Paleogeographic Models Research Articles

Upper Cretaceous sediments in the Russian Plate: Sequence stratigraphy and Milankovitch cycles

The paper discusses integrated sequence-cyclostratigraphic analysis of Upper Cretaceous sediments based on 9 sections in the Russian Plate (the Voronezh Anteclise and the Ulyanovsk-Saratov Basin). Tracts (transgressive and highstand) were for the first time recognized in sections of this region and compared with member-by-member subdivision, bed rhythmites, and Milankovitch cycles indicating their orders. Certain rhythmites and the Milankovitch cycles, which produced them, were tied to the Cyclostratigraphic Scale of the Upper Cretaceous sediments. Data on paleogeographic sedimentation environments, such as sedimentation depth, temperature, sedimentation rate, etc. and paleocoenoses are presented and paleogeographic models of rhythmic structure formation were reconstructed.

Devonian paleomagnetism of the North Tien Shan: Implications for the middle-Late Paleozoic paleogeography of Eurasia

The Ural–Mongol belt (UMB), between Siberia, Baltica and Tarim, is widely recognized as the locus of Asia's main growth during the Paleozoic, but its evolution remains highly controversial, as illustrated by the disparate paleogeographic models published in the last decade. One of the largest tectonic units of the UMB is the Kokchetav–North Tien Shan Domain (KNTD) that stretches from Tarim in the south nearly to the West Siberian Basin. The KNTD comprises several Precambrian microcontinents and numerous remnants of Early Paleozoic island arcs, marginal basins and accretionary complexes. In Late Ordovician time, all these structures had amalgamated into a single contiguous domain. Its paleogeographic position is of crucial importance for elucidating the Paleozoic evolution of the UMB in general and of the Urals in particular. The Aral Formation, located in Kyrgyzstan in the southern part of the KNTD, consists of a thick Upper Devonian (Frasnian) basalt–andesite sequence. Paleomagnetic data show a dual-polarity characteristic component (Dec/Inc = 286° / + 56°, α 95 = 9°, k = 21, N = 15 sites). The primary origin of this magnetization is confirmed by a positive test on intraformational conglomerates. We combine this result with other Paleozoic data from the KNTD and show its latitudinal motion from the Late Ordovician to the end of the Paleozoic. The observed paleolatitudes are found to agree well with the values extrapolated from Baltica to a common reference point (42.5°N, 73°E) in our sampling area for the entire interval; hence coherent motion of the KNTD and Baltica is strongly indicated for most of the Paleozoic. This finding contradicts most published models of the UMB evolution, where the KNTD is separated from Baltica by a rather wide Ural Ocean containing one or more major plate boundaries. An exception is the model of Şengör and Natal'in [A.M.C. Şengör, B.A. Natal'in, Paleotectonics of Asia: fragments of a synthesis, in: A. Yin and M. Harrison (eds.), The tectonic evolution of Asia, Cambridge University Press, Cambridge (1996) 486–640], in which coherent paleolatitudinal motion of Baltica and the KNTD is hypothesized — the latter as part of the Kipchak Arc. We suggest a parallel hypothesis, which explains coherent motion of the KNTD and Baltica. In particular, we argue that if a basin with oceanic crust ever existed between the KNTD and Baltica, it was a narrow one without (significant) active spreading in Middle to Late Paleozoic time. Notably, the paleogeographic position of Siberia during the Middle Paleozoic and hence, the width of the Khanty–Mansi Ocean between Siberia, on the one hand, and Baltica–KNTD, on the other hand, remains largely unconstrained, because of the paucity of high-quality Silurian, Devonian and Carboniferous paleomagnetic results from Siberia.

Dinosaur Tracks as Paleogeographic Constraints: New Scenarios for the Cretaceous Geography of the Periadriatic Region

A really unexpected finding of sauropod and theropod footprints in southern Latium raises to four the number of the trampled levels recognized in central and southern Italy. After the recent findings in Latest Jurassic and Early, mid and Late Cretaceous carbonate platform deposits of the Periadriatic region, dinosaur footprints seem to provide very important paleogeographic constraints for reconstructing the geodynamic history of the Mediterranean area. The presence of a varied ichnoassociation makes acceptance of the current paleogeographic models concerning the relative and absolute position of the Laziale-Abruzzese-Campano and of Apulian-Dinaric domains during the Late Cretaceous more and more problematic. Dinosaur footprints, combined with other paleontological data, demonstrate that these areas were never completely pulled apart by deep seaways, while frequent or continuous links between them, and to southern and northern mainlands, probably persisted. These data also allowed us to improve our understanding of the timing of the Mesozoic plate motion in this segment of the Western Tethys.

