Major Marine Incursions Research Articles

Lithofacies of early Jurassic successions derived from spectral gamma ray logging in the Mandawa Basin, SE Tanzania

Spectral gamma ray log (SGRL) data has been widely employed to derive subsurface lithological information and to perform well correlation when the core is not available. Previous studies mainly targeted homogenous facies whereas few attempts addressed heterolithic successions. The SGRL investigation here aimed at deriving lithofacies and further constrain conditions of deposition prevailing during the early Jurassic (Mbuo, Nondwa, and Mihambia Fms.) in the Mandawa Basin, onshore Tanzania, where core and outcrop data is rare. By use of binary discrimination diagrams of U, Th, and K, composition fields were defined for various lithofacies types (black shale, red shale, siltstone, sandstone, chert, anhydrite, halite, and carbonate) compiled from literature data. Sediment composition for two wells in the Mandawa Basin was then deduced from cuttings and SGRL data and compared with literature-derived geochemical provenance data from adjacent regions and strata. Binary discrimination diagrams revealed the presence of multi-mineral components in the heterolithic facies type, which in concert with diagenetic and detrital mineral dilution effects, led to low compositional contrasts between provenance fields and to overlap of different lithofacies types, complicating interpretation and limiting the effectiveness of the SGRL-based facies differentiation. For the early Jurassic strata, an upward increase in the Th/K ratio suggests an enhanced terrigenous influx into the basin due to intensified weathering. High Th/U and Th/K in the study region indicate a relatively shallow, well mixed, and oxic depositional setting in proximity to the source area opposing the more distal, deeper, and reduced depositional settings that prevailed in the southern central part of the basin. Th/U and Th/K excursions in the Nondwa and Mihambia formations suggest a shift towards less oxic depositional conditions upon two major marine incursions, corroborated by a decline in the clastic fraction and concomitant increase in carbonate.

Multi-proxy evidence of Caribbean-sourced marine incursions in the Neogene of Western Amazonia, Brazil

The timing of continental-scale marine flooding events in Western Amazonia during the Neogene is still an unsolved question. Despite broad proxy-based evidence of such events, the pathways and duration of late Miocene marine incursions remain controversial. We provide coupled calcareous and organic microfossil and geochemical data from six onshore cores from Neogene sequences of the Solimões Basin, Brazil. Our records support minor marine influence in the early Miocene (23.0, 21.1, 18.6, and 16.3 Ma), middle Miocene (14.9, 13.7, and 12.9 Ma) and early Pliocene (4.7, 4.2–4.1, and 3.8 Ma), and conspicuous marine incursions in the late Miocene (11.1–8.8 Ma) suggested by the consistent presence of salinity-indicative microfossils and geochemical data. Our findings challenge the view of major marine incursions in the early and middle Miocene in the studied area. We propose for the first time a new late Miocene incursion (LMI) event as the main marine flooding event in Western Amazonia during the Neogene. These onshore records are compared with three offshore cores from the Atlantic and Pacific Oceans. The similarity between microfossil assemblages of the Solimões Basin and the Caribbean Sea, and evidence of increased runoff from the Orinoco river drainage system, strongly suggest the Caribbean Sea as the primary source area of the marine incursions, supporting a Venezuelan seaway. We further show for the first time the potential linkage between Neogene marine incursions (mainly the LMI) into the Solimões Basin and major disturbances in the global carbon cycle.

