Bayhead Delta Research Articles

Sedimentary organic matter and paleoenvironmental reconstruction of the Santana Group (Lower Cretaceous), Araripe Basin, Northeast Brazil

Depositional controls of the Santana Group in the Araripe Basin are still being debated. The main controversial subject is their marine influence and paleoenvironmental evolution. In this study, palynofacies analysis was performed on 59 samples from one core drilled at Sítio Romualdo in the Araripe Basin to investigate sedimentary organic matter. Three palynofacies associations (structureless, continental particles, and aquatic + opaque) were identified. The Santana Group consist of Barbalha, Crato, Ipubi and Romualdo formations. Based on palynofacies associations we conclude that the depositional setting of the Barbalha Formation reflects a fluvially-fed lacustrine system under oxic conditions. The Crato Formation, associated with tidal flats, coastal lakes, and lagoons within an open bay environment, represents a bayhead delta system. The Ipubi Formation corresponds to a coastal plain typical of sabkha paleoenvironments, characterized by low energy under reducing conditions, during drier climate conditions. The Romualdo Formation records mainly an estuarine paleoenvironment characterized by dysoxic to anoxic conditions. The presence of palynomorph groups typifies a late Aptian age for the Santana Group, which is based on the range of guide species marked mainly by the presence of Sergipea variverrucata biozone (P-270), as well as the occurrence of Complicatisaccus cearensis and Equisetosporites maculosus. The identification of marine palynomorphs (dinoflagellate cysts Subtilisphaera and foraminiferal test linings) are indicative of several, local marine incursions.

Bayhead delta succession as a stratigraphic marker of sea‐level changes during the early to late Holocene – the Nakdong valley of south‐eastern Korea

ABSTRACTBayhead deltas form at the interface between fluvial and estuarine systems. As such, they are sensitive to processes operating in both the fluvial catchment and the marine realm, including past relative sea‐level changes. The Nakdong valley is a small and confined incised valley containing a bayhead delta that provides a record of fluvial sediment input and accommodation changes. Based on a facies analysis that included 833 sediment textures and 118 optically stimulated luminescence and radiocarbon ages within five cores up to 70 m long, this study reconstructed the development of a fully filled bayhead delta in response to relative sea‐level changes through the early to late Holocene. Sixteen facies are defined and grouped into eight facies associations: alluvial plain; fluvial channel and floodplain; oxidized fluvial channel; tidal flat; tidal bar; central basin and bayhead prodelta; bayhead delta front; and bayhead delta plain. The sequence stratigraphy of the Nakdong valley fill can be divided into three systems tracts: a lowstand systems tract, a transgressive systems tract and a highstand systems tract. Overall sedimentary analyses suggest six depositional stages corresponding to variations between sediment inputs and sea‐level rise of the developing bayhead delta in the Nakdong valley. The depositional model tracks relative sea‐level rise in the Nakdong valley after 12 ka during the early to late Holocene. These analyses on the Nakdong bayhead delta succession illustrate the balance between sediment inputs and periods of accelerating and decelerating relative sea‐level rise during the Holocene. The architecture of the valley fill also records two abrupt disequilibrium events from 9 to 8 ka and 5 to 3 ka, likely resulting from rapid sea‐level rise during the deglacial outburst flooding of the early Holocene and decreased sediment inputs during regional climatic changes associated with the middle Holocene, respectively.

Lithofacies and Sediment Sequences of a Microtidal, Wave-Dominated Tropical Estuary in Somone Lagoon (Senegal, West Africa)

Estuary sedimentary sequences have been the focus of several papers in the last decades; however, the majority these papers were centered in mesotidal and macrotidal estuaries of the middle latitudes. This present paper studies, from a sedimentological point of view, the infilling of a microtidal, wave-dominated tropical estuary, where wide tidal flats, mangroves and sabkhas are developed. Somone Lagoon is a Senegalese protected environment, very rich from an ecological point of view and with a definitive touristic vocation. For this work, 14 piston cores were studied. Additionally, the grain size, calcium carbonate and organic matter contents of 61 sediment samples vertically distributed in these cores were analyzed. The distribution of facies and the vertical sequences of sediments show the influence of the tropical seasonal fluctuations of fluvial sediment supply and evaporation processes. A high degree of bioturbation and an elevated organic content can be interpreted as the result of a high capacity of retention of the organic material into the estuary influenced by the weak tidal currents typical of a microtidal regime. These processes, acting since the last Holocene invasion of sea level, caused an advanced state of infilling of this estuarine system where both flood tidal deltas and bay head deltas prograde above the fine facies of the central domain of the estuary.

