Subsequent Sea-level Highstands Research Articles

Occurrence mode of Holocene tsunami overwash controlled by the geomorphic development along the eastern Nankai Trough, central Japan

Re-analysis of the lithological, paleontological, and geochemical data of three lagoonal sediment cores reveal the tsunami overwash history from 8000 to 6500 cal BP along the northern Suruga Bay coast, facing the eastern Nankai Trough. The coring sites are located in a transcect 0.8, 1.5, and 1.7 km inland, respectively, from the coastline at the time the overwash events occurred. The washover sand beds are distinguished from the lagoonal muds by the presence of quartz and mica, which are abundant on the outer (marine) side of the lagoon, as well as marine shell fossils. The increased C/S ratio often associated with sand beds, reflects input of open seawater to the reductive lagoon, and supports this interpretation. A maximum of eight tsunami deposits were identified between ∼7800 and 6500 cal BP, many of which can be correlated between the three cores. The magnitude of tsunami overwash implied by the thickness and grain size of tsunami deposits, and increase of C/S ratio following their deposition is relatively large at ∼7500-7200 cal BP during the post-glacial sea level rise and decreases with time during the subsequent high sea level stand. This occurrence mode (timing and scale of formation) of tsunami washover is explained by temporal changes in coastal geomorphology, rather than differences in the magnitude of individual tsunamis. Around 7500-7200 cal BP, sea level rise equilibrated with or exceeded the growth of the bay mouth barrier, and tsunamis could easily cross the small barrier and enter the lagoon, leaving behind thick and coarse-grained deposits. As the barrier becomes larger during the subsequent sea-level highstand, only the highest-amplitude waves exceeded the height of the barrier. By ∼6500 years ago, the barrier was high enough to prevent tsunamis from entering the lagoon. Similarly, the occurrence mode of tsunami deposits reported along the coast of Suruga Bay and Enshu-nada over the past 6000 years can be explained in relation to the growth of coastal barriers.

Low-accommodation foreland basin response to long-term transgression: A record of change from continental-fluvial and marginal-marine to open-marine sequences over 60,000 km2 in the western Canada foreland basin

Low-accommodation foreland basins have been of recent interest because their stratigraphic architectures provide insight into the interaction of tectonic subsidence and eustasy. Aptian-aged strata in Alberta, Canada (McMurray Formation and Wabiskaw Member) were deposited under the influence of a continental-scale river that longitudinally transported and debouched sediment into the southward-transgressing Boreal Sea in the distal, low-accommodation Western Canada Foreland Basin (WCFB). In this study, we present a high-resolution stratigraphic framework for the McMurray-Wabiskaw interval, which was deposited during a long-term transgression. Using wireline logs from >20,000 wells and more than 500 cores, regional mapping focused on resolving stratigraphic relationships and the distribution of depositional environments across approximately 60,000 km2 in the Athabasca Oil Sands Region of northeastern Alberta. The studied stratigraphic interval is part of a third-order sequence: the McMurray Formation developed during a period of relative sea-level fall, lowstand, and early transgression, while the overlying Wabiskaw Member was deposited during late transgression. Within the McMurray Formation, superimposed fourth-order sea-level fluctuations in this low accommodation setting created a complex amalgam of deltaic strata vertically and laterally juxtaposed with valley-fill deposits. Relative sea-level rises resulted in rapid transgressions in which the shorelines migrated >400 km landward. Thin (5–15 m thick), widespread deltaic parasequence sets were deposited during subsequent sea-level highstands. During relative sea-level falls, fluvial meandering channel belts developed in downcutting valleys, with associated shorelines dramatically prograding basinward. In the Wabiskaw Member, continued sea-level rise coupled with potentially decreasing sediment supply resulted in the development of tide-influenced/dominated channels and bars in estuaries. The final phase of the third-order transgression was marked by a regional transgressive ravinement event and several southwestward backstepping wave-dominated shorefaces/strandplains. This low-accommodation foreland basin succession demonstrates a transition from deposition in continental fluvial and marginal-marine settings subject to high-frequency eustatic changes, to open-marine conditions over a long-term transgression.

