Outcrop Belt Research Articles

Stratigraphic uncertainty in sparse versus rich data sets in a fluvial-deltaic outcrop analog: Ferron Notom delta in the Henry Mountains region, southern Utah

The Turonian Notom delta is one of the Ferron fluvial-deltaic wedges deposited in the Cretaceous foreland basin of North America and is well exposed in the Henry Mountains region, Utah. This article uses variable sampling of measured sections of the fluvial-deltaic wedge along a 19-mi (30-km) outcrop belt to address correlation uncertainties in sparse data sets, such as are common in subsurface studies. A high-resolution sequence-stratigraphic analysis based on 58 measured sections, extensive walking out of beds, and field photomosaics shows that the wedge is composed of 43 parasequences, 18 parasequence sets, and six depositional sequences. Resampling the outcrop belt with 13 sections provides a sparse data set. Correlations using the data set show considerable uncertainties. Fluvial complexity is greatly underestimated, and in the extreme case, only 40% of the fluvial bodies are identified. Overcorrelation results in as much as 400% exaggeration of sandstone width and thickness. Some of the uncertainties can be reduced by incorporating nonmarine sequence-stratigraphic models and various techniques that allow a more reasonable evaluation of fluvial body geometry. With sparse data, uncertainties also arise in marine successions. A key challenge is to identify and correlate the small-scale units, particularly forced regressive deposits. Many of their diagnostic features that can be easily recognized from outcrops and cores may not be as easily detected from well logs and on seismic data. In subsurface studies where sparse data are observed, integrating detailed facies analysis cores and sequence-stratigraphic concepts is critical to predict the occurrence and understand the architecture of such deposits.

On the last mosasaurs: Late Maastrichtian mosasaurs and the Cretaceous-Paleogene boundary in New Jersey

Abstract New data regarding the placement of the Cretaceous-Paleogene (K/Pg) boundary in New Jersey is presented based on a recent campaign of drill coring boundary sections in the Atlantic coastal plain of southern New Jersey. The occurrence of late Maastrichtian mosasaurs worldwide and in New Jersey is reviewed in light of the continuing controversy over the rate and cause of the extinction at the K/Pg boundary. At the Meirs Farm site in Monmouth County, NJ, the biostratigraphic position of mosasaur specimens (Halisaurus platyspondylus, Mosasaurus hoffmann) is related to the occurrence of an iridium excursion of 0.5 ppb (5x background levels) in the basal Hornerstown Formation just above the upper Maastrichtian New Egypt Formation. Other specimens in museum collections obtained during the acme of nineteenth century marl mining are from this area of the Maastrichtian outcrop belt in New Jersey. It is concluded that late Maastrichtian mosasaurs show no diminution of their biogeographic ranges and are not particularly rare in New Jersey in comparison to older mosasaur faunas. Mosasaurs became extinct in association with the collapse of the marine food web at the K/Pg boundary, and were replaced as apical marine predators in the early Danian by a variety of crocodilians.

Slope channel sedimentary processes and stratigraphic stacking, Cretaceous Tres Pasos Formation slope system, Chilean Patagonia

The Cretaceous Tres Pasos Formation of southern Chile records a slope system characterized by >800 m of paleo-bathymetric relief. Channel deposits are exposed in an outcrop 2.5 km long by 125 m thick and are located in proximity to the toe of a slope clinoform. Exquisite exposures of channel strata offer a unique opportunity for high-resolution analyses of channel stacking patterns and provide insight into the evolution of conduits that transport sediment from continents to the deep ocean.Eighteen slope channels, or channel elements, are present in the strata studied. They are 6–15 m thick and comprised of stacked turbiditic sedimentation units. Channel fills are characterized by a gradational transition from amalgamated sandstone-rich facies in the channel axes to thinly interbedded sandstone and siltstone at the channel margins over distances of 10–30 m. These elements are generally considered to be ∼300 m wide and were formed by punctuated periods of incision and sedimentary bypass, followed by in-filling by collapsing turbidity currents. Out-of-channel deposits consist primarily of fine-grained facies, which are typically covered by vegetation in the study area.The channel strata of the mapped portion of the Tres Pasos Formation can be grouped into three channel complexes 25–70 m thick. Complexes are differentiated based on the preservation of siltstone-dominated deposits (bypass drapes and channel margin), which persist across the entire outcrop belt and coincide with shifts in channel stacking pattern. The oldest four channel elements (channel complex 1) are characterized by the highest lateral offsets, relative to one another. These are interpreted to record the most unconfined channel-stacking pattern present. As the channel system evolved (channel complexes 2 and 3), channel elements began to stack on top of one another, due to the increased confinement imparted on the slope channel system. The amount of vertical offset between successive channel elements preserves the record of channel aggradation as well as erosional degradation. The greatest vertical offset observed is associated with the oldest channels; as the system matured, vertical offset decreased. This decrease in vertical offset is coincident with the decrease in lateral offset of channels. The lateral offset decrease is attributed to establishment of constructional confinement and is the consequence of increased focusing of successive channel-initiating gravity flows. As confinement establishes, channels are predisposed toward underfilled conditions upon abandonment. The capture of channel-initiating currents along channel abandonment relief fairways focused incision and resulted in increased erosion and decreased vertical offset. The consequence of these conditions is an upward increase in channel element amalgamation.The organized stacking of slope channels observed in the Tres Pasos Formation is comparable to that of seismically imaged channel-levee or entrenched slope valley systems. By analogy to these 3-dimentionally constrained systems, a portion of the poorly exposed out-of-channel facies in the Tres Pasos Formation is attributed to aggradational internal levee deposits. The facies insight derived from the studied outcrop provides insight into analogous hydrocarbon-bearing units from numerous continental margins.