Sedimentary evolution of the Nakdong River deposits on the Korea Strait shelf since the Last Glacial Maximum

We investigated the development of depositional systems and the evolution of the depositional sequence in the Korea Strait shelf deposits since the Last Glacial Maximum (LGM), integrating a dense grid of seismic data and multiple long-core datasets from the delta plain to offshore. This depositional sequence includes the lowstand systems tract (LST), transgressive systems tract (TST), and highstand systems tract (HST) over an incised valley sequence boundary. These tracts developed discrete depositional environments in both the bay and the open shelf area demarcated by the island arc. The evolution of these tracts can be delineated into five distinct phases based on the sea-level curve for the Korea Strait since the LGM. First, in the LST (the LGM lowstand; 25–17 kyr BP), fluvial deposits with upward-fining characteristics were observed in bedrock-incised valleys within the bay, while amalgamated fluvial deposits were identified in unconfined incised valleys over Pleistocene sequences on the open shelf. A shelf-margin delta was also detected along the continental shelf break. The TST is composed of three subunits: the lower, middle, and upper TSTs. In the lower TST (the post-LGM transgression; 17–13 kyr BP), estuarine deposits were occupied in the bay, while a broad sand sheet with a few moribund sand ridges was present on the open shelf. In the middle TST (the Younger Dryas (YD) event; 13–12 kyr BP), bay barrier islands were observed growing along the island arc, but a mid-shelf delta and a sand ridge developed on the open shelf. In the upper TST (the Early to Mid-Holocene transgression; 12–6 kyr BP), the fining-upward retrogradational bayhead delta–estuarine complex within a bay encircled by barrier islands developed toward the head of the estuary. Finally, in the HST (the Late Holocene highstand; 6–0 kyr BP), the coarsening-upward delta was prograding seaward, with its prodelta initially distributed off the river mouth and later as a mud belt along the eastern coast. The TST deposits, encompassing diverse sedimentary systems that make up a substantial portion of the depositional sequence on the shelf, were significantly influenced by a combination of topographical characteristics limiting the discharge of sediment from the river into the open shelf and the displacement of accommodation space with stepwise sea-level rise.

The Messinian reservoir in El-Tamad oil field, Nile Delta, Egypt: Seismic interpretation and 3D modeling

El-Tamad Field, the first oil discovery in the onshore Nile Delta, is one of the promising areas in the Nile Delta for oil and gas. Therefore, thirty seismic lines, core data, and real-time flowline resistivity measurements of the Messinian Qawasim Formation were analyzed to characterize and model the reservoir in El-Tamad Field. The structure model shows that there are three main faults in the investigated area; F1 fault is a major growth fault trending E-W with a downthrown to the north, F2 trending NW-SE with a downthrown northeast, and the NE-SW trending F3 fault with a downthrown southeast. The depth-structure contour map on the reservoir's top shows that there is a rollover anticline fold formed at the downthrown side of the major growth fault. The petrophysical model illustrates that effective porosity increases towards both of southeastern and southwestern parts, while water saturation decreases in the southeastern part of the field, and the net-to-gross increase towards the center. Penetrating more vertical and sidetrack wells along the line connecting wells 1, 3, and 4 in El-Tamad Field, especially in the crest of the anticline fold that was recorded in the area is highly recomended. The spatial variation of the reservoir quality was attributed to the depositional setting, where many branches of larger distributary channels (sand facies) accumulated within a muddy estuary (shale facies) on the front of a bay-head delta.

