Peak Transgression Research Articles

Stratigraphy and hydrocarbon resources of the San Juan Basin: Lessons for other basins, lessons from other basins

This paper examines the relationships between stratigraphy and hydrocarbon production from the San Juan Basin of New Mexico and Colorado. Abundant data and the long production history allow lessons to be learned, both from an exploration and development perspective, that can be applied in other basins. Conversely, as new play types and technologies are defined and developed elsewhere, the applicability of those tools in the San Juan Basin needs to be understood for well-informed exploration and development activities to continue. The San Juan Basin is a Latest Cretaceous – Tertiary (Paleogene) structure that contains rocks deposited from the Lower Paleozoic to the Tertiary, but only the Upper Cretaceous section has significant hydrocarbon, mostly gas, production. Herein I make the case for studying depositional systems, and the controls thereon (e.g., basin development, eustasy, sediment supply), because they are the first-order controls on whether a sedimentary basin can become a hydrocarbon province, or super basin as the San Juan Basin has recently been defined. Only in the Upper Cretaceous did a suitable combination of forcing mechanisms combine to form source and reservoir rocks, and repeated transgressive-regressive cycles of the Upper Cretaceous stacked multiple successions of source and reservoir rocks in a way that leads to stacked pay potential. Because of the types of depositional systems that could develop, the source rocks were primarily gas prone, like those of other Rocky Mountain basins. Oil-prone source rocks are present but primarily restricted to episodes of peak transgression. A lack of suitable trapping mechanisms helps to explain the relative dearth of conventional oil pools. Although gas production has dropped precipitously in the past decade, driven primarily by overabundance of gas supply associated with the shale-gas boom, the combination of horizontal drilling and multi-stage hydraulic fracturing is being applied to revive oil production from some unconventional stratigraphic targets with success.

Assessing the contribution of the La Luna Sea to the global sink of organic carbon during the Cenomanian-Turonian Oceanic Anoxic Event 2 (OAE2)

Epicontinental seas were important features of the paleogeographic landscape during the Cretaceous; however, the role they played as sinks of organic carbon is still poorly understood. The La Luna Formation (Albian-Coniacian) is a series of organic-rich limestones deposited in northwestern South America on an epicontinental sea (the La Luna Sea). This formation offers a forty-million-year continuous record of environmental change characterized by periods of oceanic anoxia in an epicontinental sea. The La Luna Sea, may have played an important role –although so far unexplored– in carbon cycling through the ocean during the Cretaceous, specifically during short-term, global-scale disruptions in the carbon cycle known as oceanic anoxic events (OAEs). To evaluate the role of the La Luna Sea in global carbon cycle perturbations, we conducted a detailed lithological and chemostratigraphic analysis of two stratigraphic sections from the Upper Magdalena Basin of Colombia, both of which encompass the Oceanic Anoxic Event 2 (OAE2) at the Cenomanian–Turonian boundary. Compared to deposits in the modern ocean, the La Luna Formation has high total organic carbon (TOC) before, during, and after OAE2. Foraminifera and nannoplankton assemblages also imply a stressed upper water column during OAE2. Geochemical and paleontological evidence suggests that the sediment-water interface was anoxic across the late Cenomanian and early Turonian. Strata deposited just after OAE2, however, contain inoceramid bivalves, consistent with short-lived re‑oxygenation of the benthic layer. Estimates of primary productivity, the covariation of Mo and U enrichment factors, and relations between Cd, Mo, Co, and Mn also reveal that the La Luna Sea was biogeochemically similar to the modern Cariaco Basin. Despite high concentrations of organic carbon found in the La Luna Formation, mass-accumulation rates of organic carbon are low, a finding that can be explained by a reduction in the accumulation rate of sediments caused by the peak of sea-level transgression that took place at the Cenomanian–Turonian transition. Based on the areal extent of the La Luna Sea and mass-accumulation rates of organic carbon, 1.7 Eg of C were removed from the ocean over 500 ky and deposited in the La Luna Sea. Interestingly, although the La Luna Sea was one-third the size of the Western Interior Seaway (WIS), the amount of organic carbon buried in the WIS during OAE2 was similar (1.4 Eg of C). In these two epicontinental seas, 3.1 Eg of C were removed from the ocean during OAE2, accounting only for 3.4% of the total C needed to cause a perturbation of the carbon cycle similar to that observed during OAE2. The low amount of organic carbon buried in the La Luna Sea and the WIS suggests that neither of these inland seas were responsible for the efficient removal of organic carbon from the ocean during OAE2. This conclusion challenges the explanation that epicontinental seas were major sinks of organic carbon—and therefore they did not play a significant role in the carbon cycle during the Mesozoic OAEs and other disruptions of the carbon cycle in Earth's history.