Re-interpretation of the age and environment of deposition of Paleogene turbidites in the Andaman and Nicobar Islands, Western Sunda Arc

Turbidites composed of sandstone-shale alternations on the Kalipur–Shibpur coast, North Andaman Island, classified under the Palaeocene–Eocene Mithakhari Group in several recent papers are identified as and compared with the type section of the Oligocene Andaman Flysch exposed at Corbyn’s Cove, South Andaman Island. The Kalipur turbidites were interpreted as the inner fan and the latter as the distal mid fan facies of a forearc submarine fan. The turbidites of these two locations are separated by ∼250 km. Detailed studies concerning the identification and comparison, have however, revealed significant differences, and the turbidite outcrops are not continuous between these two localities. Turbidites at Kalipur–Shibpur and adjacent areas are part of a melange terrane, normally gritty and coarse grained, massive to locally graded bedded, calcareous, intercalated/interstratified with conglomerates and reefoidal limestones and show a framework composition varying from volcanolithic to lithic-poor arkosic sandstones, deposited in several isolated basins, fed by transverse supply of detritus from an accreted and uplifted ophiolite and arc massif. The Kalipur–Shibpur coast exposes marginally deformed, sandstone-dominated turbidites (coherent units) containing abundant ichnotraces, late Palaeocene foraminifera, and rip-up shale clasts, indicating deposition in shallow water accretionary slope basins. South of Kalipur, the Ramnagar coast exposes mud-rich turbidites showing accretion-related deformation. These deformed turbidites are interpreted as offscraped trench deposits. Further south, on the Rampur coast, turbidites intercalated with reefoidal limestones containing late Palaeocene foraminifera indicate deposition on the upper slope or on top of the accretionary slope basin. In contrast classical Bouma sequence-bearing sandstone-shale turbidites at Corbyn’s Cove, are part of a continuous outcrop belt of siliciclastic turbidites, lack fossils, carbonate facies and conglomerates, and consist of compositionally uniform greywackes, deposited in an open deep sea fan, fed by axially transported detritus derived from the continental blocks of western Burma. These key differences together with previous mapping and stratigraphic studies confirm the incorrect identification of Andaman Flysch in North Andaman Island in recent papers. The turbidites of these two locations were neither produced by the same sediment gravity flows nor deposited in the same forearc fan during the Oligocene, instead, they were derived from different palaeographic domains, deposited in different tectonic and sedimentary environments and also at different times. This provides new insights into the Paleogene turbidite deposition in this part of Western Sunda Arc.

High-resolution stratigraphy and correlation of Cambrian strata using carbon isotopes: an example from the southern Appalachians, USA

This study presents an example of using well-constrained stratigraphic trends in carbon isotope composition of marine carbonate deposits as a tool for high-resolution correlation of successions that lack prominent biomarkers. A large, positive carbon isotope excursion is recorded within the Steptoean (Furongian, Upper Cambrian) strata of the southern Appalachians in Tennessee. A coeval excursion (also known as SPICE) has been reported for the western United States (the Great Basin area), China, Kazakhstan, and Australia, indicating that this is a global phenomenon. The anatomy of this excursion was determined through extensive sampling of homogenous micrite and dolomicrite from strata at the Thorn Hill locality in northeastern Tennessee. The δ13C values show an increase from the upper Nolichucky Shale into the overlying Maynardville Formation (Conasauga Group). The most positive δ13C values (+4 to +5 ‰ VPDB) correspond to the transition between the Maynardville and the Copper Ridge Dolomite (Knox Group). The δ13C values decline in the lower part of the Copper Ridge. This characteristic carbon isotope record is compared with the δ13C record from the Lee Valley locality, which is about 30 km away and contains a coeval succession of strata that represent similar carbonate platform depositional environments. A comparison of the δ13C records shows a remarkable similarity and provides the means for detailed correlation of these two stratigraphic successions. Next the results are compared with those from the Tazewell locality, which is in the northwesternmost outcrop belt of lower Paleozoic rocks in Tennessee. This section is separated from the Thorn Hill and Lee Valley sections by three thrust faults and is truncated at the base by a thrust fault. The strata at the Tazewell locality differ from those at Thorn Hill and Lee Valley in that they were deposited more closely to the carbonate platform margin. Despite these differences, carbon isotope records proved very useful for correlation among the three successions. The results of this study encourage application of similar approaches in correlation of Steptoean strata at more widely spaced outcrops within and beyond the Appalachian basin.

