Erosional Sequence Research Articles

Sequence stratigraphy of a wave-dominated, tidally influenced delta in the Danian of Seymour Island, Antarctica: An integrated sedimentological–palaeoecological approach

Lower Paleocene marine siliciclastics of the Sobral Formation (Seymour Island, Antarctica) form an important component of a key southern high latitude reference section for the Maastrichtian–Eocene. The formation comprises a coarsening-upward, regressive succession (270 m thick), shallowing from prodeltaic, mud-rich to sandy nearshore sediments. Sedimentary facies analysis, integrated with macrofaunal and microfloral palaeoecological studies, indicate a wave-dominated, tidally influenced deltaic environment. Four depositional sequences (1–4) record the role of relative sea-level changes in controlling the stepwise progradation of the system. Prodeltaic and distal delta-front deposits (Sequences 1, 2), comprise coarsening-upward parasequences of mud-rich heteroliths and bioturbated fine-grained muddy sands displaying draped slump scars. The overlying erosional sequence boundary is marked by incised valleys filled with fluvio-estuarine sediments; succeeding sand-rich facies (Sequences 3, 4) represent the proximal delta front and delta platform. The macro-invertebrate and palynological datasets display contrasting responses to the regressive history of the formation. The macrofaunal record in the distal deltaic sediments reflects the marine influence, though a stratigraphic increase in diversity was controlled both by ecological factors and biotic recovery following the K–Pg event. The associated palynological assemblage also displays a strong marine signal and includes typical outer neritic – oceanic dinoflagellate cyst taxa. A sharp decline in macrofossil diversity and abundance in Sequences 3, 4 is compatible with ecological stress in shallow-water, restricted inshore settings. Though the palynological record at this level is terrestrially dominated, the marine component reflects the cosmopolitan nature of dinoflagellates, displaying the highest diversities in the formation. The regressive Sobral Formation ended a protracted c. 60 Myr history of regional subsidence and marine sedimentation; although amplified by two Danian sea-level falls of probable eustatic origin, the regressive trend heralded mid-Paleocene basin inversion potentially linked to the onset of regional tectonic reorganization that ultimately led to opening of the Scotia Sea.

Stratigraphy of the Gorstian and Ludfordian (upper Silurian) Hemse Group reefs on Gotland, Sweden

ABSTRACT The Hemse Group is one of the least understood stratigraphic units of the Silurian sequence of Gotland, Sweden. New results from airborne transient electromagnetic (ATEM) measurements in combination with previously published data from field studies and geophysical investigations shed new light on carbonate platform development during the early- to mid-Ludlow Hemse Group. ATEM reveals a transgressive phase that began near the Wenlock-Ludlow boundary, which resulted in deposition of marls and corresponds roughly to the Hemse limestone units a-c and the Hemse Marl NW. In this phase little or no reef development occurs. The end of the transgressive phase coincides with the weak Linde Event. The following highstand favoured extensive reef growth forming a reef barrier system of both fringing reefs and more rampiform settings with stromatoporoid biostromes and occasional biohermal buildups. The Kuppen-Snabben Unconformity Complex marks an erosional (karstic) sequence boundary and rocky shoreline and the transition from a rampiform setting with reef biostromes towards a more rimmed setting with patch reefs.

Eco-geomorphological connectivity and coupling interactions at hillslope scale in drylands: Concepts and critical examples

The diagnosis of land degradation requires a deep understanding of ecosystem functioning and evolution. In dryland systems, in particular, research efforts must address the redistribution of scarce resources for vegetation, in a context of high spatial heterogeneity and non-linear response. This fact explains the prevalence of eco-hydrological perspectives interested in runoff processes and, the more recent, focused on connectivity as an indicator of system resource optimisation. From a geomorphological perspective and reviewing the concepts of eco-hydro-geomorphological interactions operating in ecosystems, this paper explores the effects of erosion on vegetation configuration through two case studies at different spatio-temporal scales. We focus on the structure-function linkage, specifically on how morphological traits relate with different stages in the erosional sequence, both in the abiotic and the biotic domain. Results suggest that vegetation dynamics are affected by structural boundary conditions at both scales, i.e. by surface armouring related with rock fragments at the patch scale, and by the degree of hillslope-channel coupling at the hillslope scale. Our preliminary results can serve as new working hypotheses about the structure-function interplay on hillslopes. All this, taking advantage of the recent technological achievements for acquiring very high-resolution geospatial data that offer new analytical possibilities in a range of scales.