Genesis and paleoenvironmental analysis of the ironstone facies of the Maastrichtian Patti Formation, Bida Basin, Nigeria

The recently constructed Lokoja-Abuja highway and a quarry which cut through the Patti Formation, provided opportunity for the field study and sampling of the exposed ironstone interbeds within the carbonaceous shales and clays in Ahoko-Idu axis of the southern Bida Basin, Nigeria. The origin and paleoenvironmental significance of the ferruginous intervals were examined in this paper. Investigation reveals that the ferruginous bands occur as three lithofacies comprising of laminated, bioturbated and spheroidal concretionary facies. The primary components and depositional features of the host shales (detrital minerals, wood and herbaceous fragments, laminations) are largely preserved in the ironstones indicating compositional similarity and therefore a diagenetic origin for the ironstones. Results of XRD reveal substantial amount of siderite and illite but minor amounts of kaolinite, hematite, calcite and magnetite. Major elements such as Fe2O3, SiO2, Al2O3, CaO, MgO and P2O5 range from 41.50 to 54.60%, 6.10–23.30%, 3.64–7.00%, 1.40–13.60%, 0.30–1.20% and 0.10–8.80% respectively. The current data is suggestive that kaolinite was diagenetically replaced by siderite at early stage of burial in a reducing environment. Migrating pore water, probably enriched with alkali metals like Ca and Mg, sourced from sea water in the depositional basin and abundance of organic matter in the primary sediments are interpreted to be part of the factors responsible for the siderite precipitation. Both the host shale and the ferruginous beds contain significant amount of MgO, CaO, relics of dissolved bioclasts and body fossils. The geochemical proxies combined with the presence of magnesian siderite in the ironstones indicate that the host shales were deposited in marine environment with occasional freshwater influence. The study concludes that the ironstones are marine to non-marine, sideritic and diagenetic. The laminated and fossiliferous ironstone facies was deposited at the peak of marine transgression during the Upper Cretaceous time and represents maximum flooding surface. Shallowing up and regression into fresh water swamp condition was marked by the occurrence of freshwater sideritic ironstone.

The Cretaceous marine onlap on Palaeozoic deposits (Smara–Lâayoune Basin, South Morocco). Comparison with neighbouring regions

The Cretaceous marine transgression proceeded through successive steps from the Albian to the Turonian (dated with ammonites). The onlapping wedge begins with coastal transgressive–regressive short-term sequences on massive, probably fluvial sandstones to be correlated with the very thick continental Lower Cretaceous succession found in the Puerto Cansado well in the Tarfaya sub-basin to the north. A second step, of probable Cenomanian age, reached the Palaeozoic basement. A third, more pronounced step occurred during the earliest Turonian with platy laminated limestone overlain by marlstone bearing pyritized ammonites. At early Turonian peak transgression, a marine connection was possibly established between the Atlantic and the Tethyan margins, between the Anti-Atlas and the Reguibat Shield. From large-scale correlation integrating what occurred along the southwestern shoulder of the Atlas rift, the South Moroccan Atlantic margin may have undergone a short-lived tectonic uplift around the Cenomanian–Turonian boundary.

Controls on accommodation during the early-middle Campanian in southern Alberta, western Canada foreland system

The Western Canada Sedimentary Basin is a retroarc foreland system produced by thrust sheet loading in the Canadian Cordillera and resultant flexural displacement of the lithosphere east of the orogenic front. Theoretical models of flexural tectonism and dynamic loading have long been used to predict the effects of these controls on the creation/destruction of accommodation in retroarc foreland settings, however, few field studies have attempted to evaluate their relative contributions for a given point in geologic time. As key regional drivers of subsidence/uplift and implicitly, relative sea level rise and fall, flexural tectonism and dynamic loading impart unique signatures (subsidence geometries) which can be discerned via the spatial and temporal analysis of event-significant surfaces preserved in the geologic record. We use magnetostratigraphy to constrain the timing of a major marine incursion (transgressive phase of the second-order Campanian Claggett Cycle, which spanned ∼8 Myr in its entirety) at three locations in the southern Alberta plains where the onset of this transgression is marked by a widespread ravinement surface. A chronostratigraphic framework is presented which includes new empirical ages for stratigraphic units above and below this key surface, informally termed the Milk River ‘shoulder’. The framework provides a basis for determining the earliest extents of the Claggett (Pakowki/Lea Park) sea in southern Alberta, and for characterization of its subsequent expansion during the early-middle Campanian. Although commonly depicted as a simple, westward-expanding seaway, we observe that the initial expansion of the Claggett sea in southern Alberta was more complicated and proceeded in multiple directions through time, prior to maximum transgression and the ensuing second-order regression. We interpret the Claggett sea as restricted to the Alberta foredeep at ∼82.0 Ma, before inundating a paleotopographic high associated with a prominent basement feature in southernmost Alberta (Sweetgrass Arch) at ∼81.0 Ma, likely establishing a connection with the Claggett sea in northern Montana. As relative sea level continued to rise, the sea reached the Rocky Mountain foothills in the west while also advancing more than 100 km eastward into the forebulge region during peak transgression (∼80.0 Ma). With this history established, we ascertain the overriding cause for the Claggett transgression was dynamic subsidence because (1) the magnitude, direction, and timing of relative sea level rise and the associated expansion of the Claggett sea fits published dynamic subsidence models for the Campanian of North America, and (2) several hundred metres of marine sediments are preserved on the early-middle Campanian forebulge, requiring accommodation which cannot be explained by widely accepted flexural models or eustatic estimates.