Laterally extensive modified placer gold deposits in the Paleoproterozoic Mississagi Formation, Clement and Pardo Townships, Ontario

Local gold concentrations are found in laterally extensive pyrite-bearing, framework-supported, cobble and boulder conglomerates in the basal 30 m of the Mississagi Formation in the south eastern part of the Huronian outcrop belt. These were deposited as part of a valley-fill succession, in shallow gravel-bed braided rivers with local hyperconcentrated flows. The basal contact with underlying Archean psammites is typically highly irregular and shows no obvious sign of weathering suggesting that deposition took place soon after retreat of the glaciers responsible for deposition of the Ramsey Lake Formation. Highest gold concentrations are associated with moderately well-sorted medium to large pebble conglomerate that show some signs of reworking during low flow events. Depositional elements are typically lenticular and of limited lateral extent. Unlike the older pyritic quartz-pebble conglomerates at the base of the Matinenda Formation in the Elliot Lake and Blind River areas, these conglomerates contain no uraninite and are polymict with material derived from a highly restricted catchment area with marked local and regional topographic relief. Porous detrital allogenic pyrite and euhedral post-depositional pyrite have overlapping, generally positive δ34S values, indicating a closed system during diagenesis and metamorphism. The presence of biotite-enriched rims on many of the metavolcanic and metasedimentary clasts in the conglomerates suggests that gold was partly leached from the allogenic pyrite grains at the peak of the Penokean Orogeny at 1.85–1.5 Ga. The potential source of the Au-bearing detrital allogenic pyrite appears to be an as yet undiscovered Archean volcanic-hosted massive sulfide (VHMS) deposit located 1 to 8 km north of the deposit.

Age of ferroan A-type post-tectonic granitoids of the southern part of the Keimoes Suite, Northern Cape Province, South Africa

The granitoids of the Keimoes Suite represent part of a complex group of late Mesoproterozoic intrusions of granodioritic to potassic granite composition that occupy an outcrop belt striking approximately NW–SE to the immediate west of Upington, Northern Cape Province, South Africa. The intrusions are believed to mark the contact between the Kaapvaal Craton and the Namaqua Sector of the Namaqua-Natal Province. Published geological, geochemical and geochronological knowledge of the Keimoes Suite is poor in general, with most data restricted to the central portion of the outcrop belt. In this contribution new ages and geochemical data for three granitoids of the southeastern Keimoes Suite in the Putsonderwater – Van Wykspan area, the Klip Koppies, Gemsbokbult and a member of the Kleinbegin Subsuite, are presented. All three intrusions show petrological and geochemical characteristics that agree well with literature data for the central and northern outcrop portions of the Keimoes Suite. They are weakly foliated to unfoliated, medium- to coarse-grained ferroan, alkali-calcic late- to post-tectonic A-type biotite granitoids intruded within a collisional within-plate tectonic setting. Zircon U–Pb SHRIMP ages for the intrusion of the three granitoids are within error of one another at ca. 1100 Ma. Marked differences between the three granitoids include the abundance and age of inherited zircons and Sm–Nd isotope systematics. With an ε Nd(t) value of 1.83, the Gemsbokbult Granite is dominantly sourced from partial melting of material with a mantle derivation with a minor crustal component. The other two intrusions, on the other hand, show a strong crustal source component, as indicated by ε Nd(t) values of −2.03 for the Klip Koppies Granite and −8.46 for the Kleinbegin Granite. The Keimoes Suite granitoids provide important clues as to the timing and tectonic evolution of the western margin of the Kaapvaal Craton at its junction with the Proterozoic Namaqua Sector of the Namaqua-Natal Province. A contribution to these melts from the host rocks, the ∼1.24–1.30 Ga Areachap Group volcanic belt is traced through the presence of inherited zircon cores of this age that is particularly evident in the Klip Koppies Granite. Inheritance also reflects the major tectono-magmatic events which affected the western margin of the Kaapvaal Craton during Mesoproterozoic times, with the bulk of the inheritance in the zircon populations due to peak metamorphism and partial melting caused by continental collision between the Kaapvaal Craton and the Kakamas Terrane and Bushmanland Subprovince to the west at approximately 1.18–1.2 Ga. Post 1.1 Ga ages reflect isotopic disturbance due to a later collisional event to the west at ∼1.08 Ga and subsequent exhumation of the western margin of the Kaapvaal Craton.