Evolution of the Urgonian shallow-water carbonate platform on the Helvetic shelf during the late Early Cretaceous

Urgonian platform carbonate (Cretaceous; Barremian, Aptian) forms an important lithostratigraphic unit in the Helvetic fold- and thrust unit of the northern Swiss Alps. Its widespread distribution and ubiquity allow for an integrated high-resolution study of macro- and microfacies, benthic foraminiferal biostratigraphy, sequence stratigraphy, and carbon-isotope and phosphorus records. The resulting data confirm the importance of environmental forcing and sea level change on the style of carbonate production and accumulation along the margin of the northern Tethys during the late Early Cretaceous. Stratal geometries, the succession of microfacies, the identification of major emersion surfaces, and biostratigraphy observed and analysed in twelve sections through the inner, middle, and outer platform, and in the panorama of the Churfirsten range, permit subdivision of the analysed succession into eight depositional sequences. The succession starts with a phase of sedimentary condensation (Altmann Member (Mb); late Hauterivian – late early Barremian; sequences H7, H8, and B1), followed by the deposition of hemipelagic sediments (Drusberg Mb; restricted to sequence B2 of the late early Barremian on the inner platform; and covering the late early Barremian to the middle late Barremian on the outer platform, and to the early Aptian on the outer shelf, thereby showing an important diachroneity of its upper boundary related to the inception and progradation of the Urgonian platform), and the development of predominantly lagoonal carbonate (Schrattenkalk Formation (Fm); early late Barremian – early Aptian; sequences B3, B4, B5, and A1). The Schrattenkalk Fm documents important progradation and aggradation of the carbonate platform, and the change from a ramp-like to flat-topped geometry. The oldest, allochthonous remains of the shallow-water carbonate platform were identified intercalated in and on top of the lower Barremian Drusberg Mb (sequence B2). Sequence boundary (SB) B3 resulted from an important regressive phase near the early-late Barremian boundary, which led to the emersion of the hemipelagic sediments of the Drusberg Mb in the inner part of the shelf and to the deposition of a lowstand systems tract at the base of the Lower Schrattenkalk Mb (late Barremian; sequences B3–5) in intermediate and distal domains. Deposition of in-situ platform carbonate started during the following transgressive phase in the middle late Barremian, which flooded the entire investigated area. The associated faunal assemblages and phosphorus contents indicate a concomitant increase in nutrient input, which led to a mixed photozoan-heterozoan platform association dominated by annelids and flat orbitolinids, and the formation of a condensed phosphate-rich bed on the outer shelf (Chopf Bed; middle late Barremian). The subsequent sea-level highstand allowed for the deposition of the first typical Urgonian carbonates rich in corals and rudists. This depositional sequence (B3) terminated by the important infilling of accommodation space combined with sea-level fall of at least 15 m. Later on, close to the Barremian-Aptian boundary, a further, major emersion phase (SB A1) was triggered by sea-level fall, estimated here as at least 16 m, which terminated this first phase in the deposition of rudist and coral-rich platform carbonates covering the middle late to latest Barremian (B3–B5). The overlying Rawil Mb (lowermost Aptian; transgressive systems tract A1) resulted from progressive deepening and document a phase of increasing eutrophication of the depositional environment, resulting in a mixed siliciclastic-carbonate platform build-up, characterized by sea-grass facies and the massive occurrence of Palorbitolina lenticularis. The overlying Upper Schrattenkalk Mb (lower Aptian; highstand systems tract A1) records recovery of the rudist-rich photozoan Urgonian platform. Its subsequent demise occurred well before the Selli oceanic anoxic episode (OAE 1a). It was initiated by emersion of the platform due to high-amplitude sea-level fall (SB A2), followed by eutrophication during the subsequent transgressive phase.The carbon-isotope records show an increase towards more positive values during the Lower Schrattenkalk Mb and the base of the Rawil Mb, interrupted in most sections by an excursion to lower values near the Barremian–Aptian boundary. A shift to lower values occurred also in the uppermost part of the Rawil Mb, followed by variable trends in the Upper Schrattenkalk Mb. These long-term trends are well correlated with the basinal record (Angles, La Bédoule). Deviations in the correlations are related to the influence of facies and microfacies, primary mineralogy, emersion phases, and post-depositional alteration.