Earlier onset of the Early Cretaceous Equatorial humidity belt

The long-standing view on the Cretaceous climate zones suggests the inception of an Equatorial Humid Belt (EHB) in the Albian. This inception contrasts with recent sedimentological and palynological evidence of sustained low-latitude, wetter paleoenvironment in the South American and African landmasses during the Aptian. The trigger for this early-stage EHB stands on two hypotheses: [1] the first major marine incursions to the equatorial Gondwana; [2] the southward displacement of a proto-Intertropical Convergence Zone (ITCZ). We provide new palynological, ichnological, and sedimentological data from the Codó Formation obtained from borehole 2-ANP-5-MA, drilled in the São Luís Basin, Brazilian equatorial margin. This integrated analysis allowed recovering well-preserved palynomorph assemblages of the upper Aptian Codó Formation. Based on three palynomorph-based climate indicators (sporomorph genera richness, the relative abundance of reworked palynomorphs, and climate-indicative palynofloras), and independent climate-indicative sedimentological indicators, our results show an interval of increasingly humid climate, which precedes the first major marine incursion. It also confirms that sporomorph genera richness scales directly with precipitation levels. Based on that, we reconstructed coeval late Aptian humidity patterns from thirteen South American and African sedimentary basins. Our findings support the establishment of the EHB in the late Aptian and challenge the view of a dominantly arid Equatorial Gondwana before the Albian. We argue that decreasing Proto-North Atlantic/South Atlantic sea-surface temperature (SST) gradient may have triggered a southward EHB displacement, leading to wetter, low-latitude biomes with broad and complex ecosystems. For the first time, a plausible mechanism is raised to explain the linkage between changing interhemispheric SST gradient and the dynamics of a proto-ITCZ in the Early Cretaceous.

Cretaceous Evolution of the Central Asian Proto‐Paratethys Sea: Tectonic, Eustatic, and Climatic Controls

AbstractThe timing and mechanisms of the Cretaceous sea incursions into Central Asia are still poorly constrained. We provide a new chronostratigraphic framework based on biostratigraphy and magnetostratigraphy together with detailed paleoenvironmental analyses of Cretaceous records of the proto‐Paratethys Sea fluctuations in the Tajik and Tarim basins. The Early Cretaceous marine incursion in the western Tajik Basin was followed by major marine incursions during the Cenomanian (ca. 100 Ma) and Santonian (ca. 86 Ma) that reached far into the eastern Tajik and Tarim basins. These marine incursions were separated by a Turonian‐Coniacian (ca. 92–86 Ma) regression. Basin‐wide tectonic subsidence analyses imply that the Early Cretaceous sea incursion into the Tajik Basin was related to increased Pamir tectonism. We find that thrusting along the northern edge of the Pamir at ca. 130–90 Ma resulted in increased subsidence in a retro‐arc basin setting. This tectonic event and coeval eustatic highstand resulted in the maximum observed geographic extent of the sea during the Cenomanian (ca. 100 Ma). The following Turonian‐Coniacian (ca. 92–86 Ma) major regression, driven by eustasy, coincides with a sharp slowdown in tectonic subsidence during the late orogenic unloading period with limited thrusting. The Santonian (ca. 86 Ma) major sea incursion was likely controlled by eustasy as evidenced by the coeval fluctuations in the west Siberian Basin. An early Maastrichtian cooling (ca. 71–70 Ma), potentially connected to global Late Cretaceous trends, is inferred from the replacement of mollusk‐rich limestones by bryozoan‐ and echinoderm‐rich limestones.

Jurassic break-up of the Peri-Gondwanan margin in northern Colombia: Basin formation and implications for terrane transfer