Sedimentologic, stratigraphic, and structural evolution of minibasins and a megaflap formed during passive salt diapirism: The Neoproterozoic Witchelina diapir, Willouran Ranges, South Australia

ABSTRACTThis study documents the growth of a megaflap along the flank of a passive salt diapir as a result of the long-lived interaction between sedimentation and halokinetic deformation. Megaflaps are nearly vertical to overturned, deep minibasin stratal panels that extend multiple kilometers up steep flanks of salt diapirs or equivalent welds. Recent interest has been sparked by well penetrations of unidentified megaflaps that typically result in economic failure, but their formation is also fundamental to understanding the early history of salt basins. This study represents one of the first systematic characterizations of an exposed megaflap with regards to sub-seismic sedimentologic, stratigraphic, and structural details. The Witchelina diapir is an exposed Neoproterozoic primary passive salt diapir in the eastern Willouran Ranges of South Australia. Flanking minibasin strata of the Top Mount Sandstone, Willawalpa Formation, and Witchelina Quartzite, exposed as an oblique cross section, record the early history of passive diapirism in the Willouran Trough, including a halokinetically drape-folded megaflap. Witchelina diapir offers a unique opportunity to investigate sedimentologic responses to the initiation and evolution of passive salt movement. Using field mapping, stratigraphic sections, petrographic analyses, correlation diagrams, and a quantitative restoration, we document depositional facies, thickness trends, and stratal geometries to interpret depositional environments, sequence stratigraphy, and halokinetic evolution of the Witchelina diapir and flanking minibasins. Top Mount, Willawalpa, and Witchelina strata were deposited in barrier-bar-complex to tidal-flat environments, but temporal and spatial variations in sedimentation and stratigraphic patterns were strongly influenced from the earliest stages by the passively rising Witchelina diapir on both regional (basinwide) and local minibasin scales. The salt-margin geometry was depositionally modified by an early erosional sequence boundary that exposed the Witchelina diapir and formed a salt shoulder, above which strata that eventually became the megaflap were subsequently deposited. This shift in the diapir margin and progressive migration of the depocenter began halokinetic rotation of flanking minibasin strata into a megaflap geometry, documenting a new concept in the understanding of deposition and deformation during passive diapirism in salt basins.

Formation mechanisms of channels on Titan through dissolution by ammonium sulfate and erosion by liquid ammonia and ethane

Data obtained from the Cassini Visual and Infrared Mapping Spectrometer (VIMS), Imaging Science Subsystem (ISS), and Synthetic Aperture Radar (SAR) instruments have revealed an array of fluvial channels on Titan's surface, often several hundreds of kilometers in length. The paucity of impact craters on Titan's surface suggests a formation by fluvial erosion into the water-ice bedrock. Additionally, at the landing site, the Huygens Probe Descent Imager and Spectral Radiometer (DISR) imaged Earth-like rounded cobbles 0.3–15cm in diameter composed of water ice, reminiscent of rounded stream clasts on Earth. In this paper we examine different fluvial features on Titan, identified by the Cassini spacecraft, and evaluate the possibilities of channel formation by dissolution of ice by a concentrated solution of ammonium sulfate, and by mechanical erosion by flow of liquid ammonia and liquid ethane. We find that chemical erosion of Titan's channels could be completed in 280 to 1100 years (all units of time in this paper are Terrestrial, not Titanian), much shorter than the period of about 84,000 years that a concentrated (NH4)2SO4-H2O solution could exist as a liquid on the Titan surface. Mechanical erosion of Titan's channels is generally a much slower process, on the order of 102 to 105 years to completion, and is also slower than mechanical erosion of a model river on Earth, averaging 103 to 104 years. The erosional sequence of the channels on Titan may have started after the formation of water-ice on the surface by the process of chemical dissolution by (NH4)2SO4-H2O, overlapping, or followed by, a period of mechanical erosion by liquid NH3. A final stage on the cooling surface of Titan might have been characterized by liquid C2H6 as an agent of mechanical erosion.

SEDIMENTARY FACIES AND DEPOSITIONAL ENVIRONMENTS OF DIVERSE EARLY PALEOCENE FLORAS, NORTH-CENTRAL SAN JORGE BASIN, PATAGONIA, ARGENTINA

We here investigate the sedimentology of the early Danian (ca. 66–64 Ma) Salamanca Formation in the north-central San Jorge Basin, southern Chubut Province, Patagonia, Argentina, in order to place the outstandingly diverse and well-preserved fossil floras it contains into specific environmental settings. These assemblages are among very few of Danian age from the entire Southern Hemisphere and thus provide critical data about geographic variation in recovery from the end-Cretaceous extinction. Understanding the depositional context of the Salamanca floras is necessary for comparison with other assemblages and for interpreting their exceptional preservation. The Salamanca Formation was deposited above a widespread erosional sequence boundary (SB-1) resulting from a relative base level rise and widespread marine transgression during the early Danian (Chron C29n). In response to this increase in accommodation space, a broad, shallow estuary formed that most likely extended westward at least as far as the San Bernardo belt. A transgressive systems tract was deposited in this estuary, consisting of bioturbated sand fining upwards to silt. The maximum marine flooding surface at the beginning of the highstand systems tract is defined by well laminated, unburrowed, clay deposits of a low energy, deep shelf. The Salamanca highstand systems tract (HST) consists of sandy and silty facies capped by accreting subtidal bars and sandy shoals containing an abundance of tidal indicators, suggesting deposition proximal to the San Jorge paleo-estuary head. A second sequence boundary (SB-2), formed during Chron C28r and early C28n, separates the older highstand deposits from younger lowstand and transgressive deposits. These consist of estuarine sand shoals, trough cross-bedded sands deposited in aggrading, fluvially influenced tidal channels, tidal flat muds, and bayhead deltas. The best preservation of compression floras and petrified trees occurred near the tops of subtidal bars below SB-2; at the end of the shallowing-upward cycle that caps the second HST; and in fluvially-influenced tidal channels, tidal flat mudstones, and bayhead deltas of the lowstand and transgressive systems tracts that lie above SB-2. These settings were proximal to the source forests and had rapid rates of burial. We interpret the dark muds of the Banco Negro Inferior, which cap the Salamanca Formation, as a late transgressive and highstand systems tract deposited during a time of rising groundwater table and declining river slopes in a widespread, lowland coastal forest.