Late Jurassic–Early Cretaceous palynostratigraphy of the onshore Mandawa Basin, southeastern Tanzania

New palynostratigraphic data are presented for Upper Jurassic and Lower Cretaceous formations of the Mandawa Basin, southeastern Tanzania. A Late Jurassic (Kimmeridgian‑Tithonian) age is confirmed for the Kipatimu Formation, from which the terrestrial sporomorphs allow a rough correlation to the mid Oxfordian‑Tithonian Classopollis‑Araucariacites‑Shanbeipollenits Assemblage Zone. A dinoflagellate cyst assemblage recorded in the Mitole Formation correlates to the Dingodinium jurassicum–Kilwacysta assemblage as described from the Tithonian Trigonia smeei Bed of the Tendaguru Hill, southeast Tanzania. Dinoflagellate cyst assemblages in the upper Nalwehe Formation contain species typical of the Hauterivian‑Barremian, while the Kihuluhulu Formation contains marine microfloras of Aptian to Albian ages. The palynological records confirm the presence of sediments related to profound marine transgressions and subsequent sea-level high-stands in the Late Jurassic (Kimmeridgian‑Tihonian) and in the late Early Cretaceous (Aptian‑Albian).

Response of subtropical submarine-cave ecosystem to Holocene cave development and Asian monsoon variability

AbstractA submarine cave is a unique environment that is dark, food limited, semi-isolated from the outside, and sheltered from wave action. However, our knowledge of the long-term change in submarine-cave ecosystems remains limited. We document here the community-scale responses toward long-term change in a submarine cave, Daidokutsu in Okinawa in southern Japan. Using both metazoans (ostracods and bivalves) and protozoans (larger benthic foraminiferans) in two sediment cores obtained from the cave, we reconstruct the faunal and diversity changes of the past 7 Kyr. All taxonomic groups showed long-term, gradual linear change of faunal composition from predominantly open-water taxa to predominantly cave taxa, and ostracods showed short-term variability of species diversity. The long-term faunal trend probably reflects gradual isolation of the cave ecosystem due to coral reef development (i.e., development of the cave ceiling) during periods of the Holocene transgression and subsequent sea-level highstand. The short-term diversity changes show substantial similarity to centennial- to millennial-scale Holocene Asian monsoon variability. Ostracod species diversity peaks tend to correspond with periods of strong East Asian winter monsoons. The results indicate that limestone submarine-cave ecosystems, an important cryptic habitat, developed gradually during the Holocene and may be sensitive to rapid climate changes.

The sedimentary infill of incised valley systems on the southeast coast of Australia: a tectonically stable coastal margin

Integrated sedimentological and micropaleontological (foraminifers and ostracods) analyses of two 55 m long borehole cores (S3 and S4) drilled in the subsurface of Lesina lagoon (Gargano promontory—Italy) has yielded a facies distribution characteristic of alluvial, coastal and shallow-marine sediments. Stratigraphic correlation between the two cores, based on strong similarity in facies distribution and AMS radiocarbon dates, indicates a Late Pleistocene to Holocene age of the sedimentary succession.Two main depositional sequences were deposited during the last 60-ky. These sequences display poor preservation of lowstand deposits and record two major transgressive pulses and subsequent sea-level highstands. The older sequence, unconformably overlying a pedogenized alluvial unit, consists of paralic and marine units (dated by AMS radiocarbon at about 45–50,000 years BP) that represent the landward migration of a barrier-lagoon system. These units are separated by a ravinement surface (RS1). Above these tansgressive deposits, highstand deposition is characterised by progradation of the coastal sediments.The younger sequence, overlying an unconformity of tectonic origin, is a 10 m-thick sedimentary body, consisting of fluvial channel sediments overlain by transgressive–regressive deposits of Holocene age. A ravinement surface (RS2), truncating the transgressive (lagoonal and back-barrier) deposits in core S4, indicates shoreface retreat and landward migration of the barrier/lagoon system. The overlying beach, lagoon and alluvial deposits are the result of mid-Holocene highstand sedimentation and coastal progradation.

