Late Aptian Early Albian Research Articles

Caracterização palinoflorística e paleoambiental da Formação Santana (Cretáceo Inferior), Poço 4-BO-1-PE, bacia do Araripe, Nordeste do Brasil

A paleoenvironmental characterization of the Santana Formation sedimentary rocks (4-BO-1-PE well), Araripe Basin, Late Aptian-Early Albian, was accomplished through qualitative and quantitative analyses of the palynofl ora. Palynological and lithological evidence indicate a predominantly continental depositional environment for this section, composed by a fl uvial, lacustrine deltaic system in a semi-arid climate. The detection of a single dinofl agellate cyst suggests that small sea ingressions took place during Aptian-Albian times in the region, as also verifi ed in other coeval strata of marginal Brazilian Cretaceous basins. Comparisons with palynofl oras from other continents support the hypothesis that this assemblage belongs to the North-Gondwana microfl oristic province.

Fossil Kajanthus lusitanicus gen. et sp. nov. from Portugal: floral evidence for Early Cretaceous Lardizabalaceae (Ranunculales, basal eudicot)

A new fossil flower, Kajanthus lusitanicus gen. et sp. nov, is described from the Early Cretaceous (late Aptian–early Albian) Chicalhão site near the village of Juncal, western Portugal, based on a single coalified specimen. The flower is small, actinomorphic, trimerous and bisexual, slightly compressed and with floral organs tightly adhering. The perianth is organised in more than two whorls. The inner two whorls consist of six bulky, apparently fleshy parts. The outer perianth whorls consist of narrow parts. There are six stamens, arranged in two whorls. The filaments are thick and anthers tetrasporangiate. The pollen sacs are protruding with extrorse dehiscence. Pollen observed in situ is tricolpate, tectate, finely punctate-perforate, compressed and more or less spherical in polar view with a diameter of about 15 µm. The gynoecium is superior and composed of three free carpels. Non-destructive virtual sectioning of the single flower using synchrotron radiation X-ray tomographic microscopy revealed the presence of several curved ovules in each carpel, arranged in two longitudinal rows on marginal placentae. The character suite of the Kajanthus flower is only found in extant Lardizabalaceae (Ranunculales), where it is particularly close to Sinofranchetia, a monotypic genus that is now endemic to China.

Climatic and environmental indications of carbon and oxygen isotopes from the Lower Cretaceous calcrete and lacustrine carbonates in Southeast and Northwest China

Carbon and oxygen isotopic ratios were determined from ~100 Lower Cretaceous samples from four carbonate-bearing facies in Southeast (SE) and Northwest (NW) China for palaeoclimatic and palaeoenvironmental analyses. The samples were interpreted as sediments within sublithofacies of distal alluvial ponds (SF1), open shallow lakes (SF2), littoral lakes (SF3), and marshes (SF4). Results of analyses show in SE China, δ13C values range between −5.0‰ and 3.0‰ with a negative trend through time, and δ18O values are all negative (−19.3‰–−7.4‰); in NW China, δ13C values range from −4.0‰ to 4.0‰ with periodic change, and δ18O values range between −18.0‰ and 1.5‰.Both relatively heavy values and pronounced covariances of most δ13C and δ18O indicate semiarid climate and domination of closed brackish lakes, marshes, and ponds in NW and SE China during the Early Cretaceous. Of them, positive δ13C (0.0‰ to 4.0‰) and relatively heavy δ18O (−5.0‰ to 1.5‰) values suggest interruptions of arid-evaporation in intermittences of the early Hauterivian in SW Fujian, of the middle Aptian in West Jiuquan basin, of the late Aptian in SW Ordos basin, and of the late Aptian–early Albian in Liupanshan basin. In a short interval of the early Aptian, the hot and humid climate occurred in local SW Zhejiang by high kaolinite content and in West Jiuquan basin by warm flora could be an exception. Particularly, extremely negative δ18O values (mainly −19.0–−9.0‰) indicate relatively low temperature in SE Fujian in the Berriasian–Barremian, in Liupanshan basin in the late Aptian, in SW Ordos basin in the late Albian, and in West Jiuquan basin in the mid-late Aptian, which is supposed to attribute to the presence of 2500m–4500m in elevation. More positive values and more covariance ratios of δ13C and δ18O in NW China than in SE China and in mainland China than in America indicate a arider climate in former than in later, probably attributing to the palaeogeography in distance to sea.A tentative correlation of the Early Cretaceous δ13C value excursion of calcretes shows a good compatibility between marine OAEs and terrestrial carbonate sediments and between NW China and America continents, implying a global response to carbon cycle in both marine and terrestrial environments. The feature suggests the potential of global correlation and application of terrestrial calcrete δ13C excursions in pre-Cenozoic.

