Regional Sea-level Fall Research Articles

Silurian deltaic progradation, Tassili n’Ajjer plateau, south-eastern Algeria: Sedimentology, ichnology and sequence stratigraphy

The economic potential for unconventional shale oil and gas production in the Silurian of the Berkine – Ghadames and Illizi basins (BGI) in south-eastern Algeria has been recently confirmed through exploration drilling. The aim of the present paper attempts a better understanding of the Intra-Tassilian depression within the entire Silurian of the Tassili n’Ajjer plateau. The continuous deposits of the Silurian are exposed at the southern margin of the prolific BGI basins, in the Tassili n’Ajjer plateau, offering the chance to understand the sedimentology, ichnology, and to present a detailed sequence stratigraphy framework for the region.The 410 m-thick clastic Silurian sedimentary strata are subdivided into three formations in the context of sequence stratigraphy, namely: (i) the Oued Imihrou Fm. (Llandoverian) overlain by (ii) the Atafaïtafa Fm. (late Llandoverian to Wenlockian), and (iii) the Oued Tifernine Fm. (late Wenlockian to Pridolian). These can be also distinguished across the entire investigated area and laterally traceable over kilometers. Clear cyclic stacking patterns are identified within the four studied sections showing progressively a general trend of thickening- and coarsening-upward, over a complete 2nd-order megasequence (SIL-1 MS). This transgressive-regressive succession suggests deltaic progradation, shallowing and basin infilling as evidenced by numerous diagnostic sedimentary features and trace fossils, largely from eastern-to western-Tassili plateau. Indeed, the wealth of outcrop data in the Silurian siliciclastic succession enables us to distinct thirteen facies (facies A-M), ranging from shallow-to marginal-marine facies, and in turn, grouped into six facies associations (FA1-FA6). The lowermost part of the succession, which is the most prolific sources of hydrocarbons in North Africa, consists of thick organic-rich graptolite-yielding black ‘hot’ shales and ‘lean’ shales with sparse bioturbation with small Thalassinoides belonging to the distal Cruziana ichnofacies. In contrast, the uppermost part of the Silurian deposits becomes progressively coarser and fluvial in response to the progradation of the North African Akakus deltaic system, during regional sea level fall and uplifting of the region. These progradational deposits exhibit well-preserved trace fossils with moderate to high degree of bioturbation, such as Skolithos or the so-called “Tigillites” pipe-rock, Cruziana isp., Rusophycus isp., Monocraterion isp., and Syringomorpha.The SIL-1 MS is bounded by a post-glacial latest Hirnantian unconformity on the basal (SB1), as confirmed by the moderately diverse early Silurian graptolite faunas, and by the Caledonian unconformity on the top (SB7). Each of the three formations of SIL-1 MS reveals two major 3rd-order progradational sequences, commonly delineated by discontinuity surfaces (in ascending order, SB1 to SB7), and in turn, these six sequences (i.e. Si-1 to Si-6) are subdivided into at least ten shorter-term cycles. The regional extent of each unconformity is directly linked to significant facies changes and to inflection points on the global sea level curve.

Mapping Sea Level Rise Behavior in an Estuarine Delta System: A Case Study along the Shanghai Coast

Abstract Sea level rise (SLR) is a major projected threat of climate change that is expected to affect developing coastal cities located in estuarine delta regions. Shanghai is one such city, being located in the Yangtze River Delta (YRD). It is difficult, however, for decision-makers to implement adaptation due to the uncertain causes, magnitudes, and timings of SLR behaviors. This paper attempts to map the causes and magnitudes of SLR behaviors on a decadal scale. We analyze the tidal level records from 11 tidal gauge stations and the corresponding bathymetry measurements around these stations since 1921. We identify three new SLR behaviors along the Shanghai coast due to anthropogenic geomorphologic changes (AGCs), besides the well-known eustatic sea level rise (ESLR), tectonic subsidence (TS), and urban land subsidence (ULS). The first new behavior is regional sea level rise (RSLR), which occurs as a result of land reclamation and deep waterway regulation. The second is regional sea level fall (RSLF), which occurs because the channel bed is eroded due to sediment supply decline in the river catchment. The last SLR behavior is local tidal datum rise (LTDR). Thus, we project that the magnitude of SLR for the Shanghai coast ranges from 10 cm to 16 cm from 2011 to 2030. Clarifying SLR behaviors is important to aid local decision-makers in planning structural and non-structural measures to combat escalating flood damage costs in an estuarine delta system; this field is full of future challenges.