Jurassic extensional basins developed along the northwestern margin of South America during the break-up of Pangea. Presently, these basins are dispersed in several tectonic blocks of the northern Andes and Mexico, hindering reconstruction of western equatorial Pangea before break-up. This is the case of the Cosinas Basin (Guajira block) and the Machiques Basin (Perijá Range), in northern Colombia, which are filled by Jurassic sedimentary and volcano-sedimentary successions. Autochthonous and para-autochthonous hypotheses on the origin of this basins have been proposed. The purpose of this research is to document the sedimentological evolution, depositional age (Sr-isotope + U-Pb geochronology), sediment provenance and paleogeography of the Cosinas and Machiques basins in order to constrain whether these basins formed within a single extensional margin or they formed as extensional basins in different tectonic blocks. Volcanic detrital zircon U-Pb ages documented in La Quinta Formation in the Machiques Basin and at the base of Rancho Grande Formation in the Cosinas Basin suggest that extensional basins were active in Early Jurassic time. However, a significant difference exists in their subsequent history. Whereas in the Machiques Basin dominates the accumulation of Lower and Middle Jurassic volcanoclastic deposits with abrupt lateral thickness changes, accumulation in the Cosinas Basin is dominantly of siliciclastic strata, with the record of two major marine incursions in Late Jurassic time. Integration of provenance results indicates that the Santander Massif supplied sediments to the Machiques Basin. In contrast, Middle to Upper Jurassic sandstones of the Cosinas Basin document unroofing of basement blocks that include metamorphic, sedimentary and plutonic rocks from the Guajira and Maya blocks. The similarity in age and composition of pre-Jurassic rocks in northwestern South America and the so-called peri-Gondwana blocks in the Mexican subcontinent (i.e., Maya and Oaxaquia blocks) challenge the use of detrital zircon population as an indicator of the autochthonous or para-autochthonous origin of the Guajira block. Large uncertainty of paleomagnetic results, and the lack of constraints for the time magnetization acquisition preclude estimating paleolatitudes for the Guajira block in Jurassic time but support previous interpretation of ca. 70°-90° clockwise rotation of the Guajira block relative to stable South America craton.Our preferred paleogeography considers that the Cosinas and Machiques basins were close to each other along the western continental margin of Pangea during the onset of extension in Early Jurassic time. The change from continental to marine depositional environments in Middle to Late Jurassic time along the Cosinas Basin, which have not been identified in the Machiques Basin or other autochthonous Jurassic basins in northwestern South America, allow us to propose that these blocks were separated during the Callovian - Tithonian interval, with the Cosinas Basin remaining closer to a conjugate Mexican margin, that we interpret as the Maya block. Collision of the Guajira block with the South American margin occurred near the Jurassic-Cretaceous boundary, as documented by deformation of Jurassic units previous to deposition of Berriasian strata in the Guajira block.

Identifying marine incursions into the Paleogene Bohai Bay Basin lake system in northeastern China

The extent of marine influence on the Paleogene Bohai Bay Basin in northeastern China, which accumulated mainly continental (e.g., fluvial and lacustrine) facies, remains under debate. Evidence from geochemical, paleontological, and lithological data have documented several marine incursions, but no systematic study has been conducted to investigate their timing and frequency and their significance for understanding the Paleogene tectonic evolution of northeastern China. In order to address the marine influence on sediment accumulation and to identify variations in paleosalinity during deposition of the mid-Eocene lower Es3 (Es3L) Member of the Shahejie Formation, this study analyzed multiple paleosalinity proxies (B/Ga, Sr/Ba, and S/TOC) in fine-grained siliciclastic (argillaceous) sediments of the Luo-69 drillcore from the Dongying Depression. All three proxies yielded similar paleosalinity interpretations, with peak salinity and marine influence (i.e., maximum B/Ga, Sr/Ba, and S/TOC values) occurring at the base of the Es3L Member, and progressively declining salinities, representing a shift toward brackish conditions, upsection over an interval of 185.5 m representing ~2.2 Myr of sedimentation. This paleosalinity record suggests gradual long-term variations that were probably due to changes in tectonically controlled gateways linking the Bohai Bay Basin to the Yellow Sea. In addition to analysis of the mid-Eocene Es3L Member, published data on the stratigraphic distribution of marine fossils, glauconite, and elemental paleosalinity proxies are applied to evaluate the history of marine incursions into the Bohai Bay lake system throughout the mid to late Paleogene (~50–24 Ma). This analysis revealed four intervals of probable major marine incursions dating to the early Eocene, the middle Eocene, the early Oligocene, and the late Oligocene.

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.

Evolutionary bottlenecks in brackish water habitats drive the colonization of fresh water by stingrays.

Species richness in freshwater bony fishes depends on two main processes: the transition into and the diversification within freshwater habitats. In contrast to bony fishes, only few cartilaginous fishes, mostly stingrays (Myliobatoidei), were able to colonize fresh water. Respective transition processes have been mainly assessed from a physiological and morphological perspective, indicating that the freshwater lifestyle is strongly limited by the ability to perform osmoregulatory adaptations. However, the transition history and the effect of physiological constraints on the diversification in stingrays remain poorly understood. Herein, we estimated the geographic pathways of freshwater colonization and inferred the mode of habitat transitions. Further, we assessed habitat-related speciation rates in a time-calibrated phylogenetic framework to understand factors driving the transition of stingrays into and the diversification within fresh water. Using South American and Southeast Asian freshwater taxa as model organisms, we found one independent freshwater colonization event by stingrays in South America and at least three in Southeast Asia. We revealed that vicariant processes most likely caused freshwater transition during the time of major marine incursions. The habitat transition rates indicate that brackish water species switch preferably back into marine than forth into freshwater habitats. Moreover, our results showed significantly lower diversification rates in brackish water lineages, whereas freshwater and marine lineages exhibit similar rates. Thus, brackish water habitats may have functioned as evolutionary bottlenecks for the colonization of fresh water by stingrays, probably because of the higher variability of environmental conditions in brackish water.