Water-mass evolution in the Cretaceous Western Interior Seaway of North America and equatorial Atlantic

Abstract. The Late Cretaceous Epoch was characterized by major global perturbations in the carbon cycle, the most prominent occurring near the Cenomanian–Turonian (CT) transition marked by Oceanic Anoxic Event 2 (OAE-2) at 94.9–93.7 Ma. The Cretaceous Western Interior Seaway (KWIS) was one of several epicontinental seas in which a complex water-mass evolution was recorded in widespread sedimentary successions. This contribution integrates new data on the main components of organic matter, geochemistry, and stable isotopes along a north–south transect from the KWIS to the equatorial western Atlantic and Southern Ocean. In particular, cored sedimentary rocks from the Eagle Ford Group of west Texas (∼ 90–98 Ma) demonstrate subtle temporal and spatial variations in palaeoenvironmental conditions and provide an important geographic constraint for interpreting water-mass evolution. High-latitude (boreal–austral), equatorial Atlantic Tethyan and locally sourced Western Interior Seaway water masses are distinguished by distinct palynological assemblages and geochemical signatures. The northward migration of an equatorial Atlantic Tethyan water mass into the KWIS occurred during the early–middle Cenomanian (98–95 Ma) followed by a major re-organization during the latest Cenomanian–Turonian (95–94 Ma) as a full connection with a northerly boreal water mass was established during peak transgression. This oceanographic change promoted de-stratification of the water column and improved oxygenation throughout the KWIS and as far south as the Demerara Rise off Suriname. In addition, the recorded decline in redox-sensitive trace metals during the onset of OAE-2 likely reflects a genuine oxygenation event related to open water-mass exchange and may have been complicated by variable contribution of organic matter from different sources (e.g. refractory/terrigenous material), requiring further investigation.

Intrinsic processes control late Turonian calcareous dinoflagellate cyst assemblages – A case study from the Sussex chalk (England)

Changes in late Turonian calcareous dinoflagellate cyst assemblages (c. 6400 specimens from 45 samples of the 27 m-thick Bridgewick section; Sussex, southern England) were plotted against four sedimentary sequences to test its response to eurybatic sea level changes. Only seven taxa occur, and Pithonella sphaerica, P. ovalis and P. spiralis dominate the flora (c. 98%). Pithonella cardiiformis, P. discoidea, N. circumperforata and Pirumella sp. are rare. The composition is identical to that of Salzgitter-Salder (NW Germany). The ratio P. sphaerica/P. ovalis (Ps/Po ratio) shows positive peaks during eurybatic sea level lows (max. 1.6) and negative peaks during maximum flooding (min. 0.2). The cumulative c-dinocysts composition, CCC (P. sphaerica, P. ovalis, P. spiralis, others), shows a proportional decrease of P. sphaerica and P. ovalis during eurybatic sea level falls and predominance of P. ovalis during sea level high. The species richness curve shows lowest species numbers in the context of peak transgression. Higher species numbers are associated with lower eurybatic sea levels. In conclusion, the Ps/Po ratio, the CCC and species richness in conjunction aid the recognition of eurybatic sea level changes. Due to the remote position of the working area on the well-oxygenated, drowned Turonian shelf, we exclude variations of external nutrient input, salinity, temperature and oxygen changes as a possible trigger for the observed floral changes. Instead, we suggest that intrinsic mechanisms drive the floral dynamics. In the sense of the intermediate disturbance hypotheses, the progressive increase of P. sphaerica, P. spiralis and other during progressive eurybatic sea level fall is triggered by higher disturbance frequencies (episodic water turbulence, resuspension events, increased internal cycling rate of Corg); vice versa, maximum flooding is characterized by lower disturbance frequencies (decrease of episodic turbulences, less internal cycling) and the progressive predominance of the optimal competitor, which is P. ovalis. The presence of seafloor topography (intra-shelf depressions and swells) could have created downslope currents where parautochthonous assemblages would be expected and could explain the offset of depositional dynamics at the seafloor leading to changes in the floral assemblages.

Paleocirculation and foraminiferal assemblages of the Cenomanian–Turonian Bridge Creek Limestone bedding couplets: Productivity vs. dilution during OAE2