Position of the Triassic–Jurassic boundary and timing of the end-Triassic extinctions on land: Data from the Moenave Formation on the southern Colorado Plateau, USA

Strata of the Moenave Formation on and adjacent to the southern Colorado Plateau in Utah–Arizona, U.S.A., represent one of the best known and most stratigraphically continuous, complete and fossiliferous terrestrial sections across the Triassic–Jurassic boundary. We present a synthesis of new biostratigraphic and magnetostratigraphic data collected from across the Moenave Formation outcrop belt, which extends from the St. George area in southwestern Utah to the Tuba City area in northern Arizona. These data include palynomorphs, conchostracans and vertebrate fossils (including footprints) and a composite polarity record based on four overlapping magnetostratigraphic sections. Placement of the Triassic–Jurassic boundary in strata of the Moenave Formation has long been imprecise and debatable, but these new data (especially the conchostracans) allow us to place the Triassic–Jurassic boundary relatively precisely in the middle part of the Whitmore Point Member of the Moenave Formation, stratigraphically well above the highest occurrence of crurotarsan body fossils or footprints. Correlation to marine sections based on this placement indicates that major terrestrial vertebrate extinctions preceded marine extinctions across the Triassic–Jurassic boundary and therefore were likely unrelated to the Central Atlantic Magmatic Province (CAMP) volcanism.

Lithogeochemistry as a tracer of the tectonic setting, lateral integrity and mineralization of a highly metamorphosed Mesoproterozoic volcanic arc sequence on the eastern margin of the Namaqua Province, South Africa

The Areachap Group represents the medium- to high-grade metamorphic and deformed remnants of a Mesoproterozoic (ca. 1.24–1.30 Ga) volcanic arc that was accreted onto the western margin of the Kaapvaal Craton during the early stages of the Namaqua-Natal Orogeny. Few regional geochemical studies of the succession have been undertaken and despite recent geochronological work from throughout the outcrop belt there is still uncertainty as to the lateral integrity and correlation of the succession from one area to another. This uncertainty has been aided by minimal outcrop and extensive deformation and dislocation of one area from another. This study uses trace element geochemistry combined with Sm–Nd-isotopic data from the northern and southern ends of the succession to test its lateral integrity. Minor differences, including the abundance of certain lithologies, varying influences of a crustal component and differences in age between different areas are apparent, but the succession, as a whole, displays lateral integrity as a continental island arc. All lithologies are characterised by enrichment in Th, U, Pb and the large ion lithophile elements relative to primitive mantle, along with Nb–Ta depletion relative to La, and strong light rare earth element (LREE) enrichment relative to heavy REE depletion, which, along with positive ε Nd(t) values of mafic gneisses from both ends of the succession, indicates mantle-derivation within a subduction-related environment with a substantial amount of crustal assimilation. Despite some minor geochemical and isotopic differences in the lithological architecture on a local scale the Areachap Group exhibits coherent geochemical characteristics along its entire strike length. Localised remobilization of elements, particularly in areas characterised by granulite grade metamorphism, as well as in the altered footwall zones of volcanogenic massive sulphide lenses, is present. These alteration trends, along with primary geochemical signatures are remarkably well preserved despite the high grade of metamorphism, as illustrated in clearly defined hydrothermal alteration trends on the alteration box plot, and serves as an efficient tool to delineate areas that have undergone hydrothermal alteration spatially associated with massive sulphide ore bodies. The systematic association of hydrothermal alteration and sulphide ore bodies provides support for the isochemical character of rock compositions during high-grade metamorphism and the lack of extensive post-metamorphic alteration processes.