Fluvial to bay sequence stratigraphy and seismic facies of the Cretaceous to Paleogene successions in the MITI Sanriku‐oki well and the vicinities, the Sanriku‐oki forearc basin, northeast Japan

AbstractThis paper describes the significant depositional setting information derived from well and seismic survey data for the Upper Cretaceous to Lower Eocene forearc basin sediments in the central part of the Sanriku‐oki basin, which is regarded as a key area for elucidating the plate tectonic history of the Northeast Japan Arc. According to the results of well facies analysis utilizing cores, well logs and borehole images, the major depositional environments were of braided and meandering fluvial environments with sporadically intercalated marine incursion beds. Seismic facies, reflection terminations and isopach information provide the actual spatial distributions of fluvial channel zones flowing in a north–south trending direction. The transgression and regression cycles indicate that the Upper Cretaceous to Lower Eocene successions can be divided into thirteen depositional sequences (Sequences SrCr‐0 to SrCr‐5, and SrPg‐1 to SrPg‐7). These depositional sequences demonstrate three types of stacking patterns: Types A to C, each of which shows a succession mainly comprising a meandering fluvial system, a braided fluvial system with minor meandering aspects in the upper part, and major marine incursion beds in the middle part, respectively, although all show an overall transgressive to regressive succession. The Type C marine incursion beds characteristically comprise bay center and tidal‐dominated bay margin facies. Basin‐transecting long seismic sections demonstrate a roll up structure on the trench slope break (TSB) side of the basin. These facts suggest that during the Cretaceous to Eocene periods, the studied fluvial‐dominated forearc basin was sheltered by the uplifted TSB. The selective occurrences of the Type C sequences suggest that when a longer‐scale transgression occurred, especially in Santonian and early Campanian periods, a large bay basin was developed, creating accommodation space, which induced the deposition of the Cretaceous Kuji Group along the arc‐side basin margin.

The Cretaceous of the South Kyzylkum and Nuratau Region, Western Tien Shan, Central Uzbekistan

Abstract The Kyzylkum and south Nuratau areas of Central Uzbekistan form the westernmost parts of the Tien Shan and contain rare outcrops of Cretaceous rocks that were deposited along the northern margin of the Amu Darya Basin. The succession can be subdivided into 18 clastic and carbonate facies, and these were deposited in a series of contrasting depositional environments from Aptian through to Maastrichtian times. Broadly, the region comprised a coastal–marine transitional zone, with the coastline advancing and retreating over time, these changes being related to major marine incursions to the east (as far as the Tarim Basin). Tectonic activity along the basin margin, with subsequent uplift of parts of the mosaic of boundary blocks, most probably influenced the development of coeval clastic/carbonate successions in the late Cretaceous.

Facies evolution and stratigraphic correlation in the early Oligocene Tard Clay of Hungary as revealed by maceral, biomarker and stable isotope composition