The limestone–marlstone (or limestone–calcareous shale) bedding couplets of the lower Bridge Creek Member of the Greenhorn Formation coincide with Oceanic Anoxic Event 2 and the Cenomanian–Turonian stage boundary at 93.9 Ma, and are characterized by fluctuations in microfossil and macrofossil biofacies, and organic carbon. Since G.K. Gilbert (1895), these strongly alternating lithofacies have been attributed to climate and/or productivity cycles. Heretofore, only the calcareous shale and marlstone parts of the Bridge Creek bedding couplets have been quantitatively analyzed for planktic and benthic foraminiferal assemblages. In this study, foraminiferal assemblages extracted from the hard limestone beds are comparable with the muddier lithologies thereby allowing a quantitative evaluation of the foraminiferal response to cyclically changing conditions in the U.S. Western Interior Sea (WIS) that resulted in the deposition of these lithologic couplets. The results reveal a modest cyclical response of foraminiferal assemblages extracted from limestone beds compared to adjacent calcareous shale or marlstone. These include the absence of planktic planispiral morphotypes (Globigerinelloides), increase in the proportion of planktic biserial and triserial morphotypes (Heterohelix and Guembelitria, respectively), and an increase in the proportion of benthics relative to total foraminifera (decrease in percent planktics) in the limestone beds. Such conditions suggest that the limestones may have been more productive than the adjacent shales and marlstones. Reduced surface salinity and greater stratification of the upper water column may have also contributed to the differences in assemblages preserved in the marlstones and calcareous shales. The onset of OAE 2 in the late Cenomanian is marked by an abrupt benthic oxygenation event (‘Benthonic Zone’) as Tethyan waters were drawn well north into the WIS, and cool Boreal waters spread across northwest Europe, known as the Plenus Cold Event. At this time, the WIS became an important ocean gateway for surface ocean circulation with rising sea level that helped facilitate the development and spread of OAE 2. A cyclonic (counterclockwise) gyre circulation in the WIS during deposition of the lower part of the Bridge Creek was driven by the difference between precipitation in the north and evaporation in the south. The gyre is represented by two modes, strong and weak, responsible for deposition of the limestone and marlstone, respectively. For the middle and upper parts of the studied section representing the plateau of OAE 2 and subsequent peak transgression of the WIS, the counterclockwise gyre was driven less by E-P gradient but by the amount of surface runoff from both margins of the WIS with deposition of limestone beds during the wetter (strong) phase and marlstones during the drier (weak) phase. Highest levels of TOC redevelop after OAE 2 in the early Turonian with the incursion or development of an oxygen minimum zone at the time of peak transgression.

Late Quaternary palaeoenvironmental changes documented by microfaunas and shell stable isotopes in the southern Pearl River Delta plain, South China

A high resolution study of sediments, microfaunas (ostracods and foraminifers), and stable isotopes of ostracod shells from core PRD05, sampled from the Da'ao plain of the Pearl River Delta, provides new data about palaeoenvironmental changes during the Late Quaternary. The basal fluvial gravelly sediments of the core, representing the lowest deposits of the Quaternary at the core site, were possibly formed prior to the transgression associated with the last interglacial stage. This fluvial setting changed into a marginal marine setting during the last interglacial stage, any record of calcareous fossils were destroyed by subsequent dissolution during the last glacial phase, when the upper part of the deltaic sediments experienced subaerial oxidation, indicated by a mottled clay layer in the core. The postglacial sea-level rise in the South China Sea began at ~16,700 cal yr B. P. During the first phase of sea-level rise (from ~16,700 to 10,100 cal yr B. P.), a swamp environment developed at the core site. At ~10,100 cal yr B. P., marine waters intruded onto the Da'ao plain and reached the core site through the Modaomen channel. This timing was earlier than in other parts of the Pearl River Delta. Since then, a semi-enclosed estuarine environment developed and was maintained until ~5560 cal yr B. P. During this transgressive interval, short-term environmental fluctuations can be recognized based on microfauna and shell geochemical data. From ~10,100 to 8630 cal yr B. P., the sea-level rose, which was followed by a fall from ~8630 to 8520 cal yr B. P. An expanded transgression occurred between ~8520 and 7900 cal yr B. P. The time intervals from ~8520 to 8200 cal yr B. P. and from ~8080 to 7900 cal yr B. P. were marked by peak transgression. From ~7900 to 5560 cal yr B. P., the core site generally showed a reduced marine influence and enhanced freshwater input. A fluvial environment developed from ~5560 to 3100 cal yr B. P., and was succeeded by an alluvial plain setting.

A Middle Ordovician (Darriwilian) Dysaerobic Brachiopod Assemblage from the Precordillera Terrane of Argentina: Implications for Early Colonization of Deep Waters

A rapid increase of water depth associated with dysoxic bottom conditions took place in the central part of the Precordillera basin during the middle Darriwilian. The peak of transgression and drowning of the carbonate ramp are marked by the transition from the uppermost carbonate beds of the San Juan Formation to a rhythmic alternation of graptolitic black shales and marlstones (base of the Los Azules Formation). This lithofacial change is associated to a drastic faunal shift involving a renewal of the ecologic structure. The carbonate facies dominated by filter-feeding organisms were rapidly replaced by new communities characterized by benthic and pelagic trilobites and low-richness assemblages of minute brachiopods. Two main associations are recognized in the Los Azules Formation: a basal ephemeral Sivorthis-Mendolaspis Association and a more persistent Chilcorthis-Palaeoglossa Association inhabiting deeper and more dysoxic sea-bottoms. The new plectorthoid family Chilcorthidae is established to include the new genus Chilcorthis. Five new species are described: Palaeoglossa minima sp. nov., Cyrtonotreta vasculata sp. nov., Philhedra pauciradiata sp. nov., Sivorthis precordillerana sp. nov., and Bockelia canalis sp. nov. Associate taxa are Anomaloglossa? sp., Cyrtonotreta? sp., Protohesperonomia? sp., Syntrophia? sp. and unidentified Finkelnburgiidae, Polytoechiidae and Aegiromenidae. The first successful deep-water incursions are Floian—Dapingian in age, but from the Darriwilian they have been documented in several paleoplates. These pre-Sandbian brachiopod communities were more restricted geographically than the global Foliomena Fauna with which they share some taxa and similar morphotypes but differ in the higher proportion of linguliform brachiopods and less diverse plectambonitoids.

