Atlantic Coastal Basin Research Articles

Cretaceous geology and stratigraphy of Morocco and adjacent coastal basins

The Cretaceous marine sediments of Morocco and adjacent coastal basins provide an outstanding archive of environmental diversity from extended shelf seas and marginal basins along the Atlantic and Tethyan margins to deep oceanic basins of the western Tethys and eastern Atlantic Ocean. The geological highlights of Morocco's fascinating landscape include records of Lower Cretaceous Tethyan marginal and deep-water clastic sequences in the Rif mountain chain (submarine fan systems of the Massylian and Mauretanian flysch units), as well as siliciclastic sedimentary sequences in subsiding coastal basins (TanTan Delta), which extend offshore along the NW African Atlantic margin. Vestiges of the Aptian to Turonian greenhouse climate, sea-level highstands and oceanic anoxic events are exceptionally well preserved in Tethyan marginal and deep-water sedimentary successions of the Rif, in Atlantic coastal basins and as transgressive pulses on the Moroccan Meseta, Sahara platform and High Atlas rift system. Furthermore, sedimentary expressions of the tectonic movements between the African and European plates associated with the end Cretaceous climate and sea-level changes are documented in the Rif mountain chain, in coastal basins, and in the massive marginal marine phosphorite sedimentation in the Middle and High Atlas, on the Moroccan Meseta and Sahara platform. The Cretaceous sedimentary archives from Morocco and adjacent coastal basins still retain untapped potential to further contribute to our understanding of global eustatic sea-level changes and the response of the oceans and marine biota in upwelling driven oxygen minimum zones under greenhouse climate conditions.

River drainage rearrangements and the phylogeographic pattern of the annual fish Austrolebias arachan (Cyprinodontiformes, Rivulidae)

AbstractPalaeogeographical and climatic processes are among the main factors affecting biological diversity and distribution patterns. In freshwater systems, major dispersal processes are caused by river drainage rearrangements where the direction of flow of a stream changes, allowing range expansions and connection of previously isolated communities. In the Neotropical region, this process has been known to connect part of the southwestern Amazon basin with La Plata basin during the formation of the Bolivian Orocline, and La Plata basin with Atlantic coastal basins since the split from Africa. Several species of annual fishes of the genus Austrolebias are known to inhabit seasonal ponds both in the Rio Negro basin (southern tip of Brazilian shield, Lower Uruguay ecoregion), and the headwaters of rivers draining into the Merín lagoon (coastal drainages, Laguna dos Patos ecoregion). The aim of this article was to analyse the phylogeographic pattern of Austrolebias arachan in the context of putative river rearrangements. Two hypotheses were tested: (a) the spatial configuration of river basins determines the genetic structure and distribution of this species, and (b) coastal drainages captured sections of upland shield river drainages. We analysed a fragment of the mitochondrial cytochrome b gene and a fragment of a nuclear intron locus. Results support the prediction of the first hypothesis: geographic variation within A. arachan is structured according to geography. The second hypothesis was in part confirmed: the migration analyses showed that Rio Negro basin populations could have acted as a source for Laguna Merin basin. However, migration estimates also support gene flow in the opposite direction. The cytochrome b haplotype network configuration and its phylogenetic pattern suggests at least two independent events of capture, with divergence time estimated at the onset of Pleistocene glacial cycles.

A systematic revision of the five-spotted Hemichromis complex (Cichliformes: Cichlidae) from West Africa and Lower Guinea, with the description of a new species from Cameroon

The group of large five-spotted Hemichromis species from West Africa and Lower Guinea was revised using an integrative approach combining morphometry (measurements, meristics and colour patterns) and genetics (cytochrome b gene). Specimens from 100 populations were sorted into three main groups on the basis of morphological criteria. Samples from Lake Bosumtwi clustered together with others from West Africa in a large well-supported clade (bootstrap values and posterior probabilities: 98/96/100), both in morphometric and genetic analyses, raising again the hypothesis of synonymy of H. frempongi with H. fasciatus. Samples from southern Lower Guinea, including the type locality (Gabon) of H. elongatus, constituted a second weakly resolved cluster (64/38/51), resulting from a potential admixture of populations of diverse origins which share common morphological traits. Populations from northern Lower Guinea (Atlantic coastal basins) and the upper Chad basin in Cameroon formed a third strongly supported clade (99/100/100), which corresponds to a new species, Hemichromis camerounensis sp. nov., herein described.