Paleoenvironmental reconstruction of the Permian coal beds in the Paraná Basin (Brazil): evidence from organic geochemical and sedimentological analyses

The Rio Bonito Formation (Paraná Basin) contains the thickest coal beds in Brazil, that are related to peat-swamp developed in coastal depositional systems during the early Permian. This study aims to understand the source of the coal organic matter and to distinguish depositional sub-environments along the coal-bearing interval from the southern border of the basin. The analysis was performed on sedimentological and stratigraphic data from 10 wells combined with organic petrology and geochemistry of the three coal beds. Samples were collected in different key beds from the SG-14 well, in different and strategic depths at the base (218.0 m), at the intermediate (209.0 m), and at the top (181.0 m) of the coal-bearing interval. The vitrinite reflectance measurements of the selected samples, Tmax values, C29 ααα 20 S/(20 S + 20 R) ratio, and the absence of peak Ts (and presence of peak Tm) revealed the organic matter in the coal-bearing interval is thermally immature, suggesting low burial depth for the Rio Bonito Formation in the study region. Maceral composition showed predominance of vitrinite and inertinite in the three coal samples. Rock-Eval pyrolysis data indicate that the coal beds are composed of type III kerogen in the three samples, while the biomarkers indicate type II-III kerogen mixtures. The n-alkane distribution profiles suggest dominance of terrestrial organic matter in the samples, with a possible marine contribution at the top of the coal-bearing interval. The pristane/phytane (Pr/Ph) and C27/C29 ααα20 R ratios (regular steranes) revealed sub-anoxic condition and land plants as major organic matter source for basal and intermediate parts of the coal-bearing interval. They also indicate an increased anoxic conditions and possible marine algal contribution toward the uppermost coal level. The presence of cone-in-cone-shaped structures (SG-07 well, depth 157.5 m) and Orbiculoidea fossils (SG-14 well, depth 192.5 m) are indicative of brackish water and support river-marine source mixtures at the top of the coal-bearing interval. The evolutive sedimentation model comprises four paleoenvironments for the Rio Bonito Formation along geological time, in which the basal part of the coal interval is interpreted as overbank deposits in a fluvial depositional system. On the other hand, the intermediate part of the coal-bearing interval was formed in bay-head delta, and the upper part was deposited in the central estuary, both in a tide- and wave-influenced estuarine context.

Ostracoda and foraminifera biostratigraphy and palaeoenvironmental evolution of the Aptian Santana Group, post-rift of the Araripe Basin, Brazil

The Santana Group, consisting of the Barbalha, Crato, Ipubi and Romualdo formations, records the post-rift sequence of the Gondwana break-up in the Araripe Basin. The post-rift phase of the South Atlantic rifting is dated as the Alagoas Stage, which corresponds to the ostracod biozone 011. A detailed ostracod stratigraphical distribution along the Santana Group, allowed to standardise a common ostracod for the biozone name and a code, the Pattersoncypris micropapillosa Biozone (OST-011). Four different ostracod associations were recognized establishing the subzones: Pattersoncypris cucurves (OST-011.1), Pattersoncypris cucurves-Neuquenocypris berthoui (OST-011.2), Damonella grandiensis (OST-011.3) and Pattersoncypris crepata (OST-011.4). Planktic foraminifera were also recovered in different associations, leading to identify two international biostratigraphic intervals and calibrate the ostracod subzones, the Early Aptian Leupoldina cabri Zone includes the OST-011.1 and OST-011.2 subzones, and the upper Late Aptian Hedbergella infracretacea–Microhedbergella miniglobularis composite zone correlated to the OST-011.4 subzone. Between these zones there is an interval without foraminiferal biostratigraphy resolution, the OST-011.3 subzone which is assigned to the lower Late Aptian. The palaeoenvironmental evolution of the Santana Group was reconstituted from the integrated study of ostracods, planktic and benthic foraminifera. Through the Aptian, the Araripe Basin evolved from a transitional to a marine environment, recording estuarine, fluvial to bayhead deltas, a bay coastal watershed that reached better-marine conditions under extreme aridity and the full installation of an epeiric sea flooding the area.