THE OCCURRENCE OF VERTEBRATE AND INVERTEBRATE FOSSILS IN A SEQUENCE STRATIGRAPHIC CONTEXT: THE JURASSIC SUNDANCE FORMATION, BIGHORN BASIN, WYOMING, U.S.A

Previous studies of the sequence stratigraphic distribution of fossils have focused on the record of relatively abundant marine invertebrates. Only a handful of studies have examined how sequence stratigraphic architecture influences the occurrence of vertebrates, particularly large and rare tetrapods. The Jurassic Sundance Formation of the Bighorn Basin, Wyoming, USA, contains a rich suite of invertebrate and vertebrate fossils, including large and rare marine reptiles, and this allows the sequence stratigraphic controls on the distribution of these groups to be compared. The Sundance Formation consists of four depositional sequences, with the lower two being carbonate dominated and the upper two siliciclastic dominated. Two incised valley fills are also present. The presence of multiple depositional sequences and strongly erosional sequence boundaries is the likely cause of the complicated lithostratigraphic nomenclature of the Sundance. Invertebrates (mollusks and echinoderms) in the Sundance conform to well-established patterns of occurrences, including strong facies control and fossil concentrations at maximum flooding surfaces, in the upper portion of parasequences, and within lags overlying sequence boundaries. As expected from their rarity, marine reptiles (ichthyosaurs, plesiosaurs, and pliosaurs) show a weaker connection to sequence stratigraphic architecture. Nonetheless, they do display facies control and are found primarily in offshore mudstone, rather than shoreface and estuarine sandstone. They are also more common at hiatal surfaces, including a zone of concretions at the maximum flooding surface and in lag deposits overlying sequence boundaries. These associations suggest that sequence stratigraphic architecture may be a useful approach for discovery of marine vertebrates and that sequence stratigraphic context should be considered when making paleobiological interpretations of marine vertebrates as well as invertebrates.

Deciphering the impact of sea-level changes and tectonic movement on erosional sequence boundaries in carbonate successions: A case study from Tertiary strata on Grand Cayman and Cayman Brac, British West Indies

The stratigraphic architecture of carbonate successions that develop on geographically isolated islands reflects the balance between sea-level highstands, lowstands, and tectonic activity. This is readily apparent in the Tertiary carbonate sequences on the Cayman Islands that are formed of the Bluff Group, which includes the Brac Formation (Lower Oligocene), Cayman Formation (Middle Miocene), and Pedro Castle Formation (Middle Pliocene). These strata are overlain by the Pleistocene Ironshore Formation. The unconformities that define the boundaries between these formations are variable because some developed during one cycle of past erosion, others developed through two or more past erosional cycles, and some are still developing today. Some unconformities, like the one at the top of the Cayman Formation, are geographically variable because they underwent different developmental histories in different areas. The challenge with architectural complex successions, like those on the Cayman Islands, is that of deciphering the impact of sea-level changes as opposed to tectonic influences.During sea-level lowstands, the older carbonate successions were exposed on land and modified by surface and subsurface karst development. At the same time, marine erosional processes affected the coastal areas. Surface karst modification, which commonly produced rugged topographies with erosional relief at least 62m, was controlled largely by rainfall, runoff, and stratal dip. Weathering on Grand Cayman at the end of the Miocene, for example, produced a dish-shaped topography with elevated peripheral rims. In contrast, uplift of the east end of Cayman Brac between the Late Pliocene (3.6Ma) and ~400ka, elevated the basal part of the Cayman Formation 33m above sea level. Subsequent karst development, which is still ongoing today, removed most of the Cayman Formation on the eastern part of the island and produced peripheral rims that are higher than those on Grand Cayman. During some lowstands, like that between the Late Pliocene and ~400ka, coeval coastal erosion led to the development of coastal benches that cut into the older carbonate strata. The combination of karst development in the islands' interiors and coastal erosion produced complex, rugged topographies that strongly influenced patterns of deposition during the following highstand. The complex stratigraphic architecture of the carbonate successions on the Cayman Islands reflects the variable impact of tectonics, karst development, and coastal erosion that was associated with each lowstand–highstand cycle.