Calibrating Mid-Permian to Early Triassic Khuff sequences with orbital clocks

ABSTRACTThe Middle East Geologic Time Scale (ME GTS) seeks to document and age-calibrate Arabian Plate transgressive-regressive (T-R) depositional sequences using: (1) Geological Time Scale of the International Commission on Stratigraphy (GTS), and (2) Arabian Orbital Stratigraphy time scale (AROS). AROS is based on an orbital-forcing glacio-eustatic model that offers three orbital clocks to date T-R sequences: (1) Stratons @ ca. 405 Ky; (2) Dozons @ ca. 4.86 My (12 stratons); and Orbitons @ ca. 14.58 My (36 stratons, three dozons). The Earth today is in Orbiton 0, which started ca. 1.5 Ma (SB 0); the ages of lower boundaries of orbitons can be estimated with the formula SB n = n × 14.58 + 1.5 Ma. This scheme was used to calibrate the Arabian Plate’s Mid-Permian to Early Triassic Khuff sequences, which contain one of the largest gas-bearing carbonate reservoirs in the World.The Khuff and equivalent formations have been interpreted by several authors in terms of six long-period sequences in outcrop belts and subsurface sections (Khuff sequences KS6 to KS1 in ascending order). Their type sections are briefly reviewed with emphasis on their boundaries, higher-order architecture and stage assignments. The age calibration starts at the basal Khuff Sequence Boundary (Khuff SB, Sub-Khuff Unconformity) defined in a type section in Al Huqf outcrop in Oman. Above the Khuff SB (ca. 268.9 Ma) the type sections of the oldest Khuff sequences KS6 (ca. 268.9–264.0 Ma) and KS5 (ca. 264.0–259.1 Ma) are defined in Oman and interpreted to each consist of twelve subsequences (stratons) with the predicted architecture of two consecutive dozons. By biostratigraphy they span the Mid-Permian (Guadalupian Epoch), Wordian and Capitanian stages.Type-Sequence KS4 (ca. 259.1–254.2 Ma) is defined in Iran and corresponds to the Wuchiapingian Stage. The Iranian type-Khuff Sequence KS3 (ca. 254.2–249.4 Ma) contains nine subsequences (stratons) grouped between two major exposure surfaces. By correlation to the Changhsingian Stage and Permian/Triassic Boundary (PTrB) type section in South China, it is interpreted as a dozon with three missing stratons. Khuff Sequence KS2 (ca. 249.4–247.8 Ma) contains the PTrB with an orbital age of ca. 249.0 Ma, compared to 251.0 ± 0.4 in GTS and 249.0–253.0 Ma by radiometric dating in its type section. Khuff sequences KS2 and KS1 contain 13 subsequences (stratons) between ca. 249.4–244.1 Ma spanning latest Permian and Early Triassic. The boundary of the Khuff with the overlying Sudair Formation, Sudair Sequence Boundary, is defined in Borehole SHD-1 (Central Saudi Arabia) and calibrated at ca. 244.1 Ma falling near the age of the Early/Mid-Triassic Boundary in GTS. The enclosed Chart shows a work-in-progress correlation of the six Khuff sequences across the Arabian Plate.

Significance of channel-belt clustering in alluvial basins

Research Article| June 01, 2010 Significance of channel-belt clustering in alluvial basins Elizabeth A. Hajek; Elizabeth A. Hajek 1University of Wyoming Department of Geology and Geophysics, 1000 E. University Avenue, Laramie, Wyoming 82071, USA2Saint Anthony Falls Laboratory, University of Minnesota, 2 3rd Avenue SE, Minneapolis, Minnesota 55414, USA Search for other works by this author on: GSW Google Scholar Paul L. Heller; Paul L. Heller 1University of Wyoming Department of Geology and Geophysics, 1000 E. University Avenue, Laramie, Wyoming 82071, USA Search for other works by this author on: GSW Google Scholar Benjamin A. Sheets Benjamin A. Sheets 3University of Washington, School of Oceanography, 1503 Boat Street, Seattle, Washington 98195, USA Search for other works by this author on: GSW Google Scholar Geology (2010) 38 (6): 535–538. https://doi.org/10.1130/G30783.1 Article history received: 15 Oct 2009 rev-recd: 12 Jan 2010 accepted: 20 Jan 2010 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Elizabeth A. Hajek, Paul L. Heller, Benjamin A. Sheets; Significance of channel-belt clustering in alluvial basins. Geology 2010;; 38 (6): 535–538. doi: https://doi.org/10.1130/G30783.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The distribution of channel deposits in alluvial basins is commonly used to interpret past changes in climate, tectonics, and sea level. Here we present preliminary evidence that long-time scale (∼103–105 yr) self-organization in fluvial systems may generate structured stratigraphic patterns spontaneously, in the absence of, or independent from, changing basin boundary conditions. A physical experiment and an ancient alluvial succession (Ferris Formation, latest Cretaceous–Paleogene, south-central Wyoming) both show stratigraphy where clusters of many closely spaced channel deposits are separated from each other by extensive intervals of overbank mudstones. Analysis using spatial point process methods shows that channel deposits in both basins are statistically clustered over intermediate basin length scales. In the experiment, external controls (base level, subsidence rate, and sediment/water supplies) were not varied, and therefore not factors in cluster formation. Likewise, the ancient system lacks stratigraphic and sedimentologic evidence of external controls on channel clustering. We propose that channel clusters, as seen in this study, reflect a scale of fluvial self-organization that is not usually recognized in ancient deposits. This type of internally generated stratigraphy should be considered when reconstructing tectonic, climate, and sea-level changes from ancient basin fills and when correlating between outcrop belts or subsurface wells. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Middle Devonian brachiopods from arctic Canada and some preliminary results of North American biogeography based on quantitative assessment