Coastal changes and human occupation in the eastern part of Thessaloniki Bay: Geoarchaeological investigations of Toumba Gona

AbstractThe Holocene marine transgression in the Aegean Sea area has significantly impacted prehistoric societies. Toumba Gona is a tell site located at the mouth of the Anthemous River, east of Thessaloniki Bay. According to earlier research, the site should be dated at most to the late stage of the Early Bronze Age. Geoarchaeological research by means of electrical resistivity tomography, vibra‐coring, sedimentological analysis, and radiocarbon dating shows, however, that the direct proximity of the site witnessed the maximum marine transgression around 3000–2500 BC and the human habitation phase before the 3rd millennium BC, before the transgression. The coastline began to recede due to the delta progradation, which resulted in the formation of extensive marshes to the south of the Toumba. The increased fluvial activity since the end of the 4th millennium BC is relatable with the progradation of the Anthemous River's bay head delta. Human occupation was recorded before and after the marine transgression thus suggesting human persistence in the coastline environment.

Large deltas, small deltas: Toward a more rigorous understanding of coastal marine deltas

Deltas are subaerial landforms that cap underlying deposits with subaqueous extensions that result from a river feeding sediment directly into a standing body of water at a rate that overwhelms any effective dispersal processes derived from the ambient basin. This definition encapsulates both the terrestrial surface expression and the geological focus on the entire sediment mass. Environmental studies also focus on the ecology of deltaic wetlands, their drowning history, and related sustainability issues including societal considerations, history, and culture. A mean 76 ± 16% drop in hydraulic energy occurs in all subaerial deltas regardless of size, given the break in gradients separating fluvial and deltaic surfaces, driving an ever-decreasing bed-material transport, shallowing of distributary channels and concomitant overbank flooding. A delta's sediment mass grows from the addition of new river loads but can also include aeolian and marine sediment derived from outside the delta domain, growth of peat and other biomass, and inputs from human action. Removal of sediment is via river plumes interacting with marine currents, wave-induced transport, sediment failures and gravity flows, high-tide inundation onto the delta plain, tidal channel widening and deepening, and human action (peat, clay, sand and gravel mining). A delta's trapping efficiency ranges from 0 for small-load rivers that discharge directly into an energetic ocean, to 80% for large deltas, and up to 100% for some semi-enclosed bayhead deltas, including fjords. The global (ensemble) subaerial delta aggradation rate is ∼1.6 mm/y if 70% of the global sediment load exits the river mouth(s), a reminder of how much sediment can be expected to be delivered to the surfaces of global deltas at a time when the 2022 CE sea level rise is ∼4 mm/y. At the planetary scale, deltas are environmentally complex given Earth's range in climate, hydrodynamics, tectonic settings, relative sea-level provinces, sediment input, redistribution processes, and human actions. Under natural conditions, the subaerial portion of deltas adapt to change by advancing, retreating, switching, aggrading, and/or drowning, whereas many modern deltas are structurally constrained by societal needs. The 89 large and mud-rich coastal marine deltas (i.e. subaerial area > 1000 km2) account for 84.3% of Earth's total deltaic area that hosts >89% of all humans occupying deltas, many living within megacities. The 885 medium-size deltas (i.e. subaerial areas 10–1000 km2) account for 15.5% of the global delta area and 10.5% of humans living on deltas, with characteristics that fall between the small and large delta categories. The 1460 small and essentially sandy deltas (1–10 km2), including all fjord deltas, are impacted less from human action (with exceptions) and most are better able to withstand climate change. Recognizing the limits of big data in capturing delta complexity, field data remains a necessary gold standard for site investigators.

Paleocene (65–63 and 58.5 ma) marine flooding and 62–60 ma sediment bypass in southern Wyoming, U.S.A.: Implications for Laramide sediment flux to the Gulf of Mexico