New Evidence of ‘Anomalous' Vertical Movements along the Hinterland of the Atlantic NW African Margin

AbstractLow‐temperature thermochronology studies revealed major exhumation events affecting domains in the hinterland of the Central Atlantic margins, where Palaeozoic and/or Precambrian basement is exposed. Thus, domains traditionally assumed to be stable since at least the Variscan and juxtaposed to subsiding Meso‐Cenozoic basins, appear to be affected by km‐scale vertical movements during the Atlantic rifting and after the Early Jurassic breakup in the Central Atlantic. In this contribution, we investigate the extent and the magnitude of these motions along the NW African margin by presenting the first low‐temperature thermochronology data from west Mauritania. The analysed 22 samples were collected along the Mauritanides, a N‐S trending Variscan Belt separating the cratonic Taoudeni Basin in the east from the Atlantic coastal basin in the west. The obtained apatite fission track (AFT) ages range between 236 and 90 Ma, with mean track lengths between 11.22 and 12.81 μm and Dpar comprised between 1.6 and 2.1 μm. The uncorrected (U‐Th‐Sm)/He (AHe) ages vary between 261 and 33 Ma. Inverse thermal modelling of the AFT and AHe data indicates that the hinterland of the Mauritanian Atlantic margin experienced (i) burial between the Permian and the Late Triassic, (ii) km‐scale exhumation during Middle‐Late Jurassic and Early Cretaceous, (iii) burial during the Palaeogene–early Miocene, and (iv) exhumation between mid‐Miocene and present‐day. We argue that these vertical movements are primarily driven by the tectonic evolution of the Atlantic rift and the subsequent geodynamic evolution of the Central Atlantic Ocean and the African plate.

Paleoclimatic and paleoenvironmental reconstruction at Tarfaya Atlantic coastal basin (Morocco) based on clay mineral records from Upper Cretaceous to Quaternary

We generated clay mineral assemblage records on Late Cretaceous to Miocene-Pliocene rocks and on Holocene river sediments at Tarfaya Atlantic coastal basin in Morocco. Results show that illite, chlorite, kaolinite, smectite, and palygorskite originated from a continental source whereas corrensite formed within the basin. A very high kaolinite content and kaolinite/(illite+chlorite) ratio during the Turonian indicate a high chemical weathering which was coeval with a global weathering event during a warm and humid climate during that time. An increase in physical erosion during the Santonian as reflected by high (illite+chlorite) content and low kaolinite/(illite+chlorite) ratio was most likely associated with the early western Anti-Atlas uplift. Abundant (palygorskite+sepiolite) contents in Early Eocene rocks reflect the development of extremely dry and warm climatic conditions over the coastal regions of Tarfaya during that time. Dominant (illite+chlorite) and smectite clay minerals and a marked increase in sedimentation during the Miocene-Pliocene suggest an increase in physical erosion of the high-relief source region (western Anti-Atlas) which was mainly controlled by tectonic denudation. High kaolinite content and kaolinite/(illite+chlorite) ratio also occurred in the Holocene river sediments indicating warm and humid climate during that time. The abundant corrensite, however, indicates a basin depositional environment and suggests that oxic conditions were dominant in the Tarfaya basin during the Campanian and the Oligocene-Early Miocene.

Middle Eocene vertebrates from the sabkha of Gueran, Atlantic coastal basin, Saharan Morocco, and their peri-African correlations

The sabkha of Gueran in the Southwest Moroccan Sahara has yielded a rich and diverse fauna of late middle Eocene vertebrates that include the world's richest Bartonian age archaeocete assemblage. Archeocete remains were described previously and here we report on the rest of the vertebrate fauna. The Gueran fauna includes abundant chondrichthyan species belonging to Lamniformes, Carcharhiniformes and Rhinopristiformes, and actinopterygian assemblage consisting of Cylindracanthus, of a siluriform, and of Perciformes. Turtles are represented by at least two marine taxa referred to as Cheloniidae and Dermochelyidae. Crocodylian remains belong to at least two longirostrine species, including gavialoid remains. Snakes are represented by Pterosphenus cf. schweinfurthi (Palaeophiidae). Seabirds are represented by a pseudo-toothed bird (Pelagornithidae). The avian fossil belonged to a gigantic soaring bird and constitutes the earliest occurrence of the genus Pelagornis. The presence of proboscideans is attested by dental fragments. This fossil assemblage from Gueran shows affinities with those of the Eocene beds of Egypt and Libya. The numerous shared taxa support a close biogeographical connection between faunas from southeastern and southwestern coasts of the Mediterranean Sea.