Storm erosion during the past 2000 years along the north shore of Delaware Bay, USA

The recent impacts of tropical cyclones and severe storms on the U.S. Atlantic coast brought into focus the need for extended records of storm activity from different geomorphologic settings. Such reconstructions are typically developed from sites that experienced repeated overwash of sand into low-energy, depositional environments. However, salt-marsh sediment may also preserve a record of repeated erosion from tropical cyclones and storms. We describe late Holocene sediments beneath the Sea Breeze salt marsh (Delaware Bay, New Jersey) from more than 200 gouge cores positioned along seven transects. The stratigraphic record documents at least seven depositional sequences consisting of salt-marsh peat and mud couplets that represent dramatic changes in sedimentation regime. There are a number of processes that could produce these stratigraphic sequences against a background of rising relative sea level including: lateral migration of tidal creeks; tidal channel network and/or drainage ditch expansion; changes in sediment delivery rates; rapid relative sea-level change; tsunami; and formation of salt pans. The abrupt contacts between the salt-marsh peat and overlying intertidal mud suggest that erosion of the peat was followed by rapid infilling of accommodation space. Correlation of erosional surfaces across 2.5km suggests a common mechanism and we propose that the erosion was caused by tropical cyclones and/or storms. We developed a chronology of repeated salt-marsh erosion and recovery using 137Cs, metal pollution (Pb concentration and stable isotopes), and radiocarbon data. Two recent episodes of salt-marsh erosion may correlate with historic tropical cyclones in AD 1903, and AD 1821/1788 that impacted the Atlantic coast of New Jersey, but the erosive nature of the Sea Breeze site hinders definitive correlation. Prehistoric erosional sequences correlate with overwash fans preserved in the regional sedimentary record. We estimated that it takes from several decades to almost 200years for salt-marshes to recover from storm erosion.

Reconstructing the Avalonia palaeocontinent in the Cambrian: A 519 Ma caliche in South Wales and transcontinental middle Terreneuvian sandstones

AbstractAn Early Cambrian caliche on the St Non's Formation (emended) is the base of the Caerfai Bay Formation (unit-term changed) at Caerfai Bay, South Wales. Subaerial exposure and the caliche mean the two formations were not genetically related units. The St Non's is an older sand sheet (likely tidalitic, not delta-related) referred to Avalonian depositional sequence (ADS) 2, and the Caerfai Bay is a shallow mud basin unit refered to ADS 4A. The similar Random Formation (upper ADS 2) in North American Avalonia has a basal age ofc. 528 Ma and is unconformably overlain by red mudstones or sandstones in fault-bounded basins on the Avalonian inner platform. Coeval British sandstones (lower Hartshill, Wrekin, St Non's, Brand Hills?) are unconformably overlain by latest Terreneuvian (ADS 3) or Epoch 2 (ADS 4A) units. Dates of 519 Ma on Caerfai Bay ashes give an upper bracket on the late appearance of Avalonian trilobites and suggest an ADS 2–4A hiatus of several million years. Post-St Non's and post-Random basin reorganization led to abundant Caerfai Bay Formation volcanic ashes and sparse Brigus Formation ashes in Newfoundland. The broad extent of erosional sequence boundaries that bracket lithologically similar to identical units emphasize that ‘east’ and ‘west’ Avalonia formed one palaeocontinent. The inner platform in southern Britain was larger than the Midlands craton, a tectonically defined later Palaeozoic area unrelated to terminal Ediacaran – Early Palaeozoic depositional belts. The cool-water successions of Early Palaeozoic Avalonia were distant from coeval West Gondwanan carbonate platforms.

Sedimentological, palynological and geochemical studies of the terrestrial Triassic–Jurassic boundary in northwestern Poland