ABSTRACTSedimentary structures unique to tidally influenced environments and unambiguously salinity-stressed marine ichnofossil assemblages in the lower Paleocene Ferris and upper Paleocene Hanna formations of Wyoming's Hanna Basin (HB) necessitate major revision of local and regional reconstructions of the Paleocene Western Interior Seaway (WIS). Preserved in sandy estuarine bars, sandy tidal flats, heterolithic distributary channels, bayhead delta, and flood-tide-delta deposits similar those in the modern Trinity River, its bayhead delta, and the San Luis Pass flood-tidal delta on the Texas coast, these these assemblages include Arenicolites, Bergaueria, Fuersichnus, Gyrochorte, Ophiomorpha, Palaeophycus, Planolites, Psilonichnus, Rhizocorallium, Rosselia, Siphonichnus, Skolithos, Spongeliomorpha, Taenidium, Thalassinoides, and tetrapod tracks. Mapping an ∼ 325-m-thick succession of lower Paleocene strata (65 to 63 Ma) around the western HB reveals a series of marine flooding events, each followed by coal accumulation. A similar, 170-m-thick succession of interfingering coastal-plain and restricted-marine strata occurs in the upper Paleocene (58.5 Ma) Hanna Formation, following accumulation of lacustrine and floodplain deposits and an episode of major gravel and cobble progradation from 62 to 60 Ma. These younger ichnofossils record the final major transgression of the WIS and have major implications for the depositional environment of the time-equivalent Waltman Shale in the Wind River Basin to the north and for sediment routing to the Gulf Coast Wilcox sands. Ichnofossils are an underutilized source of physicochemical proxy data that are lifting the veil from the cryptic Paleocene transgressions of the WIS that have for so long remained undetected because of the absence of open-marine body fossils.

An integrated sedimentological, rock typing, image logs, and artificial neural networks analysis for reservoir quality assessment of the heterogeneous fluvial-deltaic Messinian Abu Madi reservoirs, Salma field, onshore East Nile Delta, Egypt

This study introduces an integrated evaluation of geological and geophysical data, including sedimentology, diagenetic alteration, image log analysis, core measurements, formation evaluation, and a neural analysis technique (K-mode algorithm) to characterize the upper Messinian heterogeneous reservoirs of the Salma Field, Nile Delta, Egypt. It links observed reservoir permeability and flow zone indicators (FZI) to predict reservoir quality and distribution within un-cored parts of the field. Core and image log analysis show that the Abu Madi sandstone reservoir is composed of seven clastic litho-facies deposited within fluvial to deltaic environments. The reservoir is controlled by four hydraulic flow units (HFU's) and five flow units (FU). Fluvial channel facies, tidally influenced fluvial channel facies, and uppermost parts of bayhead delta facies are dominated by clean sandstone with a low clay content (avg. 20%). These facies are characterized by the high pore-throat sizes (R35 and FZI values), indicating a pore system dominated by mega-to macro-pores. The estuarine facies is composed of mudstone, siltstone, and argillaceous sandstone, with 25% average clay content and moderate R35 and FZI values, indicating a pore system dominated by macro-to meso-pores. The heterolithic estuarine and bayhead delta facies contain abundant argillaceous-rich sandstones, with 29% average clay content and low R35 and FZI values, indicating a pore system dominated by micro-pores. A neural log technique was applied to predict FZIs and permeability in un-cored intervals. Paleocurrent analysis was conducted using image log data to guide sweet spot and reservoir quality tracking across the field. Reservoir quality is controlled by both diagenetic and depositional processes, chiefly an abundance of detrital clays, grain size, and sorting. In the Salama Field reservoirs, mineral dissolution, cement dissolution, and micro-fractures enhance the pore system, while pore-filling and grain-coating detrital clays reduce reservoir quality. These results are important as they improve the wider understanding of the Messinian Abu Madi reservoir in the wider Mediterranean region.

Evolution of a single incised valley related to inherited geology, sea level rise and climate changes during the Holocene (Tirso river, Sardinia, western Mediterranean Sea)