Episodic euxinia in the Changhsingian (late Permian) of South China: Evidence from framboidal pyrite and geochemical data

A multiproxy study of a new Upper Permian–Lower Triassic section (Xiaojiaba) in Sichuan Province, China, documents large changes in marine productivity, redox conditions and detrital input prior to the latest Permian mass extinction. Marine productivity, as proxied by total organic carbon content (TOC), biogenic SiO2, and excess barium, displays a long-term decline through most of the Changhsingian stage (late late Permian), culminating in very low values around the Permian–Triassic boundary. Concurrently, redox proxies including pyrite framboid, δ34Spy, Moauth and Uauth, and Corg/P document a shift from suboxic to dysoxic/oxic conditions that was interrupted by several episodes of benthic euxinia, and detrital siliciclastic proxies (Al, Hf, Nb, and REEs) suggest an increased flux of weathered material from land areas. The long-term changes in productivity, redox conditions, and terrigenous detrital fluxes were probably caused by a regional sea-level fall across the South China Craton. On the other hand, the brief euxinic episodes occurring during the late Permian had oceanographic causes, probably related to the transient upward expansion of the chemocline at the top of the oceanic oxygen-minimum zone. These euxinic episodes may have been harbingers of the more widespread anoxia that developed concurrently with the latest Permian mass extinction and that may have played a major role in triggering the largest biotic crisis of the Phanerozoic.

Stratigraphic and palaeoenvironmental framework of the Early Permian sequence in the Salt Range, Pakistan

The Early Permian Gondwana regime succession of the Nilawahan Group is exposed only in the Salt Range of Pakistan. After a prolonged episode of non-deposition that spanned much of the Palaeozoic, the 350 m thick predominantly clastic sequence of the Nilawahan Group records a late glacial and post-glacial episode in which a range of glacio-fluvial, marine and fluvial environments evolved and accumulated. The Early Permian succession of the Salt Range has been classified into four formations, which together indicates a changing climatic regime during the Early Permian in the Salt Range region. The lower-most, Tobra Formation unconformably overlies a Cambrian sequence and is composed of tillite, diamictite and fresh water facies, which contain a floral assemblage (Gangamopteris and Glossopteris) that confirms an Asselian age. The Tobra Formation is overlain by marginal marine deposits of the Dandot Formation (Sakmarian), which contain an abundant brachiopods assemblage (Eurydesma and Conularia). Accumulation of the Dandot Formation was terminated by a regional sea-level fall and a change to the deposition of the fluvial deposits of the Warchha Sandstone (Artinskian). The Warchha Sandstone was deposited by high sinuosity meandering, avulsion prone river with well developed floodplains. This episode of fluvial sedimentation was terminated by a widespread marine transgression, as represented by the abrupt upward transition to the overlying shallow marine Sardhai Formation (Kungurian). The Early Permian Gondwana sequence represented by the Nilawahan Group is capped by predominantly shallow shelf carbonate deposits of the Tethyan realm. The sedimentologic and stratigraphic relationship of these four lithostratigraphic units in the Salt Range reveals a complex stratigraphic history for the Early Permian, which is mainly controlled by eustatic sea-level change due to climatic variation associated with climatic amelioration at the end of the major Gondwana glacial episode, and the gradual regional northward drift to a lower latitude of the Indian plate.