Complete archive of late Turonian to early Campanian sedimentary deposition in newly drilled cores from the Tarfaya Basin, SW Morocco

An expanded succession of organic-rich marlstones and limestones deposited in the Tarfaya Basin provides an outstanding opportunity to closely retrace climate evolution and sea-level changes during the Cretaceous greenhouse period. We present high-resolution X-ray fluorescence (XRF) scanning and bulk carbon- and oxygen-isotope records from two newly drilled sediment cores in the Tarfaya Atlantic coastal basin, which recovered a continuous Upper Turonian to Campanian succession of ∼290 m thickness. The XRF core scanning records reveal three long-term oscillations in the abundance of terrigenous elements (increase of Al, Ti, K, Si, and Fe normalized against Ca), which correspond to progressive transgressive phases followed by rapid regressions during the Coniacian and early Santonian. Sea-level highstands during this interval corresponding to the Coniacian−Santonian oceanic anoxic event 3 (OAE 3) are characterized by overall oxygen-depleted to anoxic conditions at the seafloor (indicated by the high organic carbon content, the presence of laminations, and low log[Mn/S], high log[V/Ca], and high log[Br/Ca]). The upper Santonian interval marks the transition from anoxic to oxic bottom-water conditions, prevalent through the early Campanian. The composite bulk carbonate δ 13 C curve exhibits strong similarities to the global stacked δ 13 C reference curve, characterized by negative excursions in the early Coniacian (Navigation and East Cliff events) and late Santonian (bracketed by the Haven Brow and Buckle events) and by positive excursions in the latest Turonian (Hitchwood event), middle Coniacian (Wight Fall event), and at the Santonian-Campanian boundary. During the early Campanian, enhanced accumulation of fine-grained carbonate and clay-rich hemipelagic sediments, increasing bulk carbonate δ 18 O, and low log(Br/Ca) and log(V/Ca) values indicate climate cooling, associated with a substantial improvement in bottom-water ventilation. Two long-term δ 13 C cycles of ∼2 m.y. duration, probably related to variations in Earth’s orbital eccentricity, are associated with the long-term cooling trend initiating the Campanian−Maastrichtian climate transition toward a cool greenhouse state.

End of a modern geological myth: there are no rudists in Brazil! Paleobiogeographic implications

Out of the few records of rudists from the Cretaceous strata of the South Atlantic coastal basins only two refer to Brazilian localities. However, petrographic analyses demonstrate that these shells should be assigned to Ostreids or to Pycnodontids rather than to Rudistids. More specifically, the domain considered herein, north of the Rio Grande Rise - Walvis Ridge barrier, was part of the warm-water tropical realm, but it was not part of the Mesogean domain because both Rudistids and Orbitolinas are missing. In addition, the scarcity of corals leads us to ascribe the taphonomic assemblage to the Chloralgal facies. Neither generalized hypersalinity or extreme sea-water temperatures seem to account for these biotic peculiarities. Instead, our alternative hypothesis favors the driving role played by oceanic circulation in the dispersal of the benthic organisms.

Coevolution between Contracaecum (Nematoda, Anisakidae) and Austrolebias (Cyprinodontiformes, Rivulidae) host-parasite complex from SW Atlantic coastal basins.

In recent years, molecular studies in host-parasite interactions in terms of coevolution have become important. Larvae (L3) of two species of Contracaecum were found parasitizing species of Rivulidae in the Atlantic coastal basins from Uruguay. The aim of this study is to determine the patterns of differentiation of this host-parasite complex in order to clarify possible coevolutionary events in such interaction throughout phylogeographic approach using both nuclear and mitochondrial molecular markers (internal transcribed spacers (ITS) and cytochrome oxidase subunit 1 (cox-1)). Based on both markers, intraspecific variation in Contracaecum species was lower than 2 %, while interspecific variation was greater than 10 %. Both species of Contracaecum constitute monophyletic groups. Contracaecum resulted in a paraphyletic genus when incorporating other Contracaecum species and closely related nematode sequences from GenBank. ITS regions showed that Contracaecum sp. 1 is more closely related to other species of the same genus than with their counterparts from Atlantic coastal basins in Uruguay. Haplotype network for both markers corroborate the existence of two distinct taxa. While ITS pairwise FST comparisons and the indirect estimate of gene flow confirm the existence of two distinct Contracaecum species, mitochondrial gene detected low levels of migrants between some of the populations from both species. Our results suggest that coevolution in this host-parasite complex species is plausible. Parasite cladogenetic events occur almost simultaneously with the separation of the hypothetical ancestors of each species complex of Austrolebias during Pliocene. Additionally, the two lineages of Contracaecum colonize differently the species within each of the Austrolebias complexes.