Hydrocarbon source rocks (i.e. Tard Clay Formation), containing type-II organic matter, were deposited in the Hungarian Paleogene Basin during Lower Oligocene. A major contribution of aquatic organisms (green algae, dinoflagellates, Chrysophyte algae) and minor inputs from macrophytes and land plants to organic matter accumulation is indicated by n-alkane distribution patterns, composition of steroids, and δ13C of hydrocarbon biomarkers. Microbial communities included heterotrophic bacteria, cyanobacteria, chemoautotrophic bacteria, as well as green sulphur bacteria. The presence of methanotrophic bacteria is indicated by 13C-depleted hop-17(21)-ene. Higher inputs of terrestrial organic matter occurred during deposition of the lower and uppermost units of the Tard Clay Formation. The terpenoid hydrocarbon composition argues for angiosperm-dominated vegetation in the area of the Hungarian Paleogene Basin. Diterpenoid hydrocarbons, derived from the resins of conifers, are about 2–3‰ enriched in 13C compared to the angiosperm biomarkers.Environmental conditions changed from marine to brackish, accompanied by oxygen-depletion in the lower parts of the water column. Organic carbon accumulation during this period was a consequence of stagnant bottom water conditions in the Hungarian Paleogene Basin due to salinity stratification. This is indicated by low pristane/phytane ratios (varying from 0.27 to 1.44), enhanced ratios of dimethylated versus trimethylated MTTCs (0.14–0.59), and the presence of aryl isoprenoids (from 0.4 to 14.1 μg/g TOC). A major marine incursion is evidenced by stable isotope ratios of organic matter and carbonates. In the uppermost member of the Tard Clay, a transition from brackish towards normal marine conditions is proposed.Up to 3 anoxic cycles are recognized in the drill core sections, separated by minima in pristane/phytane ratios and maxima in the depth trends of di-/tri-methylated MTTCs and aryl isoprenoid concentrations. In combination with the position of maxima of δ13C of carbonate and organic matter and an abrupt decrease in perylene concentrations, the cycles can be used for intra-formational correlation of the Tard Clay.

The evaporite–coal transition: Chemical, mineralogical and organic composition of the Late Triassic Abu Ruweis Formation, NW Jordan—Reference type of the “Arabian Keuper”

The Abu Ruweis Formation, NW Jordan, was studied with regard to its chemical (major, minor elements, REE, S-, O-, C isotopes, organic chemistry), mineralogical compositions (rock-forming minerals), coal petrography (maceral analyses) and palynology (age determination and palaecosystem) for the evaporite–coal facies transition which is very rarely found. The palynomorphs point to a Late Triassic/Carnian age of formation. Strong evaporation in the series is demonstrated by the presence of dolomite, gypsum and celestite. Mg, Ca, F, Cl, S, Sr and Ba reflect the variation of evaporation during basin subsidence. A detrital influx is characterized by the abundance of Si, Ti, Al, Fe, Mn Ce, Co, Cr, V, Y and the (Zr+Rb)/Sr ratio. Salinity variation may be characterized by the fractionation of LREE (La) and HREE (Lu). The element triplet Zn, Cu and Pb offers a clue to the redox regime and supports the idea of mineralogical-based proximity indicators: Zn (Eh≪0)⇒Cu (Eh≤0)⇒Pb (Eh≤0). Oxygen isotope composition (δ18O −8.3 to −0.9‰) of the carbonate fraction resulted in uniform isotope formation temperatures near 50°C. Temperature data obtained during coal petrography yielded maximum paleotemperatures of 100°C for the black shale and about 70°C for the remaining coal samples that are likely to reflect hot brine activity in some parts of the basin. The carbon isotopic composition (δ13C −12.4 to −3.9‰) of the carbonate fractions indicates a biogenic 12C-enriched CO2 source, which probably was admixed to marine pore water bicarbonate (with δ13C near 0‰) during early diagenesis. The S isotope composition (sulfide fraction −28.9 to −24.0‰) is typical of sulfides formed in the course of bacterial reduction with some sulfate (sulfate fraction +7.5 to +11.6‰) resulted from later re-oxidation of the sulfides. Phosphate distribution is a measure for the aridity while high-sulfur contents in the coal-bearing sediments point to brackish or alkaline waters. The results of the organic chemistry are interpreted in terms of a bloom of algae which were preserved during dysaerobic conditions in the course of early diagenesis. Sesqui- and diterpenoids are most probably derived from precursor molecules abundant in leaf resins of conifers. Phenanthrene and its methylated analogues derived from combustion products of fossil fuels generated by wild fires. Among the maceral types, textinites A and B represent xylem features of tree trunks and the crassi-cutinite upper leaf cuticles. The data allow for a definition of a special geological setting called the “Arabian Keuper Facies”. This facies type is characterized by an evaporite–coal transition which translates into a geodynamic setting positioned between the fully marine “Alpine Triassic Facies” of the Tethyan Ocean where submarine brines were responsible for the Alpine-type MVT Pb–Zn deposits and the epicontinental “Germanic Keuper Facies” that lacks both hydrothermal activity and major marine incursions.