The formation and evolution of the barrier islands of Inhaca and Bazaruto, Mozambique

The barrier islands of Inhaca and Bazaruto are related to the extensive coastal dune system of the Mozambican coastal plain, south-east Africa. Optically stimulated luminescence (OSL) dating of key stratigraphic units indicates that accretion of sediment within these systems is episodic. Both islands appear to have been initiated as spits extending from structural offsets in the coastline. Superposition of significant quantities of sediment upon these spits during subsequent sea-level highstands formed the core of the islands, which were anchored and protected by beachrock and aeolianite formation. At least two distinct dune-building phases occurred during Marine Oxygen Isotope Stage (MIS) 5, tentatively attributed to marine transgressions during sub-stages 5e and 5c. Although some localized reactivation of dune surfaces occurred prior to the Holocene, large quantities of sediment were not deposited on either island during the low sea-levels associated with MIS 2. Significant dune-building and sediment reworking occurred immediately prior to and during the Holocene, though it is not clear whether these processes were continuous or episodic. Significant erosion of the eastern shoreline of Bazaruto suggests that it is far less stable than Inhaca and may suffer further large-scale erosion. A model is presented for the formation of barrier islands along the Mozambican coastal plain.

Late-Quaternary paleoenvironmental evolution of Lesina lagoon (southern Italy) from subsurface data

Integrated sedimentological and micropaleontological (foraminifers and ostracods) analyses of two 55 m long borehole cores (S3 and S4) drilled in the subsurface of Lesina lagoon (Gargano promontory—Italy) has yielded a facies distribution characteristic of alluvial, coastal and shallow-marine sediments. Stratigraphic correlation between the two cores, based on strong similarity in facies distribution and AMS radiocarbon dates, indicates a Late Pleistocene to Holocene age of the sedimentary succession. Two main depositional sequences were deposited during the last 60-ky. These sequences display poor preservation of lowstand deposits and record two major transgressive pulses and subsequent sea-level highstands. The older sequence, unconformably overlying a pedogenized alluvial unit, consists of paralic and marine units (dated by AMS radiocarbon at about 45–50,000 years BP) that represent the landward migration of a barrier-lagoon system. These units are separated by a ravinement surface (RS1). Above these tansgressive deposits, highstand deposition is characterised by progradation of the coastal sediments. The younger sequence, overlying an unconformity of tectonic origin, is a 10 m-thick sedimentary body, consisting of fluvial channel sediments overlain by transgressive–regressive deposits of Holocene age. A ravinement surface (RS2), truncating the transgressive (lagoonal and back-barrier) deposits in core S4, indicates shoreface retreat and landward migration of the barrier/lagoon system. The overlying beach, lagoon and alluvial deposits are the result of mid-Holocene highstand sedimentation and coastal progradation.

Palaeogeographic and palaeoclimatic evolution of the Po Plain from 150-ky core records