A possible Late Oligocene–Early Miocene rocky shoreline on Otago Schist

Duntroonian–Waitakian limestone at Kokonga in the Maniototo district, Central Otago, New Zealand is interpreted to be a rocky shoreline facies formed near the time of maximum Cenozoic marine transgression across Zealandia. The limestone, which is correlated with the Otekaike Limestone of North Otago, contains shallow and warm water indicators such as coralline algae, oysters, echinoderms, bryozoa and a foraminiferal assemblage composed almost entirely of Amphistegina. Textural zone 2B Otago Schist cobbles, pebbles and sand within the limestone, often adjacent to invertebrate fossil and coralline algal fragments, were sourced from proximal schist exposures and may be indirect evidence of the limestone having formed near to a low-lying Otago Schist coastline. However, because the precise age of limestone deposition cannot be constrained by fossil assemblages or Sr isotopes and the timing of peak marine transgression is imprecisely known, it is not possible to state whether Otekaike Limestone at Kokonga formed before, during or after the peak of maximum Cenozoic marine transgression.

The Composition and Depositional Environments of Mesoarchean Iron Formations of the West Rand Group of the Witwatersrand Supergroup, South Africa

This paper documents the sedimentological setting, mineralogy, and geochemistry of several iron formation units interbedded with siliciclastic strata of the Mesoarchean Witwatersrand Supergroup, well known for its world-class conglomerate-hosted Au-U deposits. Four major iron formation beds, with associated magnetic mudstones, are present in two distinctly different lithostratigraphic associations, namely shale- and diamictiteassociated iron formation. The shale association is represented by the Water Tower and Contorted Bed iron formations in the Parktown Formation of the Hospital Hill Subgroup in the lower part of the succession and the diamictite association by the Promise and Silverfield iron formations in the overlying Government Subgroup. The iron formation units have been subjected to lower greenschist facies metamorphism. Oxide (magnetite and limited hematite), carbonate, and silicate facies iron formations are recognized. The iron formations typically overlie major transgressive flooding surfaces in the succession and, in turn, form the base of progradational coarsening-upward increments of sedimentation comprising magnetic mudstone, nonmagnetic shale, and interbedded siltstone-quartzite. The upward transition from iron formation into magnetic mudstone is accompanied by a change in mineralogical composition from hematite-magnetite iron formation at the base in the most distal setting through magnetite-siderite- and siderite-facies iron formation in the transition zone to magnetic mudstone. The siderite with associated ankerite displays highly depleted δ13C values, suggesting crystallization via iron respiration in presence of organic carbon. The iron formations display positive postArchean Australian shale-normalized Eu and Y anomalies with depletion in light rare-earth elements relative to heavy rare-earth elements, indicating precipitation from marine water with a high-temperature hydrothermal component. Integration of sedimentological, petrographic, and geochemical results indicates that the shale-associated iron formation was deposited during the peak of transgression, when reduced iron-rich hydrothermal waters entered the Witwatersrand Basin over a limited vertical extent due to neutral buoyancy, with the top of the plume occurring below the photic zone. It is suggested that chemolithoautotrophic iron-oxidizing bacteria, which would have been able to exploit the difference in chemistry between the iron-enriched plume water and ambient ocean water to fuel metabolic activity in the presence of limited free molecular oxygen, were responsible for precipitation of initial ferric iron oxyhydroxides. The vertical facies associations in the iron formations most likely developed in response to the limited vertical extent of the hydrothermal plume, with (from distal to proximal) hematite preserved where the base of the plume was not in contact with the basin floor, magnetite where the plume water was in contact with bottom sediment, iron-rich carbonates where organic carbon input was high, iron-rich alumosilicates where siliciclastic input became significant in more proximal settings, and iron-poor sediment above the top of the plume. Diamictite-associated iron formations in the Witwatersrand are inferred to have been deposited in a fashion similar to the shale-associated iron formations, with the exception that major transgressions and hydrothermal plume invasion were preceded by glacial ice cover. The climate warming and increased volcanic activity required could have been related to increased tectonic activity inferred for the Witwatersrand Supergroup during deposition of the glacially associated iron formations.