We performed a morpho-stratigraphic study of the Tirso River incised valley (Sardinia Island, western Mediterranean Sea), an erosional feature crossing the Sinis fault, a major normal fault bordering the Campidano basin between the Gulf of Oristano and the western Sardinia shelf. High-resolution seismic reflection profiles and multibeam echosounder data, integrated by age-constrained stratigraphic logs derived from 9 sediment cores enabled us to reconstruct the valley evolution during the Holocene. We found that the Tirso valley is the result of a single event of incision and infill during the last eustatic cycle, strongly controlled by the presence of the Sinis fault. In fact, this structure represents a geological threshold that marks an abrupt change in substrate lithology and seabed slope, which controlled the valley morphology, narrow when downcutting early Pliocene formations along the steeper open shelf, and wider inside the Gulf, in the Pleistocene alluvial deposits of the flatter Gulf of Oristano. The sedimentary record starts with alluvial sediments filling the valley along the shelf during the initial phase of sea level rise, i.e., over 10 ka. During the last ~9.0 ka, a bay head delta developed, with the formation of barriers at the gulf entrance. In the mid-late Holocene, the progressive sea-level rise led to rapid drowning of the barrier system, recorded by marine and estuarine sediments filling the valley. Analysis of ecological associations in the cores, collected along a valley-normal transect, allowed for a detailed reconstruction of the paleo-environmental conditions during the latest phase of the incised valley filling controlled by global climatic variations in the Mediterranean region between ~9.0 and ~ 4.5 ka. Together with eustasy, our work reveals that the evolution and sedimentary infill of the Tirso incised valley was strongly controlled by inherited geological constraints, which influenced the morphology of the valley and the stratigraphic pattern.

Holocene filling of a narrow estuary in a regressive coast: The Paleo-Kinu Bay region, central Japan

Estuaries are typical depositional systems of transgressive coasts; however, depositional systems with similar geometry also occur on regressive coasts. Such “regressive estuaries” are found in areas of Asia, South America, and Australia that have experienced relative sea-level lowering since the middle Holocene. Regressive estuaries form in incised valleys that were drowned by the last deglacial sea-level rise and receive relatively little riverine sediment discharge. The largest estuary in Japan, with a length of 120 km and a width of 10 km, is in the Paleo-Kinu Bay region. A partly unfilled wave-dominated estuary developed in this region prior to the artificial diversion of the Tone River during the 17th century. In this study, we document the Holocene filling of the Paleo-Kinu Bay on the basis of 14 sediment cores and 171 radiocarbon dates. Unfilled wave-dominated estuary at 7–4 ka has more pronounced wave-built morphologic elements than at 4–0 ka. As a result of sea-level lowering and weakening of tidal currents at 4–0 ka, the bay mouth became narrowed by lateral accretion of sandy tidal river sediments and the central basin was filled by progradation of muddy bayhead delta sediments. The acceleration of the bay mouth narrowing and central basin infilling similar to what happened during 4–0 ka in the Paleo-Kinu Bay region have not been identified in transgressive estuaries, but are common features of regressive estuaries.

Holocene evolution and depositional model of a bayhead delta, Lake Illawarra, Australia

AbstractExtensive research into the evolution of wave‐dominated estuaries has been documented since the 1960s. However, there has only been limited research on the prograding bay‐head deltas that are the primary drivers for the rate and stage of estuary evolution. This paper presents the findings of a high‐resolution spatial study into the evolution of the Macquarie Rivulet bayhead delta in the Lake Illawarra barrier estuary. The delta’s evolution has been established based on sedimentological analysis of 74 cores, two14C ages and 45 amino acid racemization ages. This study intersected two Pleistocene and 10 Holocene sedimentary facies associations representing both the retrogradational and progradational phases of delta development. The distribution of bayhead delta facies associations in shallow barrier estuaries is initially controlled by the antecedent morphology, but, in contrast to deeper estuaries, this influence decreases as sedimentation proceeds. Changing relative sea level also has a major control on facies distribution with transgressive facies deposited as relative sea‐level rises being replaced by prograding deltaic facies during the highstand and subsequent minor relative sea‐level fall. Fluvial sediment supply and river flood events affect the rate of delta progradation, and produce low and high flow sediment deposits within the overall deltaic sequence. The low tidal flux in barrier estuaries, the muddy cohesive nature of the sediments and the prevalence of bioturbation means that primary sedimentary structures are rarely preserved in these bayhead delta facies. The depositional model of bayhead delta evolution shown by the Macquarie Rivulet delta would be widely applicable to other similar barrier estuary settings.