Marine Incursion: The Freshwater Herring of Lake Tanganyika Are the Product of a Marine Invasion into West Africa

The spectacular marine-like diversity of the endemic fauna of Lake Tanganyika, the oldest of the African Great Lakes, led early researchers to suggest that the lake must have once been connected to the ocean. Recent geophysical reconstructions clearly indicate that Lake Tanganyika formed by rifting in the African subcontinent and was never directly linked to the sea. Although the Lake has a high proportion of specialized endemics, the absence of close relatives outside Tanganyika has complicated phylogeographic reconstructions of the timing of lake colonization and intralacustrine diversification. The freshwater herring of Lake Tanganyika are members of a large group of pellonuline herring found in western and southern Africa, offering one of the best opportunities to trace the evolutionary history of members of Tanganyika's biota. Molecular phylogenetic reconstructions indicate that herring colonized West Africa 25–50MYA, at the end of a major marine incursion in the region. Pellonuline herring subsequently experienced an evolutionary radiation in West Africa, spreading across the continent and reaching East Africa's Lake Tanganyika during its early formation. While Lake Tanganyika has never been directly connected with the sea, the endemic freshwater herring of the lake are the descendents of an ancient marine incursion, a scenario which may also explain the origin of other Tanganyikan endemics.

A restatement of the Mesozoic Atlasic Rifting (Morocco)

Dislocation of the Pangean plate resulted in the development of extensive episodes recorded in Late Triassic-Early Jurassic synrift basins that were initiated by reactivation of older Hercynian–Alleghanian thrusts. The Central Atlantic rift system evolved from rifting to drifting with the post-rift phase beginning in the early Toarcian. The present Moroccan Atlasic belt corresponds to a Mesozoic marine basin, often described as rift basins, that were inverted and uplifted as a consequence of the Alpine Orogeny. The original geometry of these basins has not been extensively obliterated, due to shortening, allowing the distinction of the synrift and post-rift sequences and their initial geometry. The ‘Atlasic rift’ concept is still equivocal since it represents two sets of conflicting interpretations. The first considers a rifting process that began during the Upper Triassic and lasted until the Bathonian or the Cretaceous. The second distinguishes two crustal extensional events: (i) an initial event, which was initiated in Carnian time and led to the development of an Upper Triassic Atlasic true rift, with fault activity that declined in earliest Liassic time; (ii) a second event, starting early in the Toarcian, was responsible for a renewed crustal extension that disrupted the Liassic carbonate platform and led to the development of the fault-bounded Middle and High Atlas troughs. They were initiated by transtensional tectonics that ended with the development of the pre-Cretaceous regional cleavage. Field and well data were used to understand the basin evolution and timing of rifting. A post-rift sequence of Liassic age that overlapped both the Triassic sediments and the titled basement blocks has been identified. It corresponds to the first major marine incursion from the Tethys sea and it gave way to the flooding of the rifted area, emphasized by thermal subsidence. The basal surface of this sequence represents a break-up unconformity beneath the marine deposits. A renewed crustal extension led to the fragmentation of the Liassic carbonate platform and the development of the fault-bounded Middle and High Atlas troughs which were controlled mainly by transcurrent faulting in the Toarcian. This development ended during the Late Jurassic. The Atlas rift regime subsequently became transpressive. The former depocentres evolving into synclines and the ridges to anticlines. Regional cleavage developed contemporaneously. Anticlinal ridges were truncated by erosion during and after their uplift. The resulting erosional surfaces were preserved by the deposition of Callovian and Cretaceous sedimentary sequences, demonstrating that this unconformity post-dates the cleavage development. The major unconformity of Callovian and Cretaceous sedimentary rocks can therefore no longer be seen as a break-up unconformity overlain by a post-rift sequence.