Late Cretaceous paleoenvironmental evolution of the Tarfaya Atlantic coastal Basin, SW Morocco

Lithological evidence, benthic foraminiferal census counts, and X-ray fluorescence (XRF) scanner-derived elemental data were integrated with planktonic foraminiferal biostratigraphy and bulk carbonate stable isotopes to retrace the Turonian to early Campanian paleoenvironmental evolution and sea-level history of the Tarfaya Atlantic coastal basin (SW Morocco). The lower Turonian is characterized by laminated organic-rich deposits, which contain impoverished benthic foraminiferal assemblages, reflecting impingement of the oxygen minimum zone on the shelf during a sea-level highstand. This highstand level is correlated to the global transgressive pulse above the sequence boundary Tu1. The appearance of low-oxygen tolerant benthic foraminiferal assemblages dominated by Gavelinella sp. in the middle to upper Turonian indicates an improvement in bottom water oxygenation, probably linked to offshore retraction of the oxygen minimum zone during a regressive phase. This interval is marked by major regressive events expressed by a series of erosional truncations associated with the prominent sequence boundaries Tu3 and/or Tu4. Dysoxic–anoxic conditions recorded in the upper Santonian of the Tarfaya Basin coincide with the eustatic sea-level rise prior to Sa3 sequence boundary. The lower Campanian transgression, only recorded in the southern part of the Tarfaya Basin, coincided with substantial deepening, enhanced accumulation of fine-grained clay-rich hemipelagic sediments and improved oxygenation at the seafloor (highest diversity and abundance of benthic foraminiferal assemblages). Stable isotope data from bulk carbonates are tentatively correlated to the English Chalk carbon isotope reference curve, in particular the Hitch Wood Event in the upper Turonian, the Navigation Event in the lower Coniacian, the Horseshoe Bay Event in the Santonian and the Santonian/Campanian Boundary Event.

Parotocinclus arandai, a new species of hypoptopomatine catfish (Siluriformes: Loricariidae) from the upper rios Jucuruçu and Buranhém, States of Bahia and Minas Gerais, Brazil

Parotocinclus arandai, new species of the hypoptopomatine catfish, is described from small creeks in the upper rios Jucuruçu and Buranhém basins, at the border of Brazilian States of Bahia and Minas Gerais. Parotocinclus arandai is distinguished from all congeners from the Atlantic coastal basins of southeastern and eastern Brazil, except Parotocinclus bahiensis, by having the branched rays and interradial membranes of the pectoral and pelvic fins unpigmented in ventral view. The new species is distinguished from most Parotocinclus species, by having a small eye, 14.8-19.3 mm HL (except P. maculicauda and P. planicauda) and by the presence of a tuft of hypertrophied odontodes on the supraoccipital (except P. cristatus and P. cesarpintoi). Parotocinclus arandai is further distinguished by having an abdomen extensively naked, with a mosaic of few rounded platelets of irregular size and distributed over the pre-anal region (except P. bahiensis, P. minutus, P. spilosoma, P. cearensis, P. cesarpintoi and P. prata). A detailed comparison with congeners on eastern Brazil hydrographical region is provided, and information on the species habitat is given.