AbstractThe Kamień Pomorski IG-1 borehole (Pomerania, NW Poland) yields a profile through the Triassic–Jurassic (T–J) transition in continental deposits. An integrated study of the sedimentology, sequence stratigraphy, palynology, biostratigraphy and geochemistry of these deposits has been carried out on the boundary interval, which represents a time of major environmental change. Two lithological units within the transitional section are distinguished: the Lower–Middle Rhaetian Wielichowo Beds of alluvial plain facies, which shows evidence of a semi-arid climate, and the Upper Rhaetian to Lower Hettangian Zagaje Formation, lying above a marked erosional sequence boundary, composed of mudstone-claystone and sandstone deposited in a fluvial-lacustrine environment. Carbon isotope values obtained from palynomaceral separates, and thus reflecting isotopic changes in atmospheric CO2, show significant fluctuations through the Rhaetian; the most conspicuous negative δ13Corgexcursion is correlated with the Rhaetian ‘initial’ excursion and shows two sub-peaks, pointing to short-term carbon-cycle disturbances of lesser magnitude. Above the ‘initial’ negative excursion, there is a positive excursion followed again by more negative values, representing subordinate fluctuation within a positive excursion and is correlated with the T–J boundary. Seventy-two miospore taxa have been determined from the studied T–J transitional section. Two major palynological assemblages have been distinguished: the lower one, typically Rhaetian, named theCingulizonates rhaeticus–Limbosporites lundblandiiassociation, which corresponds to theRhaetipollis–Ricciisporites(=Rhaetipollis–Limbosporites) Zone; and the upper one, typically Hettangian, named theConbaculatisporites mesozoicus– Dictyophyllidites mortoni–Cerebropollenites thiergartiiassociation (with the age-diagnostic pollenC.thiergartii), which corresponds to thePinuspollenites–Trachysporites(= Trachysporites–Heliosporites) Zone. The T–J palynofloral turnover occurred in a humid period and is more conspicuous then palynofloral changes observed in Greenland, the Tethyan domain or other parts of NE Europe. The osmium isotope system is studied herein for the first time from T–J continental deposits and shows marked disturbances similar to those measured in marine deposits and attributed to volcanic fallout. Carbon and osmium isotope correlation and coeval increase in polycyclic aromatic hydrocarbon (PAH) content and darkening of miospores confirm that eruptions of the Central Atlantic Magmatic Province (CAMP) contributed to the perturbances in climate and crisis in the terrestrial biosphere. A series of periodical atmospheric loading by CO2, CH4or alternatively by SO2, sulphate aerosols and toxic compounds is inferred to have caused a series of rapid climatic reversals, directly influencing the ecosystem and causing the Triassic floral crisis. A floral turnover period commenced at the ‘initial’ δ13C excursion, with the onset of CAMP volcanism. Obtained values of initial187Os/186Os between 2.905 and 4.873 and very low iridium content (about 5 ppt) lend no support to a role for an extraterrestrial impact at the T–J boundary event. The position of the ‘initial’ negative carbon isotope excursion about 12 m below the T–J boundary, position of sequence boundaries (emergence surfaces) and other isotope excursions allow reliable correlation with marine profiles, including St Audrie's Bay (UK), Csövár (Hungary) and the GSSP profile at Kuhjoch (Austria).

Numerical age calibration of the Albian/Cenomanian boundary

New biostratigraphic and sequence stratigraphic correlations of the U.S. Western Interior Albian/Cenomanian sections with North Texas and European sections calibrate the Albian/Cenomanian boundary at 97.2 Ma, which supports previous correlations in the Western Interior. This age, however, is 2.4 myr younger than the recent calibration at 99.6 Ma derived from radiometric ages of volcanic beds with diagnostic marine fossils in northern Japan. This discrepancy suggests that additional analyses are required to test the age calibration of the geologic time scale. New data of cosmopolitan dinoflagellates enable direct correlation with the European reference sections of the Albian and Cenomanian. Previous correlations in the Western Interior were based on endemic or Boreal ammonites. The first and last occurrences of dinoflagellate species bracket the Albian/Cenomanian boundary in Europe and many of these species also bracket the Clay Spur Bentonite bed previously dated 97.17±0.69 Ma. The high number of radiometrically dated bentonites in the Western Interior provides a numeric age control for the ranges of many dinoflagellates. Sequence stratigraphic correlation of the North Texas section with the U.S. Western Interior sections also demonstrates that the Clay Spur Bentonite bed correlates with the Ablian/Cenomanian boundary in Texas defined by the ammonite succession. The flooding contact SB WA6 in Texas at the Albian/Cenomanian zonal boundary of Stoliczkaia dispar and Mantelliceras mantelli correlates into the Western Interior with an erosional sequence boundary SB4 at the base of the Romeroville Sandstone in New Mexico and the Dakota Sandstone in Colorado. This contact correlates with the base of the Belle Fourche Shale that overlies the Clay Spur and the Mowry Shale in Montana. The revised correlation of the Western Interior poses two questions. 1) What is the accurate age of the boundary? Both cannot be correct. 2) What were the durations of the Albian and Cenomanian stages? Cyclostratigraphic analyses estimate that the Albian was 11.9 to 11.6 myr in duration but the Western Interior data project the duration at about 15 myr.

Environmental and biotic changes in subtropical isolated carbonate platforms during the Late Silurian Kozlowskii Event, Prague Basin

The Kozlowskii Event extinction in the Prague Basin is not prominent in number of extinct taxa among Ludfordian extinctions, but by its short duration and changes in community structure in both nektonic, pelagic and benthic faunas of different depth zones. The number of taxa going extinct is relatively low because some sensitive benthic and nectobenthic organisms (e.g., trilobites, cephalopods, gastropods, brachiopods) disappeared already at the base of the Neocucullograptus kozlowskii Zone contemporaneous with the beginning of a sea level highstand. The Kozlowskii Event includes two phases of extinction coinciding with two erosional sequence boundaries. The event also coincides with climatic changes and alterations of the ocean current regime. Sedimentary facies indicate that the first phase of extinction in the upper N. kozlowskii Zone took place during a sea level highstand situation. A lowstand with associated reduction in current activity occurred during the second phase of extinction in the lowermost Pseudomonoclimacis latilobus Zone. Recovery was completed in the upper P. latilobus Zone. The Kozlowskii Event is in the Prague Basin is accompanied by an abrupt change of benthic and pelagic communities and by immigrations from Baltica, Avalonia and Laurentia. The initial δ13C excursion correlates in the Prague Basin with the uppermost N. kozlowskii Zone after the last appearance datum of the conodont Polygnathoides siluricus.