Shallow water carbonate platforms (Late Aptian–Early Albian, Southern Apennines) in the context of supraregional to global changes: re-appraisal of palaeoecological events as reflectors of carbonate factory response

Abstract. This paper discusses the palaeoenvironmental significance of the "Orbitolina Level", the microbial carbonates and the Salpingoporella dinarica-rich deposits encased in the Aptian/Albian shallow water carbonate platform strata of Monte Tobenna and Monte Faito (Southern Italy). These facies show a peculiar field appearance due to their color and/or fossil content. In the shallow water carbonate strata, the Late Aptian "Orbitolina Level" was formed during a period of decreasing accommodation space. Microbial carbonates occur in different levels in the composite section. They reach their maximum thickness around the sequence boundaries just above the "Orbitolina Level" and close to the Aptian–Albian transition, and were not deposited during maximum flooding. S. dinarica-rich deposits occur in the lower part of the Monte Tobenna-Monte Faito composite section, in both restricted and more open lagoonal sediments. S. dinarica has its maximum abundance below the "Orbitolina Level" and disappears 11 m above this layer. On the basis of δ13C and δ18O values recorded at Tobenna-Faito, the succession has been correlated to global sea-level changes and to the main volcanic and climatic events during the Aptian. Deterioration of the inner lagoon environmental conditions was related to high trophic levels triggered by volcano-tectonic activity. Microbial carbonates were deposited especially in periods of third-order sea level lowering. In such a scenario, periods of increased precipitation during the Gargasian induced the mobilization of clay during flooding of the exposed platform due to high-frequency sea-level changes, with consequent terrigenous input to the lagoon. This and the high nutrient levels made the conditions unsuitable for the principle carbonate producers, and an opportunistic biota rich in orbitolinids (Mesorbitolina texana and M. parva) populated the platform. In the more open marine domain, the increased nutrient input enhanced the production of organic matter and locally led to the formation of black shales (e.g. the Niveau Fallot in the Vocontian Basin). It is argued that the concomitant low Mg/Ca molar ratio and high concentration of calcium in seawater could have favoured the development of the low-Mg calcite skeleton of the S. dinarica green algae. During third-order sea-level rise, no or minor microbial carbonates formed in the shallowlagoonal settings and S. dinarica disappeared. Carbonate neritic ecosystems were not influenced by the environmental changes inferred to have been induced by the mid-Cretaceous volcanism. The "Orbitolina Level", the microbial carbonates and the Salpingoporella dinarica-rich deposits in the studied Aptian/Albian shallow water carbonate strata are interpreted to be the response to environmental and oceanographic changes in shallow-water and deeper-marine ecosystems.

An overview on Callovian-Cenomanian intracratonic basins of Northeast Brazil : onshore stratrigraphic record of the opening of the southern Atlantic