Microbially induced sedimentary structures in Neogene tidal flats from Argentina: Paleoenvironmental, stratigraphic and taphonomic implications

Microbially induced sedimentary structures (MISS) result from the interaction between benthic microorganisms and sediments; MISS preferably form in upper intertidal to supratidal settings from the Archean to the modern. The identification of MISS in Neogene deposits of the Rio Negro Formation (Late Miocene to Pliocene), Rio Negro Province, Argentina, covering the transition from continental to marine facies (and vice versa), provides the base to refine the paleoenvironmental interpretation. The overall clastic paleocoast was differentiated into high-energy erosional domains and protected areas where MISS became preserved. However, during peak transgression also the initially protected areas experienced high energy sediment reworking. Because of the extent erosion in the high-energy coastal setting, the localized occurrence of tidal-flat deposits was difficult to recognize. The thin tidal flat deposits exhibit macroscopic and microscopic features that can be attributed to the interaction between microorganisms and sediment, such as cracks with upturned margins, sponge pore fabric, wrinkle structures, multidirected ripple marks, floating sand grains and so on. In addition, the biostabilization processes typical of substrates housing microbial mats led to the excellent preservation of vertebrate footprints that characterize these deposits.

Influence of inherited topography on the Holocene sedimentary evolution of coastal systems: An example from Arno coastal plain (Tuscany, Italy)

Understanding the relative control exerted by autogenic factors, such as changes in sediment supply, local subsidence and inherited topography, is of crucial interest for a thorough comprehension of the sedimentary evolution of late Quaternary coastal systems. Through an example from the Arno coastal plain, in Tuscany, we show to what extent sedimentation of the Holocene succession, even after the time of maximum marine ingression, was influenced by the presence of a buried 5–7 km-wide incised-valley system, generated by the Arno River during the late Pleistocene in response to the last glacial sea-level fall. A prominent stratigraphic marker (locally known as “ pancone”), ca. 5–15 m thick, made up of extremely soft and homogeneous organic-rich clays, is reported from the upper 20–30 m of the mid-Holocene succession buried beneath the Arno coastal plain. High-resolution stratigraphy, integrated with microfossil investigations, enables detailed facies characterization and mapping of this marker horizon. At proximal-most locations, “ pancone” consists of thin lagoonal deposits sandwiched between backswamp clays. Around the city of Pisa and at more distal locations, this stratigraphic marker is made up almost entirely of lagoonal clays with lateral transition to backswamp deposits. The development of “ pancone” was triggered by an abrupt transgressive pulse that led to the ultimate flooding of the Arno paleovalley. This transgressive event occurred shortly after a widespread episode of sub-aerial exposure and climate deterioration related to the 8200 yr BP cold event. Confinement of the mid-Holocene (“ pancone”) lagoon into an area formerly occupied by the post-glacial Arno incised-valley system, along with transition to backswamp facies in coincidence of the former interfluves, is interpreted to reflect the higher compaction of the less indurated valley-fill relative to the adjacent, stiff Pleistocene substratum. The local increase in land subsidence contributed to the prolonged persistence of a 6 km-wide lagoonal system, encompassing the peak of Holocene transgression and the early phases of decelerated sea-level rise (up to ca. 6000 cal yr BP). The Arno coastal plain thus offers a good illustration of how inherited topography, due to the presence of formerly active incised valleys, may influence the Holocene evolution of coastal plains, favoring local development and preservation of topographic lows.

Facies associations, sequence stratigraphy and timing of the earliest Jurassic peak transgression in central Spain (Iberian Range): Correlation with other Lower Jurassic sections

Facies associations and sequence stratigraphy of the Lower Jurassic shallow platform to peritidal carbonates of the Cuevas Labradas Formation (Sinemurian to Pliensbachian) have been studied in the Barranco de la Hoz section, located in the central−southern part of the Iberian Range. Four stratigraphical units (A to D) have been differentiated. Unit A, deposited in a restricted platform, is organized in aggradational thickening- and shallowing-upward sequences. Unit B, deposited in external restricted platform environments, is composed of shallowing- and deepening-upward sequences. This unit contains Polymorphites sp., Uptonia cf. jamesoni (SOW.) and Uptonia cf. angusta (QUENST.) that characterize the upper part of the Jamesoni Zone of the Lower Pliensbachian. The transgressive peak of 3rd order Cycle LJ2−2 is located in the marly facies of unit B. Unit C is composed of shallowing-upward sequences of “muddy” type in the lower part, deposited in the proximal part of the shallow restricted external platform and “grainy” type in the upper part, representing wave-dominated bioclastic shoals. Unit D is constituted by peritidal shallowing-upward sequences deposited in shallow restricted subtidal environment of the internal platform to the intertidal and supratidal environments. The 5th order cycles constituting the 3rd order facies Cycle LJ2−2, have been grouped into 4th order cycles. The duration of the 3rd order Cycle LJ2−2 is estimated as 2.6 Myr, and the possible average duration of the 4th order cycles as 0.4 Myr. This figure is close to the most frequent duration of the long-term Milankovitch orbital eccentricity cycles. Comparison of the obtained results with other Lower Jurassic sections in Spain, Europe and with the global cycles shows that the boundary between cycles LJ−1 and LJ−2, which represents the onset of the first Jurassic transgression and the peak transgression identified in this work at the Lower Pliensbachian Jamesoni Zone as well as the boundary between cycles LJ−2 and LJ−3 seem to represent good criteria for correlation between the different palaeogeographical domains.