Responses of Two New Hampshire (USA) Lakes to Human Impacts in Recent Centuries

This study contrasts the effects of human disturbances on two very different lake/catchment systems: Hatch Pond and Beaver Lake, New Hampshire. Hatch is in a steep mountain catchment remote from urban/industrial centers, and is a small, relatively deep, primarily seepage lake with moderate flushing rate. The catchment has been continuously forested, but disturbed by partial loggings and a forest fire. Beaver’s more extensive and gently sloping catchment is much closer to urban/industrial centers, and has been agricultural and recently suburban. Beaver is a larger but shallower drainage lake with much more rapid flushing than Hatch. We compared the sedimentary records and inferred limnological responses to catchment land uses and air pollution inputs of these mesotrophic lakes, and interpreted differing responses as a function of basic differences between the lakes and their catchments. Some chemical and biological variables in the sediment of both lakes respond sensitively to the first disturbances of the catchments by Euro-American settlers. Diatom-inferred Cl at both lakes indicates a modest increase of salinity at this time. Hatch sediment contains a record of soil erosion starting \(\simeq\) 1810 with first logging and fire. For ~125 years, erosion was largely of upper eluviated soil, but by \(\simeq\) 1935 it reached deeper into the illuviated (enriched) horizon. Maximal lake trophic state based on diatom-inferred limnological variables occurred in the mid-1900’s period of maximum sedimentation of illuviated soil. The sediment record since \(\simeq\) 1964 reflects forest maturation, and soil stabilization and acidification, but not lake acidification. At Beaver, the limnological effects of forest clearance and agriculture starting ~1700 were relatively mild due to gentleness of slopes and soil characteristics favoring stability. A near doubling of earth elements, and major increases in trace metal pollutants and coal combustion particles (CCP) occurred in sediment dating around 1900. Landscape analysis as well as historical and sedimentary records preclude the catchment as the major source of these increases. The most likely source is atmospheric fallout of CCP with its metal load. Despite controls on CCP and other emissions starting 1960, concentrations of earth elements and trace metals in Beaver sediment remain high due to soil mobilization by residential development of the catchment. The trophic state of Beaver Lake has increased, but the relative increase has been smaller than at Hatch, despite more intensive land uses and greater aerial inputs at Beaver. We propose that this lesser eutrophication is due to rapid flushing of Beaver Lake with stream water from relatively undeveloped parts of the extensive catchment, and lower sensitivity to nutrient inputs of this naturally more eutrophic lake. A major shift from unicellular to colonial chrysophytes at Hatch starts in the late-1800’s, and at Beaver in the early 1900’s. Colonial taxa in lesser quantities appear at the time of first settlement of the Beaver catchment. At both lakes the shifts are correlated with increasing metal fluxes from the opening of catchment biogeochemical cycles, as with the aforementioned erosional sequence at Hatch. But at Hatch aerial inputs of trace metals, and at Beaver aerial inputs major and trace metals associated with CCP are also correlated with the major chrysophyte shift. It appears that the chrysophytes have responded to stressors associated with both catchment disturbance and regional atmospheric inputs.

Depositional Patterns Across Syndepositional Normal Faults, Niger Delta, Nigeria

Abstract Niger Delta depositional patterns are complicated by structural collapse of prograding deposits into underlying mobile shale. Delta field, along the northwest margin of Niger Delta, is located on the dominantly extensional proximal margin of a major collapse structure. This structure defines a sub-basin depobelt within this prograding clastic wedge. The Miocene deltaic Agbada Formation was deposited on a succession of cuspate normal faults displaced basinward under the weight of accumulating sediments. This nearly 2-km-thick succession contains five erosionally bounded sequences. Where not deeply incised, successive sequences become progressively thinner upsection, and change from dominantly upward-coarsening deltaic successions to dominantly channel sandstones interbedded with paralic mudstones. Older sequences thicken basinward to a greater extent onto downdropped fault blocks and across crests of associated rollover anticlines. Basal erosion of younger sequence boundaries is narrower and deeper along elongate trends extending landward into footwall blocks. Along the axis of deepest sequence-boundary incision, fills contain a relatively thin layer of basal, high-sinuosity channel sandstones that are abruptly overlain by shale. The rest of the deposits within sequences are separated vertically into two parts: a lower part defined by hundred-meter-thick, basinward-inclined bed sets that coarsen upward, and an upper part with higher-frequency parallel seismic reflections that record vertical aggradation of growth strata after erosional basin-floor topography was buried. Broad constraints on the age of deposits from biostratigraphic studies suggest a tentative link between sequence-boundary development and sea level but do not constrain the timing of sequence incision and filling relative to sea-level stage. Vertical facies changes within individual sequences do not support formation of erosional sequence boundaries by lowstand fluvial incision; rather, sequence boundaries are interpreted to form by faulting of the sea floor, the slumping of rapidly accumulated sediments and erosion by submarine mass-flow channels into footwall blocks. Deeply incised sequence boundaries may be more likely to develop during initiation of a new depobelt, as deltas first prograde to the shelf edge onto undercompacted shales.