A number of small Callovian-Albian sedimentary basins occur in the NE-trending Araripe-Potiguar topographic lowlying area. These basins developed in a fairly narrow zone of Precambrian supracrustal rocks from the Middle Jurassic onwards, as a consequence of the reactivation of ancient fault lines. The basin formation and subsidence took place during five tectonic stages related with the opening of the Southern Atlantic Ocean and the split of South America from Africa. The tectonic-sedimentary sequences that fill these basins record successive evolutionary stages. 1) Proto-rift stage, with minor episodes of a) pre-rift ( Callovian -Tithonian) with basin formation and deposition of coarse to conglomeratic sandstones in alluvial fan and braided river systems; (b) tectonic quiescence with continuing subsidence, resulting in hundreds of meters of pelites accumulated mainly in lacustrine and foodplain environments (Tithonian-Berriasian). 2) Syn-rift stage: a) tectonic reactivation with erosion and deposition of sandstones in alluvial fan and braided to low-sinuosity river systems; (b) during more quiet periods, accumulation of fine-grained deposits in lacustrine, palustrine and foodplain environments took place (Berriasian-early Barremian). 3) Erosional stage that would correspond to a transitional proto-oceanic gulf phase in the southern Atlantic and a shift of rifting to the equatorial Atlantic area, causing erosion in the study area (late Barremian-early Aptian). 4) Post-rift stage: (a) related to the beginning of drift in the Atlantic Ocean. This stage is characterised by deposition in the area of shallow lacustrine fine-grained siliciclastics, limestones and gypsum under dry climate (late Aptian-early Albian); (b) two short marine transgressions affected the area and, as a consequence, shallow marine limestones were deposited (late Aptian and Albian); (c) a marine regression resulted in a return to continental conditions with deposition of coastal and lagoonal limestones, lacustrine rhythmites and fluvial sandstones (Albian-early Cenomanian?). Some of the lacustrine to fluvial dominated sequences as well as the marine deposits in these basins include highly fossiliferous units (i.e. Crato and Romualdo Fms. of the Santana Group), with very diverse, exceptionally well preserved fossil assemblages (insects, fishes and reptiles, among others), although they are mainly endemic. 5) Uplift stage: last stage of tectonic calm and slow uplift and erosion (since Cenomanian). On the basis of our proposals further sedimentological and paleontological studies have been undertaken, which confirm the existence of an alternative connection between the equatorial and southern parts of the Atlantic Ocean in Aptian-Albian times. Moreover, the last break-up stages between Brazil and Africa took place probably in the Campanian.

Palynostratigraphic study of the Early Cretaceous Río Mayer and Kachaike formations at the Quebrada El Moro Section, Austral Basin, southwestern Argentina

Palynomorph assemblages have been recovered from the upper levels of the Río Mayer Formation and the basal levels of the Kachaike Formation exposed at the Puesto El Moro Creek, southwest of Santa Cruz Province, Argentina. Forty-five spore species and 25 pollen taxa (including six angiosperms) have been identified. The palynological assemblages of the upper levels of Río Mayer Formation are dominated by pteridophytic spores, while Classopollis is a common element. In the Kachaike Formation, the angiosperm pollen grains are more diverse and abundant than in the underlying unit, and Cheirolepidiaceae pollen continues to dominate. The presence of scarce angiosperm pollen, represented by Clavatipollenites sp., allows reference of the assemblage recorded in the upper levels of the Río Mayer Formation to the Aptian Antulsporites- Clavatipollenites Zone. In addition, the lower levels of the Kachaike Formation are referred to the late Aptian–early Albian, based on the presence of Asteropollis asteroides, Pennipollis peroreticulatus, Clavatipollenites sp. and scarce tricolpate pollen. From the three major stages of the early angiosperm evolution in southern South America that have been previously recognized, the palynoflora of the upper levels of the Río Mayer Formation can be referred to Stage I (late Barremian–Aptian), whereas the assemblages recognized in the Kachaike strata are comparable to Stage II (latest Aptian– earliest Albian).

Correlation of Aptian-Albian Carbon Isotope Excursions in Continental Strata of the Cretaceous Foreland Basin, Eastern Utah, U.S.A.

