur in both the lower and upper parts of the Monterey Formation are higher-level taxa, which accounts for their broad temporal range. The recognition of two distinct marine mammal assemblages in the Monterey Formation of Orange County is an important step toward a better-calibrated sequence of faunal evolution in the region while improving the utility of marine mammals for regional biostratigraphy.

ABSTRACT North Yukon lies at the intersection of two major tectonic domains that define the western and northern edges of the North American continent—the northern Cordilleran mountain belt and the Arctic Ocean. The pre-Carboniferous geology in North Yukon includes the Neoproterozoic−lower Paleozoic North Slope subterrane of the Arctic Alaska terrane and, south of the Porcupine shear zone, Mesoproterozoic−Paleozoic rocks of the Yukon stable block. The North Slope subterrane was deformed prior to deposition of Carboniferous and younger strata, and its paleogeographic origins are debated. North Yukon was deformed again during Cretaceous−Cenozoic development of the northern Cordilleran−Brookian orogen. To help refine understanding of the geological evolution of the region, we present detrital zircon U-Pb and Hf isotopic data for 21 sandstone and conglomerate samples from Neoproterozoic to Cenozoic strata collected across North Yukon, between ~69°15′N and 67°11′N. Neoproterozoic−Cambrian strata in the British Mountains are characterized by a dominance of Paleoproterozoic zircons (peak at 1.7–1.8 Ga), whereas samples from the Barn Mountains to the south have abundant Mesoproterozoic grains (1.0–1.5 Ga), suggesting these rocks may have been deposited along different segments of the northern Laurentian margin. Regional geophysics suggest these domains could be separated by a fault. Northeastern Laurentian origins are indicated by distinct early Neoproterozoic and Ordovician−Silurian zircons in Upper Silurian−Lower Devonian immature sandstone and conglomerate of the Clarence River Group and provide the most compelling evidence for large-scale translation along northern Laurentia. Precambrian detrital zircons in Carboniferous and younger strata reflect mostly recycling of local older strata. Carboniferous conglomerates all show Late Devonian peaks (365–378 Ma) consistent with erosion of nearby granitoid plutons. Triassic to Paleocene samples yielded a range of Neoproterozoic−Paleozoic zircons recycled from nearby Devonian flysch. Most significantly, these samples also yielded juvenile zircons that are close to depositional age, but for which arc sources are only known in southern Yukon and Alaska, more than 700 km away. These source regions are distinct from NE Russian sources inferred for early Brookian (Early Cretaceous) foreland deposits in Alaska.

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 To the east of the Welsh Borderland the Ordovician is exposed only in a few scattered outcrops, but it has a wide subsurface distribution, being known from many boreholes. The outcrops all appear along the Malvern lineament and the structural features at the edges of the Warwickshire and South Staffordshire coalfields, and, apart from the Lickey Hills inlier, are all known to be of Tremadoc age. Although all the Tremadoc rocks show general similarity to the Shineton Shale Formation, they have been described under several local names, as shown in Fig. 16. The succession in the Tortworth Inlier (Column 38) is from Curtis (1968), the age of the Micklewood Beds being revised by Fortey & Owens (1992). The Bronsil Shales of the Malvern Hills (Column 39) have yielded Cressagian fossils including Rhabdinopora fabelliformis socialis and the trilobite Boeckaspis mobergi (Worssam et al. 1989, p.6). The Lickey Quartzite SW of Birmingham (Column 40) was formerly attributed (with doubt) to the Cambrian and the Barnt Green Volcanic Formation to the Precambrian (Cowie et al . 1972), but Molyneux ( in Old et al . 1991, p.4) showed that these divisions are post-Cambrian and probably Ordovician in age. The Merevale Shale Formation of the Nuneaton and Dosthill Inliers (Column 41) overlie the Merioneth Series conformably and have yielded Rhabdinopora fabelliformis socialis from a horizon some 90 m above its base (Taylor & Rushton 1972, p.35), but the Merioneth-Tremadoc boundary is not closely constrained in the area (Bridge et al . 1998, p. 36).