INTRODUCTION The upper Lower to Middle Devonian Bird Fiord Formation (late Emsian to late Eifelian) is a carbonate-siliciclastic succession that contains a diverse brachiopod-dominated fauna. A collection of over 45 000 brachiopod specimens, which came from 140 collections made at 34 locations throughout the outcrop belt of this formation, contains 22 species of brachiopods that belong to 21 genera. Among them, the atrypids form 52% of the biota, spiriferids ~13%, orthids ~8%, productids ~3%, rhynchonellids ~2%, terebratulids ~1% and the strophomenids, pentamerids, and athyridids each <1%. Spinatrypina, Schizophoria, Elythyna, Desqumatia (Independentrypa), Cupularostrum, and Atrypa collectively form more than 80% of the brachiopod biota. Biogeographically, the Devonian was a unique period because the high provincialism that was established during the Early Devonian was replaced by a very cosmopolitan biota during the Late Devonian (Boucot, 1988). Numerous worldwide and regional studies of Devonian biotas led to the qualitatively defined Old World, Eastern Americas (=Appalachian),

Oligocene-Miocene basin evolution in the northern Altiplano, Bolivia: Implications for evolution of the central Andean backthrust belt and high plateau

The upper Oligocene to lower Miocene Penas and Aranjuez formations are exposed in north-northwest–trending outcrop belts of the central Andean backthrust belt situated within the central Andean plateau along the boundary between the northern Altiplano and the Eastern Cordillera of Bolivia. Sedimentary lithofacies analyses indicate that these coarse-grained siliciclastic formations were deposited primarily in alluvial fan to braided fluvial environments. An upsection change from principally fine-grained sandstone to cobble conglomerate is consistent with increased proximity to the sediment source with time. Paleocurrent analyses reveal that flow was predominantly directed toward the west-southwest away from Cordillera Real, the elevated core of the Eastern Cordillera. Provenance data from conglomerate clast compositions and sandstone petrofacies suggest derivation from recycled quartz-rich metasedimentary and sedimentary rocks from the Paleozoic section in the Eastern Cordillera. The paleoflow orientations, sediment provenance, and increased proximity of the sediment source suggest that deposition of the Penas and Aranjuez formations was related to surface uplift of the Eastern Cordillera relative to the Altiplano. Growth strata observed in the Aranjuez Formation further indicate that shortening was synchronous with deposition, probably in a hinterland basin. New 40 Ar/ 39 Ar ages from a lowermost exposed igneous unit and interbedded ash-fall tuff beds in the Aranjuez and Penas formations show that synorogenic sedimentation and fold-thrust deformation in the frontal (west-southwestern) zone of the central Andean backthrust belt was concentrated during late Oligocene–early Miocene time. These age results are consistent with previous studies of east-derived sedimentation in the Altiplano and indicate regional uplift of the Eastern Cordillera at this time. Upsection trends in provenance data further suggest a progressively greater contribution from younger Paleozoic strata, possibly due to activation of new thrust faults during west-southwestward propagation of the backthrust belt toward the Altiplano. Such a progression of late Oligocene–early Miocene shortening along the Altiplano–Eastern Cordillera boundary likely reflects significant crustal thickening, potential isostatic uplift, and initial topographic expression of the eastern margin of the central Andean plateau.

Shark-bitten vertebrate coprolites from the Miocene of Maryland

Coprolites (fossilized feces) preserve a wide range of biogenic components, from bacteria and spores to a variety of vertebrate tissues. Two coprolites from the Calvert Cliffs outcrop belt (Miocene-aged Chesapeake Group), MD, USA, preserve shark tooth impressions in the form of partial dental arcades. The specimens are the first known coprolites to preserve vertebrate tooth marks. They provide another example of trace fossils providing evidence of prehistoric animal behaviors that cannot be directly approached through the study of body fossils. Shark behaviors that could account for these impressions include: (1) aborted coprophagy, (2) benthic or nektonic exploration, or (3) predation.

A far-field record of the end Ordovician glaciation: The Ellis Bay Formation, Anticosti Island, Eastern Canada

The end Ordovician was a critical period in Earth history. It was a time of global climatic change with large-scale continental glaciations. A wealth of recent work has documented the stratigraphical records from the near-field areas, i.e., from various Gondwanan continental relics where glaciation was more severely felt. There is an urgent need to re-examine the stratigraphic record from the far-field areas at greater levels of resolution than it has been hitherto possible in order to understand the timing, duration, and character at the glaciation centred around the Ordovician/Silurian (O/S) boundary. One of the best exposed and most complete stratigraphic records from paleotropical areas near the boundary is on Anticosti Island in eastern Canada. The east–west trending, ∼ 200 km long Ellis Bay (Hirnantian) outcrop belt, with superb coastal exposures at both ends of Anticosti Island, is slightly oblique to the paleoshoreline with near-shore siliciclastic-dominated facies restricted to the eastern part of the island and more offshore carbonate-dominated facies present along the central and western parts. These mixed carbonate–siliciclastic facies accumulated along a storm-dominated beach-to-offshore profile. They are organized into a stacked set of shallowing-upward offshore to shoreline facies successions (typically coarsening upward) separated by deepening upward facies successions (typically fining upward) formed during intervening transgressions. High-frequency glacio-eustatic fluctuations were the dominant control on the development of TR sequences observed in the Ellis Bay succession of Anticosti Island, although tectonic subsidence had a substantial secondary effect on thickness. Five TR sequences are recognized allowing for precise correlation along the Ellis Bay outcrop belt. In the eastern sections, relatively thin TR sequences are locally bounded by unconformities. A typical sequence begins with thin transgressive limestone overlain by a maximum flooding, calcareous shale grading up into regressive, storm-influenced, proximal to transitional zone fine sandstone/grainstone and shale, which are sharply overlain by shoreface to foreshore sandstone/grainstone. In the central and western sections, the thicker TR sequences have more symmetrical transgressive–regressive components reflecting greater subsidence compensated by a higher carbonate sediment supply. Two discrete glacial periods are recognized during the deposition of the Hirnantian Ellis Bay succession with the younger one recording large sea level changes (> 50 m in magnitude) caused by the development of very large ice sheets over western Gondwana. Abundant ice conditions on the globe during the end Ordovician, and the resulting high-amplitude eustasy, appear to have left distinctive stratigraphical imprints on mixed, paleotropical, siliciclastic–carbonate ramps.