Abstract Nodular carbonates (“calcretes”) in continental foreland-basin strata of the Early Cretaceous Cedar Mountain Formation (CMF) in eastern Utah yield δ13C and δ18O records of changes in the exogenic carbon cycle related to oceanic anoxic events (OAEs), and terrestrial paleoclimate. Chemostratigraphic profiles of both forebulge and foredeep sections show two prominent positive δ13C excursions, each with a peak value of −3%0 VPDB, and having background δ13C values of about −6%0 VPDB. These excursions correlate with the global early Aptian (Ap7) and late Aptian–early Albian (Ap12-Al1) carbon isotope excursions. Aptian–Albian positive δ13C excursions in the CMF also correspond to 3–4 per mil increases in carbonate δ18O. These phenomena record local aridification events. The chemostratigraphic profile on the thinner forebulge section of the CMF is calibrated, for the first time, by a radiogenic U-Pb date of 119.4 ± 2.6 Ma on a carbonate bed, and by detrital zircon U-Pb dates on two bounding sandstone units (maximum depositional ages of 146 Ma and 112 Ma). Petrographic observations and diagenetic analyses of micritic to microsparitic carbonates from nodules indicate palustrine origins and demonstrate that they crystallized in shallow early meteoric phreatic environments. Meteoric calcite lines derived from CMF carbonates have δ18O values ranging between −8.1 to −7.5%0 VPDB, supporting an estimate of zonal mean groundwater δ18O of −6%0 VSMOW for an Aptian–Albian paleolatitude of 34° N. Furthermore, our two chemostratigraphic profiles exhibit a generally proportionate thinning of correlative strata from the foredeep on to the forebulge, suggesting that there were consistently lower rates of accumulation on the forebulge during the Aptian–Albian. Identification of the global Aptian–Albian δ13C excursions in purely continental strata, as demonstrated in this paper, opens a new avenue of research by identifying specific stratigraphic intervals that record the terrestrial paleoclimatic impacts of perturbations of the global carbon cycle.

New Cretaceous paleomagnetic results from the foreland of the Southern Alps and the refined apparent polar wander path for stable Adria

The central-western and the eastern Southern Alps are separated by the triangular shaped Adige embayment, which belongs to stable Adria and was the site of pelagic sedimentation from the Tithonian through Maastrichtian. The first part of this study presents paleomagnetic results from the Tithonian–Cenomanian Biancone and Turonian–Maastrichtian Scaglia Rossa formations sampled at 33 geographically distributed and biostratigraphically dated localities. The new and high quality paleomagnetic results from the Adige embayment are then combined with coeval paleomagnetic directions from autochthonous Istria (Márton et al., 2008), which also belongs to stable Adria. The combined data set (which for the Late Albian–Maastrichtian time period is constructed similarly to the synthetic African curve by Besse and Courtillot, 2002, 2003) reveals an important tectonic event (Late Aptian–Early Albian) characterized by 20° CCW rotation and sedimentary hiatus. Comparison between paleomagnetic declinations/inclinations expected in an African framework (i.e. with the assumption that Adria is still an African promontory) leads to the following conclusions. The time-distributed Tithonian and Berriasian (150–135 Ma) paleomagnetic directions exhibit the “African hairpin” with an inclination minimum and a sudden change from CW to CCW rotation at 145 Ma. Concerning the younger ages, the declinations for Adria continue to follow the African trend of CCW rotation till the end of Cretaceous. However, the Tithonian–Maastrichtian declination curve for stable Adria is displaced by 10° from the “African” curve as a result of two rotations. The first, an about 20° CW rotation of Adria with respect to Africa took place between the Maastrichtian and the mid-Eocene. During this time the orientation of Adria remained the same, while Africa continued its CCW rotation. The younger rotation (30°CCW) changed the orientation of Adria relative to Africa as well as to the present North.

Halite pseudomorphs in the Crato Formation (Early Cretaceous, Late Aptian–Early Albian), Araripe Basin, northeast Brazil: further evidence for hypersalinity

Pseudomorphs after hopper-faced halite occur at several horizons and in five distinctive morphologies in carbonate dominated members of the Late Aptian–Early Albian Crato Formation of the Araripe Basin in northeast Brazil. Some forms exhibit typical hopper-faced cubes with an outer veneer of marcasite with calcite and quartz in the centre. A second morph comprises compacted marcasite veneers exhibiting a collapsed cube morphology showing that compaction predated calcite and quartz precipitation. A third morph comprises four pointed ‘stars’ in a series of concentric circles, representing halite crystals that grew within the sediment and possibly tore apart a thin bacterial mat that bound the surface of the sediment. Pseudomorph size varies from 5–10 mm diameter for those replaced by several mineral phases to aggregates of up to 20 mm diameter for those replaced by silica. The presence of abundant halite pseudomorphs suggests that hypersaline waters prevailed during deposition of the Nova Olinda Member fossil Konservat Lagerstätte and were a contributing factor to the excellent preservation of its fossil assemblage.