Turonian to Santonian paleoenvironmental changes in the Cretaceous Western Interior Sea: The Carlile and Niobrara formations in southern Alberta and southwestern Saskatchewan, Canada

The Upper Cretaceous Upper Colorado Group, including the Carlile and Niobrara formations, reveals a complex and dynamic marine basin system in southern Alberta and southwestern Saskatchewan. An integrated study of the lower offshore to shelf environment, including sedimentology, geochemistry, petrophysics and micropaleontology demonstrates how sea-level fluctuations played a major role in the facies distribution and paleogeography of the Interior Seaway. Paleogeographic maps, depicting seven time slices covering the Late Turonian to Santonian, illustrate general lithologies and their distribution in the north-central part of the Interior Seaway of North America. These maps are based on data from southern Alberta and southwestern Saskatchewan integrated with studies of time-equivalent strata in neighboring regions in western Canada and northwestern U.S. The Interior Seaway reached its maximum extent during the peak transgression of the Greenhorn Cycle (Early Turonian) and the basin was gradually infilled starting in the Late Turonian with the Carlile Formation. The boundary between the Carlile and Niobrara formations is an angular unconformity where up to 20 m of the upper Carlile Formation was eroded. At this time (late Late Turonian), the Interior Seaway was a narrow strait of a few hundred kilometers with its basin axis in eastern Alberta. Marginal marine sandstones were deposited in western Alberta (Cardium Formation) and central Saskatchewan/north-central Montana (Bowdoin sandstone). The overlying transgressive part of the Niobrara Cycle shows multiple dramatic basin changes, with two examples of relative lowstand and complete basin drainage implied by the Verger Member (Coniacian). These rapid lowstand events occurred within periods of marine mudstone deposition, which indicate shelfal to offshore conditions. During deposition of the Medicine Hat Member (Early to Middle Santonian), changes from a shaly to a sandy shelf took place with deposition of thirteen sandstones in the vicinity of the Bow Island Arch of southern Alberta. During lowstand periods with low accommodation, the Bow Island Arch affected sedimentation, whereas during high accommodation, the arch had no influence. A subtle syncline in the northwest part of the study area is the primary reason for deposition of the main gas producing sandstone of the Medicine Hat Member. The maximum extension of the Interior Seaway during the Niobrara Cycle occurred in the late Santonian with the deposition of the First White Specks Member.

Support for a Vraconnian Stage between the Albian sensu stricto and the Cenomanian (Cretaceous System)

The geological scale for the middle Cretaceous currently used throughout the world was proposed by Alcide d'Orbigny in the XIXth century between the years 1842 and 1847 and establishes the succession of stages as Albian, Cenomanian and Turonian. In 1868 Renevier proposed that a supplemental chronostratigraphic division be intercalated between the Albian and the Cenomanian: the Vraconnian stage. This term was not generally accepted and after a period when it was referred to by Breistroffer (1936) as a substage constituting the upper part of the Albian, as an equivalent of the Stoliczkaia dispar ammonite Zone, its abandonment was recommended by the Conference on the Lower Cretaceous held in Lyon in 1963. The conditions that led to this decision will be discussed herein. Historically, for almost a century the Vraconnian was studied only in the condensed levels of the platform where ammonites are abundant, but the succession is thin and not mappable. The type section of the Vraconnian in the Vaud canton of Switzerland is only 2 meters thick. In France the situation is the same in the northern Alps, in most of central Europe, in Russia, in the Crimea, and as far as the Caspian sea. The examination of a certain number of sections located in other parts of the world: in the southeastern basin of France (Salazac, Marcoule, Mont-Risou), in the Anglo-Paris basin (Folkestone, Merstham, Grandpre), in the Mons basin (Harchies, Strepy-Thieu), at the southern edge of the Tethys in central Tunisia (Kaalat Senan), in Madagascar (Diego Suarez) and North America in California (Dry Creek) demonstrates, however, that in a number of regions of the globe the sedimentary record of the Vraconnian is sometimes rather considerable, often much more important than that of the Albian sensu stricto. In addition, the Vraconnian represents a very important eustatic event between an Albian transgression and the great Cenomanian transgression (third order cycle and the peak of transgression in a second order cycle). It involves a period of rapid ecologic expansion both as regards the macrofauna (ammonites in particular) and microfauna (planktonic foraminifera). These are the main reasons why a rehabilitation of the Vraconnian as a true stage is proposed here.