Phylogeny and Biogeography of a Cosmopolitan Frog Radiation: Late Cretaceous Diversification Resulted in Continent-Scale Endemism in the Family Ranidae

Ranidae is a large anuran group with a nearly cosmopolitan distribution. We investigated the phylogenetic relationships and early biogeographic history of ranid frogs, using 104 representatives of all subfamilies and families, sampled from throughout their distribution. Analyses of approximately 1570 bp of nuclear gene fragments (Rag-1, rhod, Tyr) and approximately 2100 bp of the mitochondrial genome (12S rRNA, tRNAVAL, 16S rRNA) indicate that the monophyly of several taxa can be rejected with high confidence. Our tree is characterized by a clear historical association of each major clade with one Gondwanan plate. This prevalence of continent-scale endemism suggests that plate tectonics has played a major role in the distribution of ranid frogs. We performed dispersal-vicariance analyses, as well as analyses constrained by paleogeographic data, to estimate ancestral distributions during early ranid diversification. Additionally, we used molecular clock analyses to evaluate whether these scenarios fit the temporal framework of continental breakup. Our analyses suggest that a scenario in which the ancestors of several clades (Rhacophorinae, Dicroglossinae, Raninae) reached Eurasia via the Indian subcontinent, and the ancestor of Ceratobatrachinae entered via the Australia-New Guinea plate, best fits the paleogeographic models and requires the fewest number of dispersal/vicariance events. However, several alternatives, in which part of the ranid fauna colonized Laurasia from Africa, are not significantly worse. Most importantly, all hypotheses make clear predictions as to where to expect key fossils and where to sample other living ranids, and thus constitute a strong basis for further research.

ANÁLISE ESTRATIGRÁFICA APLICADA A PORÇÃO NORTE E NORDESTE DA BACIA BAURU (CRETÁCEO SUPERIOR)

The Bauru Basin, an Upper Cretaceous sedimentary basin in the southeastern South American Plate, has been studied since the end of 19h Century. In the north and northeast region the investigations were concentrated mainly near the city of Uberaba, due to the large fossil and limestone deposits economically explored there. Based on recent surface and subsurface data, an integrated stratigraphic analysis of the Bauru Group was developed in the Triângulo Mineiro region and extending to the neighboring areas in the northeast of Mato Grosso do Sul, south of Goias, and north of Sao Paulo states. This analysis allowed the construction of a paleogeographic evolution model, based on five depositional environments that composed an alluvial/lacustrine systems tract. The alluvial sedimentation entered the basin towards the lacustrine base level (Gurinhata, Limeira D'Oeste, Prata and western Sao Paulo region) from the northern/northeastern catchment area (Alto Paranaiba Uplift and Goias Alkaline Province). After Cretaceous sedimentation ended, the Triângulo Mineiro region was modified by Tertiary tectonic events, which resulted in the formation of various depressions, where remnants of the Bauru Group are preserved. Based on these data it was possible to make the paleogeographic model to the Bauru Basin and to establish the relationship with the Late Cretaceous South America paleogeography.

ANÁLISE ESTRATIGRÁFICA APLICADA A PORÇÃO NORTE E NORDESTE DA BACIA BAURU (CRETÁCEO SUPERIOR)