Integrated sedimentological and micropalaeontological (foraminifers, ostracods, pollen) analyses of eight continuously cored boreholes, up to 140 m deep, reveal the depositional history and the palaeoclimatic evolution of southeastern Po Plain (northern Italy) over the last 150 ky. Age assignments are supported by the chronostratigraphic sequence framework calibrated by radiocarbon and pollen data, allowing detailed correlation with the oxygen-isotope record. Facies analysis (including identification of 12 distinct microfossils associations) and detailed stratigraphic correlations across several tens of km document a cyclic sedimentation pattern (including continental, coastal and shallow-marine deposits), which defines two transgressive–regressive sequences, deposited over the last two interglacial–glacial cycles. Two prominent stratigraphic markers, corresponding to wedge-shaped coastal sand bodies, are recorded between 0–30 and 100–125 m core depths. These sedimentary bodies were deposited during the two major transgressive pulsations and subsequent sea-level highstands of the last 150 ky, assigned to the Holocene and the Tyrrhenian (oxygen-isotope Substage 5e), respectively. The stratigraphic architecture of post-Substage 5e deposits shows consistent patterns of coastal evolution with changing sea-level position. Lowering of sea level between 125 and 70 ky (onset of Substages 5d and 5b, and Stage 4) resulted in extensive and repeated basinward shifts of facies, which can be observed across closely spaced unconformity surfaces associated to alluvial plain sedimentation (falling-stage systems tract). The general phase of sea-level fall was punctuated by short transgressive phases (Substages 5c, 5a and Stage 3 interstadials), which led to widespread deposition of organic-rich (lagoonal and swamp) deposits. Upper Stage 3 and Stage 2 deposits (lowstand systems tract) are replaced across a significant part of the study area by a hiatal surface. Holocene interglacial deposits are characterized by a retrogradational stacking pattern of coastal plain and littoral facies (transgressive systems tract), reflecting invariably the landward migration of a barrier–lagoon–estuary system. Subsequent highstand deposition (highstand systems tract) was characterized by extensive progradation of wave-influenced deltas and strandplains. The two major transgressive surfaces, of Tyrrhenian and Holocene age, are considerably easier to identify than the other key surfaces for sequence stratigraphic interpretation. The lowermost transgressive deposits display a pollen signature diagnostic of warm climate (interglacial) periods, showing wide forest development and relatively high pollen concentrations. By contrast, the deposits overlying the unconformable surfaces related to successive phases of sea-level fall are fingerprinted by pollen spectra dominated by Pinus and non-arboreal pollen types, with very low pollen concentrations, reflecting the onset of a stable cold climate vegetation characteristic of stadial to fully glacial conditions. The good match between facies architecture, pollen distribution and global sea-level evolution strongly suggests that Late Quaternary sedimentation in the Po Basin developed under a predominantly glacio-eustatic control. Stratigraphic architecture of the Po Basin thus can provide a useful analog for interpretation and correlation in the stratigraphic record of very rapid glacio-eustatic (fourth-order) cycles, with frequencies of about 100 ky.

The Barrier Islands of Southern Mozambique

Two barrier island systems from southern Mozambique (Inhaca and Bazaruto) are described. Both systems comprise a composite barrier island or barrier island chain that developed on a steep continental margin as a result of initial (Pleistocene) spit progradation. Continuing aeolian dune deposition against this initial core during subsequent sea-level highstands and lowstands has given rise to the highest dunes (120m) of any known barrier island. Wave reworking of the composite barrier island sediment during the past 6000 years of the Holocene sea-level highstand has resulted in the formation of spits, barriers, barrier islands and bluffs on the downdrift and bay side sections of the barrier island systems. The relative stability in position of the barrier systems at contemporary sea level is attributed to their large sediment volume and the development of equilibrium shoreline forms under swell waves. Beachrock and aeolianite outcrops provide anchor points for the development of contemporary shoreline forms.

Hardbottom Morphology and Relationship to the Geologic Framework: Mid-Atlantic Continental Shelf

ABSTRACT High-energy, sediment-starved continental shelves of the mid-Atlantic region have abundant hardbottoms that extend from the shoreface to the shelf edge. Because of the thin and irregularly distributed Holocene sand sheet, shelf morphology is determined mainly by outcropping Tertiary and Pleistocene stratigraphic units. Each unit and combination of units produces different hardbottom morphologies that depend upon the geometry and spatial relationships of the units, lithology and patterns of stratification, and subsequent weathering and erosion. Hardbottoms vary in surface relief from smooth, flat surfaces to scarped surfaces with up to 10 m of relief. The morphology ranges from sloping and stepped erosional ramps to vertical and undercut scarps with associated broad rubble ramps. Hardbottoms associated with each of the different gently dipping Tertiary depositional sequences have distinctive morphologies. Hardbottoms developed on Pleistocene units unconformably overlie the Tertiary sequences as flat-lying marine carbonates, or cut into them as channel systems backfilled with fluvial and estuarine sediments. Initial dissection of hardbottoms produced highly convoluted surfaces that resulted from subaerial weathering, stream erosion, and karst formation during sea-level lowstands. During subsequent sea-level highstands, these rimary morphologies were greatly modified through the interaction of bioerosion and storms. Understanding continental shelf hardbottoms is critical for interpreting the sedimentology and stratigraphy of the Atlantic Coastal Plain and for reconstructing paleoceanographic conditions, for the following reasons. (1) They are an extensive part of the stratigraphic record on shelves that are not actively subsiding and have small volumes of terrigenous input with low accumulation rates. (2) They are important stages in the formation of major stratigraphic unconformities, condensed sections, and sequence boundaries. (3) They support diverse biological communities that produce primary carbonate sediments and are rapidly degraded and modified by bioerosion and physical processes supplying abundant new sediment to the continental shelf.