Mid-Cretaceous step-wise demise of the carbonate platform biota in the Northwest Pacific and establishment of the North Pacific biotic province

The global spatiotemporal distribution of the Cretaceous carbonate platform biota, which is characterized by “tropical” Mesogean (= Cretaceous Tethys) taxa, is an important aspect of Earth's paleobiogeography. All available records of this biota in the Northwest Pacific (Japan and Sakhalin Island) are summarized in order to elucidate its stratigraphic distribution patterns and faunal changes, with special attention given to the biota of the Late Aptian–Early Albian. This carbonate platform biota flourished from the Berriasian to Early Albian interval in the Northwest Pacific, indicating that the Northwest Pacific clearly belonged to the Tethyan biotic realm at that time. A step-wise demise of the carbonate platform biota transpired in the latest Aptian to middle Albian interval. Mesogean key taxa (rudists and dasycladacean algae), some Mesogean indicators (hermatypic corals and stromatoporoids) and nerineacean gastropods disappeared at the Late Aptian to Early Albian transition. Following this event, other Mesogean indicators (orbitolinid foraminifers and calcareous red algae) and coated grains disappeared at the Early to middle Albian transition. There is no record of carbonate platform biota in the Northwest Pacific during the long interval between the Middle Albian and Paleocene. The step-wise demise of the carbonate platform biota in the latest Aptian–middle Albian interval strongly implies a “vicariance event”, which separated the North Pacific from the Tethyan biotic realm and established the North Pacific biotic province.

Basin Evolution in Southern East Greenland: An Outcrop Analog for Cretaceous-Paleogene Basins on the North Atlantic Volcanic Margins

The Cretaceous-Paleogene Kangerlussuaq basin in southern East Greenland represents a unique outcrop analog for the frontier petroleum provinces along the deep-water volcanic margins of the northern North Atlantic. The 1-km-thick sedimentary succession comprises six facies associations: (1) alluvial plain and shallow marine (late Aptian?), (2) fluvio-estuarine (late Aptian-early Albian), (3) offshore marine (Late Cretaceous-early Paleocene), (4) submarine fan and channel-levee (early Paleocene), (5) fluvial (mid-Paleocene), and (6) volcanic (late Paleocene). Sedimentation was terminated in the late Paleocene by extrusion of flood basalts related to continental breakup. The basin fill is divided into two depositional megasequences related to regional tectonic events and sea level changes. The oldest megasequence (SQ1) spans the late Aptian to the earliest Paleocene with sea level rise in the late Aptian and maximum flooding in the late Albian-Cenomanian, followed by sea level highstand in the Late Cretaceous-early Paleocene. SQ1 is truncated by a basin-wide unconformity related to regional uplift and basin reorganization in the mid-Paleocene. The upper megasequence (SQ2) spans the mid- to late Paleocene and comprises sediments deposited during early sea level rise. Extensive volcanic deposits and continental flood basalts overlie it. Deep burial (> 6 km) and middle Eocene-early Oligocene(?) uplift excludes the Kangerlussuaq basin as a petroleum basin in itself; however, to evaluate the petroleum potential of similar volcanic influenced offshore basins, such as the West Shetland, Faeroe, More, and Voring basins, we discuss three conceptual play models based on the Kangerlussuaq basin: (1) early Albian stratigraphic trap, (2) early Paleocene structural trap, and (3) Paleocene stratigraphic trap.

Stratigraphy and tectonosedimentary evolution of a late Aptian-Albian carbonate margin: the northeastern Jebel Akhdar (Sultanate of Oman)

Upper Aptian-Albian shallow-water carbonates with rudists, large foraminifera and calcareous algae are described from northern Oman, in the northeastern part of the parautochton of Jebel Akhdar. The corresponding lithostratigraphic unit, the Al Hassanat Formation, is partly a time equivalent of the Nahr Umr Shales of Albian p.p. age which have a wide extent on the Arabian craton. The Al Hassanat platform developed in two main phases. First, during the late Aptian-early Albian, the platform was fringing an emerged area (central Jebel Akhdar) with a limited extent basinward. Then, during the middle Albian the flooding of the central Jebel Akhdar resulted in an extensive development of the platform and a connection with the Nahr Umr shelf. This sedimentary evolution and its palaeogeographic expressions were mainly controlled by structural patterns: extensive uplift and block faulting to the edge of the antecedent (early Aptian) platform, with a significant differential subsidence then a sag of the Arabian promontory. This tectonosedimentary dynamics was related to the migration of the oceanic adjacent Tethyan plate, and took place during the global geodynamic Albian revolution.