The Bauru Basin, an Upper Cretaceous sedimentary basin in the southeastern South American Plate, has been studied since the end of 19h Century. In the north and northeast region the investigations were concentrated mainly near the city of Uberaba, due to the large fossil and limestone deposits economically explored there. Based on recent surface and subsurface data, an integrated stratigraphic analysis of the Bauru Group was developed in the Triângulo Mineiro region and extending to the neighboring areas in the northeast of Mato Grosso do Sul, south of Goiás, and north of São Paulo states. This analysis allowed the construction of a paleogeographic evolution model, based on five depositional environments that composed an alluvial/lacustrine systems tract. The alluvial sedimentation entered the basin towards the lacustrine base level (Gurinhatã, Limeira D'Oeste, Prata and western Sao Paulo region) from the northern/northeastern catchment area (Alto Paranaiba Uplift and Goiás Alkaline Province). After Cretaceous sedimentation ended, the Triângulo Mineiro region was modified by Tertiary tectonic events, which resulted in the formation of various depressions, where remnants of the Bauru Group are preserved. Based on these data it was possible to make the paleogeographic model to the Bauru Basin and to establish the relationship with the Late Cretaceous South America paleogeography.

Heavy mineral characteristics of the Neogene–Quaternary sediments from the southern part of the Blagoevgrad Basin, SW Bulgaria

The Blagoevgrad Basin, a graben filled with Neogene–Quaternary continental sedimentary successions, is one of the most prospective placer-gold bearing regions in southwestern Bulgaria. The Quaternary sediments have been well explored and economically prospective areas have been identified. Previous studies indicated very limited occurrences of placer gold in the Neogene sediments, the genesis of which is connected with resedimentation of Paleogene sediments from the adjacent Bobov dol Basin to the north. From these previous studies, a detailed allo- and lithostratigraphic scheme, and paleogeographic models were developed. The present study aimed to evaluate the plausibility of the existing stratigraphic schemes and paleogeographic models for the area, through detailed study of the heavy mineral fraction of the Neogene and part of the Quaternary sediments from the southern part of the Blagoevgrad Basin. Additional objectives were to determine the mode of occurrence and the stratigraphic position of the placer gold in the Neogene sediments, and to investigate possible source areas. Special attention was paid to the mineral composition and the structure of the opaque detrital Fe–Ti oxide minerals, and the typologic features of the zircons. The results obtained could be used as a provenance indicator and for determination of the nature of the source area and the processes that operated therein. A very important finding is the discovery of placer gold in an additional three Neogene units. The morphologic features of the gold grains indicate short transport or proximity to the primary source. This interpretation is inconsistent with existing hypotheses on the origin and presence of the gold in the sediments of the basin. The characteristics of the magnetic heavy mineral fraction, and the morphologic features of zircon grains in auriferous samples, indicate the diorites and gabbro-diorites from the Strouma Diorite Group, which outcrop along the eastern edge of the graben, as a possible source. The stratigraphic distribution of the heavy minerals fully supports the plausibility of the existing detailed allo- and lithostratigraphic subdivision, and the paleogeographic model for the development of the southern part of the basin. Also, the results provide support for the soundness of an allostratigraphic approach for the study and exploration of placer deposits in basins similar to the Blagoevgrad.

A LIKELIHOOD FRAMEWORK FOR INFERRING THE EVOLUTION OF GEOGRAPHIC RANGE ON PHYLOGENETIC TREES

At a time when historical biogeography appears to be again expanding its scope after a period of focusing primarily on discerning area relationships using cladograms, new inference methods are needed to bring more kinds of data to bear on questions about the geographic history of lineages. Here we describe a likelihood framework for inferring the evolution of geographic range on phylogenies that models lineage dispersal and local extinction in a set of discrete areas as stochastic events in continuous time. Unlike existing methods for estimating ancestral areas, such as dispersal-vicariance analysis, this approach incorporates information on the timing of both lineage divergences and the availability of connections between areas (dispersal routes). Monte Carlo methods are used to estimate branch-specific transition probabilities for geographic ranges, enabling the likelihood of the data (observed species distributions) to be evaluated for a given phylogeny and parameterized paleogeographic model. We demonstrate how the method can be used to address two biogeographic questions: What were the ancestral geographic ranges on a phylogenetic tree? How were those ancestral ranges affected by speciation and inherited by the daughter lineages at cladogenesis events? For illustration we use hypothetical examples and an analysis of a Northern Hemisphere plant clade (Cercis), comparing and contrasting inferences to those obtained from dispersal-vicariance analysis. Although the particular model we implement is somewhat simplistic, the framework itself is flexible and could readily be modified to incorporate additional sources of information and also be extended to address other aspects of historical biogeography.