Role of the Ollo de Sapo massive felsic volcanism of NW Iberia in the Early Ordovician dynamics of northern Gondwana

The Ollo de Sapo Fm. in the Sanabria region of NW Iberia consists of subvolcanic, volcanic and volcano-sedimentary rocks underlying and partly intercalated with Early Ordovician, platform-facies sediments. Granitic plutons of the same age, presently transformed into orthogneisses, are genetically related. The metavolcanic rocks are dacites and rhyolites with potassic and peraluminous character, and volcanic arc signature, possibly inherited from the melting of calc-alkaline crustal material. In the study area, the Ollo de Sapo Fm. is made up of two large superposed volcanic domes separated by a thin screen of contemporaneous metapelites and sandstones. The domes underwent endogenous growth with subvolcanic, phenocryst-rich magma intruding and cooling under a volcanic and volcaniclastic shield. The western dome tapers to the east and underlies the younger eastern dome, suggesting that the whole formation consists of several volcanic edifices replacing each other along the 570 km outcrop belt of the Ollo de Sapo Fm. Two new U–Pb protolith ages of 488 ± 6 Ma and 472 ± 12 Ma match, within error, the known age span for the Ollo de Sapo Fm. in Iberia (495–483 Ma). The younger age dates an augen gneiss at the core of the eastern volcanic dome, whereas the older date comes from the overlying volcanic sequence, suggesting intrusion of the augen gneiss protolith into the overlying volcanic rocks. An incipient rift involving necking of crustal-scale boudinage a few hundred km landward of the edge, but still within the wide passive margin of northern Gondwana may explain the linear distribution of the Ollo de Sapo Fm. and the limited time span of massive volcanism. The heat for crustal melting was probably supplied by mafic magmas intruding or underplatting the lower crust. The Early Ordovician felsic magmatism of NW and Central Iberia seems to form part of a large siliceous province linked to break-up and terrane dispersion of the northern Gondwana realm.

Middle Devonian chitinozoan biostratigraphy and sedimentology in the eastern outcrop belt of the Parnaíba Basin, northeastern Brazil

Middle Devonian chitinozoan biostratigraphy and sedimentology in the eastern outcrop belt of the Parnaíba Basin, northeastern Brazil

Stratabound magnetite deposits from the eastern outcrop belt of the Archaean Iron Ore Group, Singhbhum craton, India

Most high-grade banded iron formation (BIF)-hosted iron ores are composed almost exclusively of microplaty haematite and martite-textured haematite, i.e. pseudomorphs of haematite after magnetite. Goethite abounds only where haematite-rich iron ores are affected by geologically recent lateritic weathering. Magnetite and kenomagnetite occur usually only as small, irregular remnants enclosed in martite. Although low-grade magnetite–carbonate ores have been discovered in spatial association with high-grade iron ore bodies, high-grade BIF-hosted iron ores composed predominantly of magnetite have as yet only been reported from the Iron Quadrangle in Minas Gerais (Brazil). In this contribution, the authors describe such magnetite-rich high-grade BIF-hosted iron ores from the Gorumahishani deposit in the state of Orissa, India. The high-grade magnetite ores occur as stratabound bodies hosted by a single prominent BIF unit of the Mesoarchaean Iron Ore Group (IOG). The ores are composed of an interlocking xenotopic mosaic of fine-grained magnetite, accompanied by trace amounts of Fe-rich talc, sulphides and carbonaceous matter. The occurrence of the magnetite ores is very similar to that of economically significant microplaty haematite–martite ore bodies hosted by the same BIF from other parts of the IOG; both are regarded as being of hydrothermal–metasomatic origin, formed at the expense of the iron formation protolith. The abundance of magnetite, combined with the presence of sulphides, carbonaceous matter, and a marked enrichment of Ni, Co and Zn are attributes that differentiate the magnetite-rich ores from haematite–martite counterparts. These differences are tentatively attributed to differences in hydrothermal fluid composition. Based on the available evidence, it is suggested that magmatic–hydrothermal fluids derived from closely associated mafic intrusives were instrumental in the formation of the magnetite orebodies of the Gorumahishani deposit. The abundance of magnetite and the presence of sulphides and carbonaceous matter indicate that the hydrothermal fluid was mildly reducing, rather than oxidising as proposed for haematite-rich assemblages. The abundance of martite in the coexisting haematite–martite orebodies of other parts of the IOG may be used to speculate that the formation of massive magnetite bodies may have been an essential intermediate stage during the hydrothermal formation of high-grade iron ore deposits of the IOG.