Lower Cretaceous Orbitolinas from Venezuela

Lower Cretaceous Orbitolinas from Venezuela

Correlation of the Lower Greensand Group (Woburn Sands and Carstone) of the Bedfordshire Province, England

The late Aptian-early Albian Lower Greensand Group of the Bedfordshire Province is a series of sands with minor sandstone, silt, clay and ironstone. The sequence includes shallow, nearshore, strongly tidally-influenced sands, sandwaves and localized estuarine sands (channel and sandbar). A detailed correlation of the deposit across southern England from Buckinghamshire to Norfolk has shown two distinct depositional areas, the Bedford and Norfolk Basins. Detailed study has also made the recognition of a more refined subdivision possible, which comprises four units. Correlation techniques were varied and include assessing the gamma-ray signature of the subdivisions of the Lower Greensand. New palynological evidence, together with the sedimentological study, further refines the environment of deposition of the sands in the south Bedfordshire area to be that of an estuary mouth. The estuarine and the marine environment alternate in dominance in different parts of the succession. This interpretation goes some way in explaining the different character of the Lower Greensand in Bedfordshire compared to the rest of southern England. A new interpretation of the controls on sedimentation in this area during the late Aptian-early Albian is possible using the new refined stratigraphic subdivision. This shows that in addition to environmental factors there is a relationship between basement structure and the sedimentary character of the Lower Greensand.

The Early Cretaceous Evolution of Carbonate Platforms from Northern Oman

In northern Oman (Jebel Akhdar and foothills) Hauterivian to early Aptian shallow carbonate platforms are widely extending and pass laterally to slope and basin environments in the Nakhl zone. Progradational geometries are identified in that zone where significant correlation between thickness and sediment types supports a prominent tectonic control. The platform records four main sedimentary breaks (drowning events). Early Barremian (lower Lekhwair Formation), Late Barremian (basal Kharaib Formation), lowermost early Aptian (upper Kharaib Formation) and middle Aptian (Shuaiba-Al Hassanat formations boundary). The late Aptian-early Albian hiatus (pre-Nahr Umr unconformity) is regarded as an early Albian tectonically driven erosion. In the Nakhl zone, coral-rudist limestones of late Aptian-early Albian (lower Al Hassanat Formation) document an east-west ribbon platform, the southward extension of which was obscured by the middle Albian erosions and rudist limestones of middle to late Albian (upper Al Hassanat Formation), a lateral equivalent of the Nahr Umr circa littoral shaly sediments, document an east-west-trending linear platform. The foregoing points out a northward progradation coeval with a southward transgressive major trend for the Hauterivian-early Aptian interval, a faulted margin corresponding with the Nakhl zone active during the Aptian-Albian, a late Aptian ribbon platform coeval with the Bab basin initiation southward,more » a regional uplifting and truncation during the early-Albian (Austrian phase), whereas shallow-water carbonates are still forming at the edge of the former platform, and an active linear platform at the northern edge of the Nahr Umr basin, the corresponding drowning contemporaneous with the onset of the Cenomanian platform eastward.« less

The Cretaceous Oceanic Events (Anoxia and Hiatus) within a Sequence Stratigraphic Framework