Paleomagnetism and tectonic significance of Albian and Cenomanian turbidites, Ochoco Basin, Mitchell Inlier, central Oregon

Understanding continental growth and convergent margin dynamics associated with terrane accretion and modification of the Cordilleran margin of North America is prevented by conflicts in paleogeographic models for major terranes, oceanic plates, and the North American margin. We present new paleomagnetic data suggesting that the Blue Mountains superterrane, located at an inboard portion of the Cordilleran margin, has undergone substantial northward translation and clockwise rotation relative to North America since mid‐Cretaceous time. Positive baked‐contact, conglomerate, and fold tests, provide evidence that the magnetization of these rocks dates from their deposition. These results yield a mean direction of D = 10.8°, I = 58.5°, k = 127, α95 = 4.1°, and N = 11. Comparison of magnetic inclinations and magnetic fabrics shows no evidence for inclination error. A paleolatitude of 39.2° +4.8°/−4.3°N is obtained, indicating a paleolatitude anomaly of 15.9° ± 4.1° (northward translation of 1760 ± 460 km) and CW rotation of 37° ± 7.3° with respect to North America since circa 93 Ma. Application of an ad hoc 5° inclination error yields translation of 1200 ± 460 km. Through comparison with other units in the western Cordillera we infer that the Mitchell Inlier was located adjacent to the Sierra Nevada arc during mid‐Cretaceous time and that the Blue Mountains province may correlate with the Intermontane superterrane in British Columbia. Other interpretations, such as large inclination errors or an independent displacement history for the Mitchell Inlier, are possible but unlikely. More work is needed to fully test these hypotheses and resolve major questions of Cordilleran tectonics.

Paleocanais na plataforma continental interna do Rio Grande do Sul: evidências de uma drenagem fluvial pretérita

Subsurface structures mapped by the Sparker® acoustic system on the inner continental shelf of Rio Grande do Sul State, in southern Brazil were interpreted as paleochannels which developed over a coastal plain environment during the upper Pleistocene and lower Holocene. Two major reflectors displaying expressive lateral continuity were identified. The first reflector, which is shallower, is buried at depths ranging from 10 to 12m and its surface is relatively homogeneous displaying few irregularities associated that channeled structures. The second reflector mapped on depths ranging from 14 a 32m is more discontinuous and irregular and display erosive structures. The overall interpretation of the shallow seismic reflection profiles indicates that this preterit drainage display an N/NE-SE/SW orientation increasing its width and decreasing its depth towards offshore. The presence of paleochannels in the inner continental shelf of RS state supports the paleogeographic models of the area proposed by early studies.

A LIKELIHOOD FRAMEWORK FOR INFERRING THE EVOLUTION OF GEOGRAPHIC RANGE ON PHYLOGENETIC TREES

At a time when historical biogeography appears to be again expanding its scope after a period of focusing primarily on discerning area relationships using cladograms, new inference methods are needed to bring more kinds of data to bear on questions about the geographic history of lineages. Here we describe a likelihood framework for inferring the evolution of geographic range on phylogenies that models lineage dispersal and local extinction in a set of discrete areas as stochastic events in continuous time. Unlike existing methods for estimating ancestral areas, such as dispersal-vicariance analysis, this approach incorporates information on the timing of both lineage divergences and the availability of connections between areas (dispersal routes). Monte Carlo methods are used to estimate branch-specific transition probabilities for geographic ranges, enabling the likelihood of the data (observed species distributions) to be evaluated for a given phylogeny and parameterized paleogeographic model. We demonstrate how the method can be used to address two biogeographic questions: What were the ancestral geographic ranges on a phylogenetic tree? How were those ancestral ranges affected by speciation and inherited by the daughter lineages at cladogenesis events? For illustration we use hypothetical examples and an analysis of a Northern Hemisphere plant clade (Cercis), comparing and contrasting inferences to those obtained from dispersal-vicariance analysis. Although the particular model we implement is somewhat simplistic, the framework itself is flexible and could readily be modified to incorporate additional sources of information and also be extended to address other aspects of historical biogeography.