Geochemical variations in aeolian mineral particles from the Sahara–Sahel Dust Corridor

The Sahara–Sahel Dust Corridor runs from Chad to Mauritania and expels huge amounts of mineral aerosols into the Atlantic Ocean. Data on samples collected from Algeria, Chad, Niger, and Western Sahara illustrate how corridor dust mineralogy and chemistry relate to geological source and weathering/transport history. Dusts sourced directly from igneous and metamorphic massifs are geochemically immature, retaining soluble cations (e.g., K, Na, Rb, Sr) and accessory minerals containing HFSE (e.g., Zr, Hf, U, Th) and REE. In contrast, silicate dust chemistry in desert basins (e.g., Bodélé Depression) is influenced by a longer history of transport, physical winnowing (e.g., loss of Zr, Hf, Th), chemical leaching (e.g., loss of Na, K, Rb), and mixing with intrabasinal materials such as diatoms and evaporitic salts. Mineral aerosols blown along the corridor by the winter Harmattan winds mix these basinal and basement materials. Dusts blown into the corridor from sub-Saharan Africa during the summer monsoon source from deeply chemically weathered terrains and are therefore likely to be more kaolinitic and stripped of mobile elements (e.g., Na, K, Mg, Ca, LILE), but retain immobile and resistant elements (e.g., Zr, Hf, REE). Finally, dusts blown southwestwards into the corridor from along the Atlantic Coastal Basin will be enriched in carbonate from Mesozoic–Cenozoic marine limestones, depleted in Th, Nb, and Ta, and locally contaminated by uranium-bearing phosphate deposits.

Cyclicity of Cenomanian-Turonian organic-carbon-rich sediments in the Tarfaya Atlantic Coastal Basin (Morocco)

Density logs from six completely cored and logged exploration wells across the Cenomanian-Turonian boundary in the Tarfaya Basin (Morocco) reveal cyclic sedimentation patterns of different frequencies. The cyclicity is mainly expressed as fluctuations in organic carbon and pelagic carbonate content. Two large scale cycles are observed, one from the top of theRotalipora cushmaniplanktic foraminiferal biozone to the upper part of theWhiteinella archaeocretaceaZone and one from the top of theW. archaeocretaceaZone to the upper part of theHelvetoglobotruncana helveticaZone. The large scale trends are related to major changes in organic carbon burial and probably reflect 3rd order sea-level fluctuations. Maximum organic carbon burial corresponds to benthos-free, laminated sediments, indicating bottom water anoxia. These periods coincide with sea-level highstands of the Vail-Juignet sea-level chart. Milankovitch-scale cyclicity is present, superimposed on these large scale cycles. The obliquity (39ky in the Cretaceous) signal is the most pronounced cyclicity and allows the duration of theW. archaeocretaceaZone to be estimated as 720ky. Individual obliquity cycles have a pronounced signature in the well logs and can be used as correlation horizons within the entire basin.

Potential Invasion of the Great Lakes by Fish Species Associated with Climatic Warming

Global warming associated with increasing atmospheric levels of carbon dioxide and other gases (i.e., the greenhouse effect) has been widely predicted. This warming process would likely alter the geographic distribution of numerous fish species in the Great Lakes region. The probability of 58 common, widely distributed species invading the upper Great Lakes (Lakes Huron and Superior) from the lower Great Lakes (Lakes Michigan, Erie, and Ontario), and the lower Great Lakes from the Mississippi and Atlantic Coastal basins was assessed by comparing ecological characteristics of possible invaders to those of 11 recently invading species, using discriminant function and principal components analyses. Twenty-seven of these 58 species were judged to be potential invaders of the Great Lakes as a response to climatic warming. These invading species would dramatically alter the present Great Lakes fish communities.

Origin and Significance of Evaporites in Basins Around Atlantic Margin

Immediately before the rifting and disruption of Pangaea, that great continent lay athwart the Triassic equator. The continental interior was arid, and isostatic adjustments following rifting produced drainage flowing away from the newly formed elongate troughs. These climatic and physiographic conditions inevitably led to the deposition of evaporites as the sea entered the rift zones whose margins now form the edges of the continents around the Atlantic. As a result, the sediment sequence in the Atlantic coastal basins shows a change from continental to evaporitic to normal marine conditions, except where evaporites were deposited directly onto newly formed oceanic crust. The age of these evaporites gives a chronology for the establishment of a permanent body of water be ween the separating fragments of Pangaea. The North Atlantic appeared in the north in Late Triassic time and became established farther south by the Middle Jurassic. The South Atlantic became established during Aptian time. The Gulf of Mexico was formed in Middle Jurassic time by the subsidence of continental material. The union of the two parts of the incipient ocean occurred during or later than Albian time and the resulting free circulation of water in the Atlantic eliminated the restriction necessary for the continued accumulation of evaporite deposits. No such deposits are known, therefore, from the northern coast of Brazil or from the northern side of the Gulf of Guinea.