Late Vendian Miaohe-type ecological assemblage of the east European platform

Upper Vendian rocks of the East European Platform (EEP) are characterized by the presence of the White Sea fossil biota, which colonized the region from the southeastern White Sea area to the Central Urals [1]. The White Sea biota includes ecological assemblages of the Avalon (Newfoundland), Ediacara (South Australia), and Nama (Namibia) types, each related to certain environmental conditions [2]. In 2006, we found a previously unknown and morphologically diverse assemblage of carbonaceous macroscopic fossils in the fine-grained aluminosiliciclastic rocks of the Perevalok Formation (Sylvitsa Group) in the Central Urals. Together with the organic-walled macrofossils from the rocks of the Lyamtsa Formation (Valdai Group) in the southeastern White Sea area, the carbonaceous fossils from the Perevalok Formation represent a new (fourth) ecological assemblage of the White Sea fossil biota. The ecological assemblage is older than 557‐558 Ma [3, 4] and includes macroscopic microbial colonies, multicellular and coenocytic eukaryotic macroalgae. In the Late Vendian history of the EEP, this assemblage predated the appearance of the world’s most diverse soft-bodied assemblage, which was found in the overlying rocks of the White Sea area (Verkhovka and Erga formations) and Central Urals (Chernokamen Formation) [1, 2, 5]. The carbonaceous fossils are confined to a thick (200‐400 m) transgressive sequence at the base of the Upper Vendian succession in the southeastern White Sea area and Central Urals (Fig. 1). The lower part of the sequence (laminated mudstones with layers of volcanic tuffs) is gradually replaced upsection by thinly interbedded siltstones and mudstones with rare layers of wave-bedded sandstones. The sequence was formed by the advance and periodic retreat of storm-dominated coastal depositional setting into subaqueous muddy planes with relatively quiet sedimentation in the course of oscillating wane in transgression. Fossiliferous intervals contain thin laminae of phosphorites and organic matter and mark the peak of shallow-water transgression over the platform. In the southeastern White Sea area, this sequence correlates with the Lyamtsa Formation and lower part of the Verkhovka Formation; in the Central Urals, with the upper part of the Staropechny and Perevalok formations [6, 7] (Fig. 1).

Geochemical arguments for identifying second‐order sea‐level changes in hemipelagic carbonate ramp deposits

AbstractSecond‐order transgressive–regressive (T–R) cycles, previously recognized using sedimentological criteria in Lower Jurassic hemipelagic deposits from northern Spain, are distinguishable based upon bulk‐rock organic geochemistry [total organic carbon (TOC) and hydrogen index (HI)] and the stable carbon isotope compositions from belemnite rostra. There is a coincidence between regressions and decreasing δ13Cbel, TOC and HI values, and between transgressions and increasing δ13Cbel, TOC and HI values. The δ18O and Mg/Ca records from the belemnite rostra are not always in phase with the T–R cycles. The δ18Obel record reveals, however, a prominent excursion towards higher values within the spinatum Zone that correlates, according to our results, with a regression and with negative shifts in Mg/Ca, δ13Cbel and TOC. On the other hand, an excursion in the δ18Obel towards lower values in the serpentinus Zone also correlates with a peak transgression and with positive shifts in Mg/Ca, δ13Cbel and TOC. These two excursions have been identified in other European regions as geochemical perturbations of the same characteristics. This suggests a link between second‐order relative sea‐level changes and variations in seawater geochemistry that may reflect local and regional palaeoceanographic perturbations in sea‐water temperature, salinity and water circulation during the Early Jurassic.Terra Nova, 18, 233–240, 2006

Biotic changes linked to a minor anoxic event (Faraoni Level, latest Hauterivian, Early Cretaceous)

A conspicuous renewal in the ammonite faunas of the Mediterranean Tethys occurred in the latest Hauterivian. This faunal turnover took place following a stepwise pattern. The first step occurred at the boundary between the Pseudothurmannia ohmi Subzone and the Pseudothurmannia mortilleti Subzone, coinciding with the base of the so-called Faraoni Level. This is a C org-rich interval that has been recognised in several basins of the Mediterranean Tethys and seems to be the expression of a short-lived oxygen-deficient event. It correlates with a well-documented second-order peak transgression. The oxygen depletion preferentially affected the deep nektic ammonites, which would explain the extinctions within this group around the Faraoni Level. On the contrary, an increase in the trophic resources in the photic zone favoured the diversification of epipelagic ammonites. Concurrently, an abrupt change took place at this level in the nannoconid assemblage composition. A minor second event, located at the base of the Pseudothurmannia picteti Subzone, was marked by the replacement of a few planktic ammonite species by closely related forms, and the structure of the ammonite assemblage was not substantially altered. The coincidence of this event with a further restructuring of the calcareous nannofossil assemblage suggests that some changes had to occur in the planktic ecosystem during the sea-level highstand subsequent to the peak transgression. The third and last stage of the renewal process started in the upper part of the P. picteti Subzone, coinciding with a drastic sea-level fall. It is characterised by the extinction of many of the species that had appeared in the two previous events, resulting in an extensive modification of the assemblage structure. The regression would probably cause a drop in the primary productivity and, consequently, an improvement in the oxygenation level of the sea bottom. This would explain the extinction of several planktic ammonite species and the appearance of new nektic and nektobenthic lineages.