The Allochthony of the Argentine Precordillera Ten Years Later (1993–2003): A New Paleobiogeographic Test of the Microcontinental Model

Recent and new faunal data from the Cambrian to Silurian rocks of the Precordillera, Famatina and Northwest Argentina basins are used to discriminate between different paleogeographic models, and especially to establish to what extent they are compatible with a previous conclusion that the Precordillera is a Laurentian-derived microcontinent. There is no paleontological evidence to support a para-autochthonous Gondwanan origin of the Precordillera. The strong differences in the Cambrian trilobite faunas and lithologic successions preclude a common origin of the Precordillera terrane, eastern Antarctica and South Africa. Recent discoveries of brachiopods and organisms of the Phylum Agmata strengthened Laurentian affinities during the Cambrian. The latest Cambrian-early Ordovician faunas that inhabited the autochthonous Northwest Argentina basin, including the western Puna volcaniclastic successions, are mostly peri-Gondwanan. The early Ordovician brachiopods, ostracods and trilobites display mixed Laurentian, Baltic and Avalonian biogeographical links supporting a drifting of the Precordillera across the Iapetus Ocean. Increasing Gondwanan elements during the Llanvirn, along with varied geological evidence, indicate that the first stages of collision may have begun at that time, involving a major change in the plate kinematics. The distribution of facies and faunas, basin development, and timing of deformation are interpreted as resulting from a north to south diachronous closing of the remnant basin during the last phases of convergence and oblique collision of the Precordillera terrane with the Gondwana margin. The high level of endemism of Caradoc faunas may be a consequence of the rearrangement and partial isolation of sedimentary areas during the strike-slip movement of the colliding Precordillera plate with respect to the Gondwana margin. Suggested relationships between facies distribution, geographic barriers and faunal migrations before and during the collision are depicted in a series of schematic reconstructions at five time slices from late Cambrian to Silurian.

Facies succession, stratigraphic occurrence, and paleogeographic context of conglomeratic shorelines within the Falher "C", Spirit River Formation, Deep Basin, west-central Alberta

The Falher “C” is a Lower Cretaceous transgressive-regressive sequence that contains mixed coarse-grained sandstone and conglomerate gas reservoirs in the Deep Basin of west-central Alberta. A total of 11 facies were identified in cored intervals of the Falher “C” and are further grouped into five facies associations representative of deposition in an offshore-marine through continental spectrum. Coarse-grained sandstone and conglomerate facies of high reservoir potential comprise facies association two (FA2), and were deposited within wave-dominated upper shoreface and foreshore settings. Finer grained non-reservoir facies (mudstone, siltstone, and fine-grained sandstone) were deposited within storm-dominated lower shoreface (FA1), tidal inlet (FA3), back-barrier (FA4), and coastal plain (FA5) environments. Several significant stratigraphic surfaces have been utilized to further sub-divide the Falher “C” into three parasequences termed C1, C2, and C3. In ascending order of stratigraphic occurrence these surfaces are defined as i) TSE1, ii) RSE, and iii) TSE2. The lower and upper contacts of conglomerate-rich parasequence C2 correspond to a regressive surface of marine erosion (RSE), and the youngest transgressive surface of marine erosion (TSE2). From correlations made between cored intervals and calibrated well logs, it can be recognized that reservoir-quality conglomerates are stratigraphically restricted to Twp. 67. A paleogeographic model constructed for the Falher “C” demonstrates the distribution of important environments of deposition at four consecutive depositional intervals corresponding to distinct changes in sea level and sediment supply to the Falher shoreline. Initially, the shoreline underwent transgression in response to a relative rise of sea level whereupon parasequence C1 began to prograde northward. Transgression and progradation was followed by a relative fall of sea level and increased sediment supply that brought about a forced regression and deposition of the reservoir-bearing conglomeratic parasequence C2. Progradation of parasequence C3 was established shortly after a second period of transgression across the study area. Continued normal regression in response to progradation of parasequence C3 northward of the study area maintained aggradation of coastal-plain and deposition of the capping nonmarine C4 unit. The results of this study serve to enhance our present understanding of the scale, distribution, and three-dimensional predictability of conglomerate gas reservoirs in the Deep Basin of Alberta.