Multiple cenozoic invasions of Africa by penguins (Aves, Sphenisciformes)

Africa hosts a single breeding species of penguin today, yet the fossil record indicates that a diverse array of now-extinct taxa once inhabited southern African coastlines. Here, we show that the African penguin fauna had a complex history involving multiple dispersals and extinctions. Phylogenetic analyses and biogeographic reconstructions incorporating new fossil material indicate that, contrary to previous hypotheses, the four Early Pliocene African penguin species do not represent an endemic radiation or direct ancestors of the living Spheniscus demersus (blackfooted penguin). A minimum of three dispersals to Africa, probably assisted by the eastward-flowing Antarctic Circumpolar and South Atlantic currents, occurred during the Late Cenozoic. As regional sea-level fall eliminated islands and reduced offshore breeding areas during the Pliocene, all but one penguin lineage ended in extinction, resulting in today's depleted fauna.

Crustal uplift and sea level rise in northern Cascadia from GPS, absolute gravity, and tide gauge data

We combine data from nine GPS, absolute gravity, and tide gauge stations to estimate the relation between sea‐level rise, vertical motion, and solid Earth processes in the Pacific Northwest. GPS vertical velocities (in ITRF2000) and absolute gravity rates are well correlated, with a gradient of 0.2 ± 0.1 μGal mm−1, but show a significant offset of 0.53 ± 0.30 μGal yr−1 (2.2 ± 1.3 mm yr−1) (95% confidence). Tide gauge and GPS data indicate a northeast Pacific regional sea‐level rise of 1.7 ± 0.5 mm yr−1, aligned to ITRF2000, or an unlikely regional sea‐level fall of −0.5 ± 0.5 mm yr−1, aligned to absolute gravity. Although we cannot rule out a bias in the GPS reference‐frame alignment, our results suggest a possible absolute gravity bias by a long‐period mass increase from an unknown near‐surface or deep‐seated source. The impact of such a mass increase on gravity, vertical motion, and sea level remains to be defined.

An eolian facies within the Upper Jurassic Smackover Formation, Tchula Lake field, Mississippi

The Late Jurassic upper Smackover Formation in Mississippi is primarily composed of an overall regressive sequence of marine carbonate ramp deposits. In central Mississippi, however, carbonate deposition was interrupted by a large influx of siliciclastic sediment. Tchula Lake field is unique not only because it produces gas and condensate from dolomitic quartz sandstone deposited in a mixed siliciclastic-carbonate environment but also because it produces from what we interpret to be an eolian dune sand facies. Throughout the Gulf Coast Basin no previous literature has cited an eolian facies within the Smackover Formation. Several characteristics of the producing zone sandstones have led to interpretations of this unit as an eolian coastal dune facies deposited on and around a Jurassic paleohigh. Facies and paleoenvironmental interpretations are based on compositions and textures described in cores and thin sections. The eolian facies consists of fine-grained, dolomitic quartz sandstone with a mean porosity of 10-20% and permeability of 0-800 md. The sand grains are rounded to subrounded and well sorted. The sandstone exhibits a sequence of upward-steepening planar laminations capped at the top by a set of small-scale wedge planar cross-laminations. The sharply defined, parallel lamina are less than 5 mm thick and are commonly inversely graded and contain probable vadose meniscus carbonate cements. This facies is interpreted to have been deposited in a backshore coastal dune environment. The unit is characterized by low-angle (<20 degrees dip) climbing translatent strata commonly produced by the migration of wind ripples along a dune. No fossils or trace fossils are found within this facies. Eolian-deposited sandstone is a potential reservoir rock along the updip limit of the Smackover Formation in west-central Mississippi. Other paleohighs along the Jurassic shoreline may have undergone similar eolian deposition during a regional sea level fall. Eolian-deposited sandstones on these paleohighs should have (Begin page 486) similar reservoir properties as the sandstone at Tchula Lake field. The updip Smackover Formation in west-central Mississippi is an underexplored horizon, and eolian-deposited reservoirs have strong potential as a new exploration target. The sea level fluctuation event associated with deposition at Tchula Lake field also has implications for exploration in Upper Jurassic sedimentary rocks across the entire United States Gulf Coast Basin and prompts a reevaluation of these reservoirs.