Palaeogeographic and palaeoenvironmental characteristics of major marine incursions in northwestern Europe during the Westphalian C (Bolsovian)

The Westphalian C was a time of marked tectonic and climatic changes within the Variscan Foreland, but our understanding of these changes is hampered by a poor appreciation of large-scale palaeogeography and palaeogeographic evolution within this key stratigraphic interval. The distribution of tonsteins, marine bands and faunal occurrences related to marine incursions or the proximity of marine conditions in Britain and on the European mainland during the Westphalian C (Bolsovian) is briefly summarised. The favoured environmental conditions of some selected fossil taxa (Lingula, arenaceous foraminifers, Geisina, conchostracan faunas and Torispora producing tree ferns) are highlighted. A palaeogeographic model shows the relationship between major sedimentary facies belts in the Westphalian C of western Europe and the influence of major marine incursions on the distribution pattern of incursion-related faunas. The frequent succession of transgressive-regressive faunal phases in beds with marine faunas and the close correlation between the distribution of these beds and the distribution of upper delta plain environments in the Westphalian C of northwestern Europe suggest that marine incursions were long-lived, related to glacio-eustatic events, and cannot be regarded as catastrophic « flash floods ».

Foraminiferal response to Albian relative sea-level changes in northwestern and central Alberta, Canada

Albian foraminiferal assemblages from three wells in northwestern (Imperial Spirit River No. 1, 12-20-78-6W6), central (AngloHome C&E Fort Augustus No. 1, 7-29-55-21W4), and southern Alberta (Amoco B1 Youngstown, 6-34-30-8W4) provide the basis to track a fluctuating sea-level history in western Canada. Two global second-order marine cycles (Kiowa - Skull Creek and Greenhorn) were punctuated by higher frequency relative sea-level cycles expressed during the time of the Moosebar-Clearwater, Hulcross, Joli Fou, and Mowry seas. A total of 34 genera and 93 subgeneric taxa are recognized in these Albian-age strata. Foraminiferal abundance and species diversity of the latest Albian Mowry Sea were higher than in the early to middle Albian Moosebar-Clearwater and Hulcross seas. The two earliest paleo-seas were shallow embayments of the Boreal Sea, and relative sea-level fluctuations caused variable marine to brackish conditions expressed in a variety of faunal assemblages. Towards the late Albian, relative sea level rose, deepening the basin and establishing increased marine conditions and more favourable habitats for foraminifera. In the deeper Joli Fou Seaway and Mowry Sea, however, reduced bottom water oxygen through stratification or stagnant circulation caused times of diminished benthic faunas. The Bluesky Formation in northwestern Alberta contains the initial transgression of the early Albian Moosebar-Clearwater Sea and is marked by a sudden faunal increase. In contrast, transgression by the late late Albian Mowry Sea was associated with a gradual increase of foraminiferal faunas. Numerous agglutinated species range throughout the entire Albian, absent only at times of basin shallowing. However, each major marine incursion throughout the Albian introduced new taxa.

Origin of depositional cycles in a Permian "saline giant": The Salado (McNutt zone) evaporites of New Mexico and Texas

Two types of metre-scale depositional cycles have been recognized within the McNutt Potash Zone of the Permian Salado evaporites. The cycles record progressive drawdown and concentration of brine in a shallow, marginal marine basin. cycles, made of basal carbonate-siliciclastic mudstone, then anhydrite-polyhalite pseudomorphous after primary gypsum, overlain by halite and capped by muddy halite, are interpreted as marine sea water dominated.They formed by massive spillover of sea water through a connection with the Permian Ocean at high relative sea-level stands (lagoonal mudstone and anhydrite-polyhalite after subaqueous gypsum), followed by relative lowering of sea level and gradual basin restriction (shallow saline-lake and salt-pan halite) and ending with a desiccated basin, isolated from the Permian Ocean (salt-pan halite and saline mudflat muddy halite cycle cap). cycles occur between the Type I cycles and contain halite overlain transitionally by muddy halite. They record a temporal evolution of environments from a shallow saline lake to an ephemeral salt-pan-saline mudflat complex and are interpreted as continental-dominated sequences sourced by meteoric inflow from surrounding land areas that mixed with variable amounts of sea water, either residual or introduced into the Salado basin by seepage. The vertical stacking of Type I cycles is best explained by periodic invasions of sea water into the Salado basin coincident with eustatic sea-level rises. The continental-dominated Type II cycles formed during intervening periods of eustatic sea-level fall and low stands. The maximum time interval between major marine incursions is an average of 10 5 yr.