Lower Ordovician sponge bioherms in the Makkol Formation, Taebaeksan Basin, mideast Korea

Isolated sponge bioherms are documented from the Lower Ordovician Makkol Formation of the Taebaek Group in the Taebaeksan Basin, mideast Korea. They are formed by an association of a lithistid spongeArchaeoscyphia, a receptaculidCalathium and stromatolitic algae, and share many features with the Lower Ordovician buildups known elsewhere. These bioherms were established in an incised bottom and reached up to about 1 m in height. As the bioherms grew upward, they were more severely affected by intense wave action and frequent storms, which eventually perished the bioherms. The occurrence ofArchaeoscyphia-Calathium association suggests a close biogeographic link between Korea and North China, supporting the paleogeographic model that the Taebaeksan Basin was connected through contiguous shallow waters to North China in the early Paleozoic.

Paleontological evidence of the Campanian to Early Paleocene paleogeography of Baja California

Fossil assemblages of Campanian to Maastrichtian dinoflagellates, invertebrates, mammals, reptilians, and planktonic foraminifera in Baja California are mainly temperate, similar to those found in the North Atlantic, Rocky Mountains and North Pacific regions. Early Paleocene assemblages are mostly subtropical ones. Danian dinoflagellates suggest deposition in a temperate to subtropical province, and benthonic foraminiferal assemblages (Midway type) are common in a region from the Gulf of Mexico to the northeastern coast of the U.S.A. The molluscan assemblages recovered from early Paleocene rocks indicate deposition mostly in subtropical environments. Although these invertebrate assemblages are related to the Caribbean province, dominant mollusks and solitary corals indicate subtropical to warm temperate paleoenvironments. Considering that the dinoflagellate, invertebrate, vertebrate and planktonic foraminiferal assemblages indicate temperate environments, together with paleogeographic models based on plate tectonics, we conclude that at least since the Campanian, Baja California was attached to western Mexico. The peninsula did not move much more than the 300 km needed to close the Gulf of California.

Neodymium isotopic composition of Ordovician conodonts as a seawater proxy: Testing paleogeography

To evaluate recent Ordovician paleogeographic reconstructions, εNd values were determined in conodonts as a proxy for the isotopic variations in ancient seawater using samples from the major cratons and microplates. Isotopic variations reveal the existence of distinct oceanic masses and epeiric seas that constrain the position of plates and terranes relative to these water masses and hence constrain models of regional and global paleogeography. The isotopic patterns show a consistent picture of the changes in the Ordovician oceans. The εNd values for water masses associated with Laurentia are strongly negative (−28 to −18) in the Early Ordovician, evolving over time to higher values in the range −13 to −5. The Early Ordovician signature of Laurentia is in marked contrast to other cratons and microplates, which have a range of values from −10 to −5. Samples from South China show interesting signals that reflect a greater similarity to Laurentia than other peri‐Gondwana terranes. The isotopic variations are a function of both regional geology and global tectonic processes, the most obvious being the Taconic Orogeny and onset of the closure of the Iapetus Ocean. Regional and global models of paleogeography are considered in light of these proxy signals. This study also reveals that conodonts are powerful geochemical tools for obtaining information on ancient water masses. The use of the Nd isotopic signatures from conodonts provides a method independent of paleomagnetism and biogeography to test paleogeographic models. By integrating such information, a more multidisciplinary approach is possible.

From Guaratuba to Babitonga: a geologicevolutionary contribution to the study of the spatial distribution of the shell mound builders on the northern coast of Santa Catarina.

O trabalho apresenta uma abordagem geológico-evolutiva dos sambaquis da planície costeira de Joinville cujos resultados indicam uma possível rota para deslocamento de sambaquianos entre o litoral sul paranaense e o litoral norte catarinense. O modelo paleogeográfico proposto e as datações absolutas de sambaquis atualmente disponíveis convergem para o canal do Palmital como região inicial de ocupação de Joinville por aquelas populações de pescadores-coletores.