Use of spectral gamma-ray data to refine subsurface fluvial stratigraphy: late Cretaceous strata in the Book Cliffs, Utah, USA

High-resolution (sub-seismic) stratigraphic correlation in fluvial strata is extremely difficult using conventional subsurface core and wireline-log data (e.g. gamma-ray, neutron, density, sonic logs). Spectral gamma-ray logs provide additional data on lithological composition that can aid identification of chronostratigraphic surfaces in a high-resolution sequence stratigraphic correlation framework. The application of spectral gamma-ray data in constructing such a framework is demonstrated using data from exposures and the subsurface from the Cretaceous non-marine Blackhawk Formation and Castlegate Sandstone of the Book Cliffs area, Utah, USA. These strata provide an ideal test case for the use of spectral gamma-ray data in fluvial stratigraphy, because interpretations can be compared directly with a high-resolution sequence stratigraphic framework from coeval shallow-marine strata exposed nearby. The stratigraphic abundance of coals and relatively uniform Th/K ratio, which indicates uniform clay mineralogy, in mudstone sections of the non-marine Blackhawk Formation implies a humid, subtropical climate throughout its deposition. Variations in facies architecture and stratigraphy are therefore interpreted to have been driven principally by fluctuations in relative sea level and hinterland tectonics. Fluvial and fluvio-tidal sandstones within incised valleys, which lie above erosional sequence boundaries, have a highly variable spectral gamma-ray character, but locally are marked by distinctive heavy mineral lags and concentrations (Th consistently >3 ppm). On the interfluves of incised valleys, sequence boundaries are marked by palaeosols that locally have a distinctive leached, potassium-depleted character (Th/K ratio >17). Lateral variability in spectral gamma-ray character of sequence boundaries is consistent with facies and sequence stratigraphic models of the non-marine Blackhawk Formation and Castlegate Sandstone, but sequence boundaries do not exhibit a clear, diagnostic character in every location. In addition, high Th/K ratios of the type noted at interfluve palaeosols may be produced by potassium depletion beneath coal seams as a result of acid groundwater percolation during early diagenesis, and by thorium enrichment in heavy mineral lags within foreshore sandstones. These alternative interpretations can be eliminated if high Th/K ratios occur in intervals lacking coal seams and shallow-marine sandstones, respectively. Thus, within the context of a sound understanding of detrital mineralogy, diagenetic history, facies and appropriate sequence stratigraphic models, spectral gamma-ray data allow sequence boundaries to be identified and correlated in non-marine fluvial strata at least 80 km from the interpreted coeval shoreline.

Depositional and Stratigraphic Architecture of the Santonian Emery Sandstone of the Mancos Shale: Implications for Late Cretaceous Evolution of the Western Interior Foreland Basin of Central Utah, U.S.A.

Abstract The Santonian Emery Sandstone Member of the Mancos Shale is a continental to shallow marine succession that prograded into the Cretaceous Western Interior Seaway of western North America. Sediment sourced from the uplifted Sevier Orogen to the west fed a shoreline in the adjacent subsiding north–south trending foredeep. These sandstones are now superbly exposed along a 120-km-long, strike-parallel, continuous outcrop in the cliffs of the Wasatch Plateau of central Utah. Shoreface parasequences are composed of coarsening-upward successions of hummocky cross-stratified and intensely bioturbated sandstones, indicating deposition at a storm-dominated shoreline. Two distinct depositional complexes record eastward and northeastward shoreline progradation at the northern and southernmost extremities of the Plateau respectively. In the subsurface, Emery strata comprise a thick succession of coal-bearing, coastal-plain deposits. The Emery Sandstone records a period of long-term paralic progradation to aggradation, containing at least 17 parasequences arranged into three, 600 kyr, regressive–transgressive cycles that are recognizable in both complexes. Each cycle comprises only highstand and transgressive systems tracts, and erosional sequence boundaries are absent. Absence of erosional sequence boundaries is attributed to high rates of subsidence in the foreland basin at this time. Santonian glacio-eustatic sea-level fluctuations are proposed to account for the high-frequency cyclicity, whilst longer-term aggradation may be a function of organic controls on shoreface stacking patterns by immobile raised coal mires lying landward of the shorefaces. When placed in a temporal and spatial context, the Emery Sandstone provides a missing link in the understanding of long-term controls on Late Cretaceous depositional systems in the central Utah segment of the foreland basin, and allows inferences to be made regarding foreland basin dynamics. In particular, thrust tectonics is postulated to have played a major role in governing the architecture of Late Cretaceous strata in central Utah. Marked along-strike thickening of the Emery Sandstone in particular is attributed to long-lived elevated rates of subsidence in northern Utah and southern Wyoming during the Late Cretaceous that is adjacent to concentrated zones of thrust nappes. A north-dipping depositional slope is inferred from this subsidence distribution, aligned approximately parallel to the structural grain of the hinterland load. Northeastward progradational directions of successive Turonian, Santonian, and early Campanian regressive wedges in this part of the Western Interior Basin are attributed to the presence of this tectonically induced regional slope gradient. Sediment sourced to these Late Cretaceous shorefaces is likely to have been sourced through structural recesses within the thrust front, resulting in a spatial alignment of several successive regressive wedges and implicates an important, long-lived kinematic control on sediment supply. Of Late Cretaceous regressive wedges, only the Emery Sandstone does not temporally coincide with reported periods of active thrusting, suggesting that the regression was a consequence of factors other than uplift-related, elevated rates of sediment supply.