Volcanic Tephra Bed Formation and Condensation Processes: A Review and Examination from Devonian Stratigraphic Sequences

The formation and preservation of airfall volcanogenic tephras is the result of various interacting factors. Explanations of the depositional history of individual tephra beds lie along a spectrum between two end-member models of a primary single event and a complex depositional history. Nonrecognition of the history of a tephra bed may lead to errors in geochronology, correlation, and interpretation of the record of explosive volcanism in sedimentary successions. Increased understanding of sequence stratigraphic principles and condensation processes favors the model of tephras with a complex depositional history. This is supported by studies that indicate many tephras in the rock record result from amalgamation and/or reworking in the depositional environment. A prediction of increased tephra bed formation/preservation within transgressive systems tracts due to condensation processes is tested through 11 Lochkovian to Eifelian (Lower to lower Middle Devonian) depositional sequences. The character and distribution of over 80 Devonian K-bentonites and tuffs in the Appalachian Basin (eastern United States) indicates a complex history of tephra deposition and preservation. Of 11 sequences, four have numerous tephra beds, one sequence yields one tephra bed, and six sequences yield no tephra beds. In total, 38 individual beds cumulatively occur within transgressive systems tracts, 39 in early highstand systems tracts, one in late highstand systems tracts, and one in lowstand systems tracts, noting that lowstands are poorly preserved or not recognized along the outcrop belt. This indicates that airfall tephras are concentrated in the transgressive and early highstand systems tracts, not just in condensed intervals of transgressive systems tracts; they do not consistently occur through all sequences. A simple sequence stratigraphic condensation model for the deposition and preservation of volcanic tephras is not applicable.

Middle to late Cenozoic basin evolution in the western Alborz Mountains: Implications for the onset of collisional deformation in northern Iran

Oligocene‐Miocene strata preserved in synclinal outcrop belts of the western Alborz Mountains record the onset of Arabia‐Eurasia collision‐related deformation in northern Iran. Two stratigraphic intervals, informally named the Gand Ab and Narijan units, represent a former basin system that existed in the Alborz. The Gand Ab unit is composed of marine lagoonal mudstones, fluvial and alluvial‐fan clastic rocks, fossiliferous Rupelian to Burdigalian marine carbonates, and basalt flows yielding 40Ar/39Ar ages of 32.7 ± 0.3 and 32.9 ± 0.2 Ma. The Gand Ab unit is correlated with the Oligocene–lower Miocene Qom Formation of central Iran and is considered a product of thermal subsidence following Eocene extension. The Narijan unit unconformably overlies the Gand Ab unit and is composed of fluvial‐lacustrine and alluvial fan sediments exhibiting contractional growth strata. We correlate the Narijan unit with the middle to upper Miocene Upper Red Formation of central Iran on the basis of lithofacies similarities, stratigraphic position, and an 8.74 ± 0.15 Ma microdiorite dike (40Ar/39Ar) that intruded the basal strata. Deformation timing is constrained by crosscutting relationships and independent thermochronological data. The Parachan thrust system along the eastern edge of the ancestral Taleghan‐Alamut basin is cut by dikes dated at 8.74 ± 0.15 Ma to 6.68 ± 0.07 Ma (40Ar/39Ar). Subhorizontal gravels that unconformably overlie tightly folded and faulted Narijan strata are capped by 2.86 ± 0.83 Ma (40Ar/39Ar) andesitic lava flows. These relationships suggest that Alborz deformation had migrated southward into the Taleghan‐Alamut basin by late Miocene time and shifted to its present location along the active range front by late Pliocene time. Data presented here demonstrate that shortening in the western Alborz Mountains had started by late middle Miocene time. This estimate is consistent with recent thermochronological results that place the onset of rapid exhumation in the western Alborz at ∼12 Ma. Moreover, nearly synchronous Miocene contraction in the Alborz, Zagros Mountains, Turkish‐Iranian plateau, and Anatolia suggests that the Arabia‐Eurasia collision affected a large region simultaneously, without a systematic outward progression of mountain building away from the collision zone.