Reconstruction of the geodynamic evolution of the Northern Calcareous Alps by means of paleomagnetism

We review the paleomagnetic studies published on the Northern Calcareous Alps (NCA) and also include data from two theses that were not published yet. Moreover, we present new data from upper Santonian to Maastrichtian Gosau deposits from the Neue Welt Area in the easternmost part of the NCA (D = 325.8. I = 37.1, α95 = 8.3, positive fold test, thus indicating counterclockwise rotation and inclination flattening). Although several of the recently published studies provide valuable data, presently the paleomagnetic data base of the NCA does not provide a sufficient frame work for a quantitatively backed paleogeographic model. By thus documenting the state of research, we want to outline major problems regarding the paleomagnetic characterization of the NCA and also specify the kind of paleomagnetic future work that is particularly needed within this mountain range.

Estuarine Deposits of the Upper Qishn Formation (Lower Cretaceous), Masila Region, Yemen

In the Masila region of Yemen, several hundred million barrels of recoverable oil occur within three Lower Cretaceous siliciclastic lithostratigraphic units of the upper Qishn Formation. The basal unit, S3, locally exceeds over 40 m in thickness. This unit overlies an erosive surface and is formed of blanket deposits of cross-bedded, fining-upward, pebbly sandstones with siliceous and dolomitic paleosols. The S3 unit reflects deposition from coarse-grained fluvial channels and related environments. The middle unit, S2, is composed of complexly interbedded sandstones and mudstones up to over 25 m thick. These deposits grade upward from S3 sediments and display cross-bedded pebbly sandstones, cross-bedded sandstones with mud drapes, and inclined heterolithic units formed of interbedded sandstones and mudstones. The S2 unit is capped by widespread churned and rooted mudstones 3-5 m thick. S2 deposition resulted from the rapid infilling of an estuary, or estuaries, created by the transgression of the fluvial systems responsible for S3 sedimentation. The uppermost unit, S1, at its base is formed of an eastern shelfal package with Helminthopsis- and Chondrites-bearing mudrocks, and western beds formed of sediments generally similar in appearance to the middle S2 unit. Above the lowest S1 deposits are marine muddy coquinas that blanket the study area. In the west, these sediments are abruptly to gradationally overlain by Ophiomorpha-bearing, fine-grained, cross-bedded shelly sandstones up to about 10 m thick. These sandstones form a north-south linear trend and are interpreted to represent a progradational pulse of subtidal shoal sediments that interrupted the general deepening trend exhibited by the upper Qishn interval. Final upper Qishn deposition was younger shelfal muddy coquinas that overlie the shoal deposits. These and younger carbonate sediments act as the uppermost seal to reservoirs in the Masila region. The upper Qishn sediments are compared to modern and Pleistocene estuarine deposits. We propose stratigraphic, depositional, and paleogeographic models for this formation and, from this analysis, we derive several implications for future petroleum operations in the region.

40Ar/39Ar laser probe dating of detrital white micas from Cretaceous sedimentary rocks of the Eastern Alps: Evidence for Variscan high-pressure metamorphism and implications for Alpine orogeny: Comment and Reply

The detritus of Cretaceous synorogenic sandstones of the northern margin of the Austroalpine microplate contains evidence for a high-pressure metamorphic basement and obducted oceanic crust exposed in early Alpine time. 40 Ar/ 39 Ar laser-probe data of detrital white micas give excellent plateau ages in a narrow range from 320 to 360 Ma. White micas cover the whole range from muscovites up to phengites (3.04 to 3.48 Si per formula unit). Heavy mineral spectra contain chrome spinel, glaucophane, chloritoid, epidote, and garnet, as well as zircon, tourmaline, and rutile. Glaucophane, chloritoid, and phengite correlate in their abundance. These minerals also correlate positively with the stable minerals zircon, tourmaline, and rutile derived from continental rock and negatively with chrome spinel, which represents oceanic crustal provenance. We propose that both glaucophane and phengite come from the same source rock, Variscan high-pressure metamorphic rocks of the Austroalpine basement. The Early Carboniferous age gives constraints for paleogeographic models concerning the collision between Gondwana and Laurussia. Furthermore, detrital glaucophane in the early Alpine sedimentary rocks cannot be used as proof for Cretaceous subduction at the Austroalpine-Penninic plate boundary.