The authors revision of the stratigraphy and organic geochemistry of the North and South Atlantic DSDP boreholes has demonstrated the large range of occurrences and settings of the Cretaceous black shales during the so-called Oceanic Anoxic Events. No simple model has yet been found to explain the exact timing of the anoxic layers and stratigraphic gaps associated with these three particular events. Sequence stratigraphic analyses of outcrop sections in the Western Interior basin (Black Hills area), in the Paris basin, and in the Southeast basin of France provide data to precisely date the timing of the anoxia and oceanic gaps and to show their relationship to long and short term changes of relative sea level. The Oceanic Anoxic Events correspond to periods of severe starvation events where several depositional sequences, and more particularly their maximum flooding surfaces, are merged. Three strongly condensed stratigraphic periods are recognized in the deep Atlantic ocean: event E1 a during the early Aptian, event E1b during the late Aptian-early Albian, and event E2 from the late Cenomanian to the middle Turonian. They coincide on the adjacent landmass with the overall backstepping of the depositional environments due to the overall rapid rising of relative sea level.

Stratigraphic implications of palynological studies on berriasian to Albian deposits from western and southern Portugal

One section in the Algarve Basin (southern Portugal) and several sections in the Western Portuguese Basin (Lisbon region) have been investigated palynologically with emphasis on the use of dinoflagellate cysts in stratigraphy. As a result, the stratigraphy of both basins can be defined more accurately. In the Western Portuguese Basin the existence of Jurassic and Late Valanginian-Early Hauterivian strata could not be established from dinoflagellate cyst associations. The existence of a hiatus comprising at least the Upper Barremian was confirmed but a continuous Late Aptian-Early Albian sequence in the basin could not be unambiguously identified. In the Algarve Basin the Upper Aptian is well-developed.

Trace fossil assemblages from the Albian phosphaterich sandstones of the Helvetic Shelf (western Alps)

In the Albian phosphate-rich sandstones of the Helvetic Shelf, three ichnofacies have been identified: 1, a Trypanites ichnofacies comprising Gastrochaenolites, Entobia, Rogerella, Trypanites and Caulostrepsis in condensed phosphatic series; 2, a Cruziana ichnofacies comprising Palaeophycus and Chondrites in relatively expanded series; 3, a Glossifungites ichnofacies characterized by glauconite-filled Thalassinoides and related to Upper Albian unconformities. The first and the second ichnofacies existed simultaneously in areas of different submarine topography created during a late Aptian-early Albian erosional period. All these ichnofacies developed in an open shelf environment and were linked to submarine unconformities and to phosphatization periods.

Relation of Unconformities, Tectonics, and Sea Level Changes, Cretaceous of Western Interior, United States and Canada: ABSTRACT

Intrabasin tectonics have influenced patterns of deposition and geographic distribution of major unconformities within the Cretaceous of the Western Interior. Eight major regional to subregional unconformities have been identified. Five of these have been related by previous workers to sea level changes and to well-documented regressive-transgressive cycles. New studies of recurrent movement on basement-controlled fault blocks suggest a synchronous relation among fault block movement, sea level changes, and unconformities. Which fault blocks moved on the basin floor, and when, can be explained by stress fields generated by direction and rates of plate motion. Unconformities associated with north-northwest fault trends are caused by more westerly movement, and those associated with east-northeast trends by more northerly plate motion. Expansion of the Gulf of Mexico, the Atlantic Ocean, or the Arctic Ocean during these plate motions may account for associated sea level changes. The 81 to 82 m.y. unconformity and shoreline regression in the Western Interior and synchronous volcanic events on the northern margin of the Gulf of Mexico illustr te the relations. Uncertainty exists in dating many of the unconformities. However, by use of the time scale of Obradovich and Cobban, the approximate dates for unconformities are estimated as follows: (1) late Neocomian to early Aptain, >100 m.y.; (2) late Aptian-early Albian, 100 m.y. ±; (3) Albian, 96 to 97 m.y.; (4) early Cenomanian, 93 m.y. ±; (5) Turonian to early Coniacian, 87 to 88 m.y.; (6) late Coniacian-early Santonian, 81 to 82 m.y.; (7) late Campanian 71 to 74 m.y.; and (8) late Maestrichtian, 64 to 69 m.y. Several billion barrels of oil have been found in sandstones associated with unconformities in the Cretaceous. Future stratigraphic trap exploration will be guided by a knowledge of tectonic influence on sedimentation during sea level changes and how these factors controlled distribution of source rock, migration patterns, reservoir rock, and seal. End_of_Article - Last_Page 1006------------