Martian hillside gullies and icelandic analogs

We report observations of Icelandic hillside gully systems that are near duplicates of gullies observed on high-latitude martian hillsides. The best Icelandic analogs involve basaltic talus slopes at the angle of repose, with gully formation by debris flows initiated by ground water saturation, and/or by drainage of water from upslope cliffs. We report not only the existence of Mars analog gullies, but also an erosional sequence of morphologic forms, found both on Mars and in Iceland. The observations support hypotheses calling for creation of martian gullies by aqueous processes. Issues remain whether the water in each case comes only from surficial sources, such as melting of ground ice or snow, or from underground sources such as aquifers that gain surface access in hillsides. Iceland has many examples of the former, but the latter mechanism is not ruled out. Our observations are consistent with the martian debris flow mechanism of F. Costard et al. (2001c, Science 295, 110–113), except that classic debris flows begin at midslope more frequently than on Mars. From morphologic observations, we suggest that some martian hillside gully systems not only involve significant evolution by extended erosive activity, but gully formation may occur in episodes, and the time interval since the last episode is considerably less than the time interval needed to erase the gully through normal martian obliteration processes.

Tectonic Control of Facies Architecture in Falling-Stage Deposits in A Forearc Basin: Upper Miocene Senhata Formation, Boso Peninsula, Japan

ABSTRACT The Upper Miocene Senhata Formation is the infill of the Middle Miocene-Middle Pliocene forearc basin that developed in response to the north ward to northwestward subduction of the Philippine Sea plate beneath the Eurasia plate at the Sagami Trough. The formation has been affected by anticlinal and synclinal deformation and provides an opportunity for analyzing three-dimensional variations in thickness, stacking patterns, and bounding-surface features of component lithofacies associations. The lower part of the formation consists mainly of shoreface and shelf deposits and indicate falling-stage deposits that are overlain by transgressive shoreface and shelf deposits of the upper Senhata Formation, with the contact defined as a wave-ravinement surface. Forced-regressive shoreface-to-shelf deposits of the lower Senhata Formation are characterized by an erosional basal surface of a Type 1 sequence boundary and lateral change to multiply stacked, thicker regressive deposits with a gradational basal contact with the underlying shelf and upper slope deposits, indicating progradational and aggradational stacking of shoreface and shelf deposits, which are interpreted to be equivalent to the shelf-margin systems tract of a Type 2 sequence. The erosional sequence boundary at the base of forced-regressive deposits coincides laterally with a transgressive surface of erosion to the northwestern upslope area, where the forced-regressive deposits are absent. The lateral variation in the falling-stage regressive deposits of the lower Senhata Formation is interpreted to indicate that falling-stage and lowstand-stage deposits of Types 1 and 2 sequences are largely contemporaneous and their development is controlled by spatial variation in the rate of basin subsidence in both depositional-dip and depositional-strike directions.

ABSTRACT: Sequence Stratigraphic Framework of a Shelfal Siliciclastic Succession in an Upper Cretaceous-Lower Tertiary Salt Diapir Influenced Basin, La Popa Basin, Northeastern Mexico

La Popa Basin, Nuevo Leon, Mexico, is a salt influenced foreland basin with excellent exposures around several salt stalks, a 25 km salt wall and syndiapiric strata. The Delgado Sandstone Tongue, Potrerillos Formation is a shelfal sandstone/siltstone deposited during diapir growth in a complex, wave-dominated, eastward-prograding deltaic depositional system. Typical parasequences comprise offshore shale overlain by lower to upper shoreface sandstone. The Delgado Sandstone contains a lower, progradational parasequence set as part of a highstand system tract that is part of the underlying Middle Siltstone Member of the Potrerillos Formation and an upper retrogradational set that is part of a transgressive system tract that persists into the Upper Mudstone Member. The two system tracts are separated by an erosional sequence boundary that incises up to 10 m of underlying strata and locally preserves a lowstand system tract within incised valleys. Sequence stratigraphic correlation of 30 measured sections documents that: 1) strata proximal to El Gordo salt stock and La Popa salt wall show depositional thinning within a radius of 1 km, higher-energy and shallower-water lithofacies, and a difference in stratal stacking patterns toward the diapirs; and 2) strata around El Papalote salt stock exhibit thinning within a radius of 0.3 km, no change in lithofacies approaching the diapir, and similar stacking patterns on both flanks. These data suggest that the salt bodies had different bathymetric expressions during deposition of the Delgado Sandstone and may have had different uplift styles and/or rates.