Cenomanian phosphorites in the former Soviet Union

Cenomanian nodular phosphorites are widespread over vast areas of the Precambrian East European Platform (EEP). They are found in various parts of the Cenomanian forming thicker and richer beds close to the Cenomian/Turonian boundary. They are encountered nearly ubiquitously, but are concentrated on synsedimentary highs in thin and condensed sequences with numerous hiatuses. All of the economic deposits are confined to the paleohighss. Phosphorite pavements made of amalgamated, roworked and concentrated concretions are typical of the condensed sequences. More complete successions occur in the adjacent lows, where Upper Cenomanian sediments have been proved. Phosphorite in the lows are found as in-situ concretions either scattered irregularly or aligned in rows and spaced rather regularly. Calculations show that some 25 Gt of P 2O 5 was buried during the Cenomanian. A positive δ 13C anomaly was found to coincide with a thin layer of sandy to silty chalk, containing phosphatic intraclasts. The layer lies directly on the youngest and thickest pavement. The anomaly is believed to represent Oceanic Anoxic Event 2 (OAE-2) on the EEP. Consequently, (1) phosphorite pavement is most probably of Upper Cenomanian age, and (2) phosphogenesis is related to OAE-2. There is no black shale amidst the Upper Cenomanian on the EEP and sea-level variation during that time might be responsible for both the spatial and temporal differentiation of phosphorites and black shales. The former were deposited during transgressive phases, whereas the latter correspond to subsequent sea-level highstand. The extent of spatial differentiation might be preconditioned by the scale of difference between corresponding sea-levels. An upwelling scenario is hardly probable to explain the origin of the Cenomanian phosphorite of the EEP. A river-input mechanism is more probable, as it was advocated by Bushinsky.

Perriwinkle and Perriwinkle North Fields, Martin County, Texas: A Cisco-Canyon Lowstand Reef Complex

During the middle-early through early-middle deposition of sediments in Canyon field, a series of glacio-eustatically controlled sea level lowstands resulted in carbonate buildups seaward of the Horseshoe Atoll in Martin County, Texas. The resulting reef tracts consist of algal-bryozoan boundstones, clean forereef talus conglomerates, and other high-energy shelf carbonates. The reef complex was chemically eroded into a tower karst terrain during subsequent sea level lowstands and associated subaerial exposure. The highly sculptured paleotopography was mechanically eroded during the onset of a sea level highstand, filling lows with locally derived conglomerates. In addition, highstand basinal foreshelf conglomerates from the atoll were deposited. These conglomerates contain at least one reservoir-size upper Strawn high-energy shelf allochthon. A transgressive-regressive shelf margin reef tract was deposited seaward of the now exhumed Canyon paleokarst surface during a lowstand that occurred early in Cisco deposition. The tract consists of an algal-bryozoan boundstone, associated forereef talus conglomerates, immediate backreef grainstones and packstones, and erosional foreshelf detritus. Although subaerially exposed, the Cisco reef tract is not as highly solutioned as the Canyon tract, resulting in a lower relief paleotopographic surface. During subsequent sea level highstands, both the Cisco and Canyon reef tracts were buried under lower Cisco basinal foreshelfmore » conglomeraters, masking the stratigraphic intricacies of the paleobathymetric surface and obscuring the relationship of time correlative reservoirs within the shelf-edge complex.« less