The Holocene evolution of Lake St Lucia, Africa's largest estuary: Geological implications for contemporary management

The Holocene evolution of one of Africa's largest estuaries, Lake St Lucia on the east coast of South Africa, is examined and juxtaposed with previous and contemporary management practices aimed at fixing the estuary state at a single point along the system's overall evolutionary pathway. The estuary has been heavily altered over the course of the last century, mostly through mouth management activities aimed at maintaining a direct connection with the ocean. Based on a high-resolution seismic reflection dataset, LiDAR and bathymetry data of the lake system, together with palaeontological (foraminifera and diatoms) and sulfur isotope geochemical data from three long (14–16 m) piston cores, we investigate the system-wide and coeval evolution of the three different sub-basins (False Bay, North Lake and South Lake) that comprise the current estuarine lake system. The northern and southern sections of the system evolved independently of each other since the Last Glacial Maximum, with each area responding differently to early sea-level forcings. In North Lake, the lagoon evolved from fully estuarine (8300 cal. BP) to a restricted lagoon with occasional marine incursions, steadily decreasing tidal prism and the development of marginal spits by wind-driven lagoonal waves. The tidal inlet closed approximately 6200 cal. BP and fluvial conditions became more dominant as main back-barrier conditions became attuned to fluvial supply. Similar conditions prevailed in the adjacent False Bay basin, albeit with an earlier onset due to sheltering by prominent bedrock peninsulas. In contrast, South Lake evolved from a fully estuarine system (9500 cal. BP) with a persistent ocean connection, to a segmented system with a diminished tidal connection by 5500 cal. BP. The inlet sealed at this point and an isolated back barrier system developed. This prompted the system-wide impoundment of waters causing back-barrier flooding and in situ drowning of bayhead deltas and segmenting spits. By 2000 cal. BP, these areas had flooded sufficiently to connect over a bedrock high that had diverted the initial palaeo-valleys to the north and south. A new, single lagoon, representative of the modern-day system formed, with a new marine connection to the modern day Mfolozi River and contemporary estuary. Periodic desiccation of the lake sub-basins since this point has been apparent. Given the complex management plans implemented to stop excess sedimentation, hypersaline events, and the continuation of “normal” lake levels, this is at odds with the natural system's evolution to a pan or wetland-type state. This geological background should be key to informing management plans that work within the natural evolutionary trajectory of the system, as opposed to attempting to maintain an unattainable status quo.

Building and Raising Land: Mud and Vegetation Effects in Infilling Estuaries.

Many Holocene estuaries were infilled to form convergent, single‐channel systems, while others remained partially or wholly unfilled. This difference in the degree of infilling depends partly on the balance between fluvial and coastal sediment input and the hydrodynamics that can export sediment. However, it remains unclear to what degree this balance is tipped by mud supply and eco‐engineering vegetation, and by what planform patterns the infilling proceeds. This study aims to explore experimentally how mud and vegetation change the degree and process of infilling, elevate and merge bars above intertidal levels and affect the planform of estuaries. To this end, three experiments were conducted in the Metronome, a flume that tilts periodically to create tidal currents, wherein forced tidal asymmetry resulted in net importing estuaries. In the second and third experiments, mud was supplied and in the third experiment seedlings were released of three vegetation species with eco‐engineering traits at a laboratory scale. With only sand, the estuary fills sufficiently to form a multi‐channel pattern with intertidal bars. Both mud and vegetation settle on intertidal bars and on the fluvial bay‐head delta, thereby contributing to bar stabilization and further estuary infilling, pointing at effective strategies to keep up with future sea‐level rise. This reduces channel mobility and effectively narrows the summed subtidal channel width toward an ideally converging funnel shape. This seems especially effective where vegetation stabilizes the mud. The experiments suggest that a range of steady states exists between the end‐members of an unfilled and a completely infilled, ideal estuary.

Does Load‐Induced Shallow Subsidence Inhibit Delta Growth?

AbstractThe ability of deltas to persist by building new land is critical to maintaining these vital ecologic environments that are often home to major economic and population centers. However, the deposition of land‐building sediment triggers load‐induced shallow subsidence which may undermine the effectiveness of natural and engineered emergent landforms. Here, we present a new method to quantify shallow subsidence in a 6,000–8,000 km2 relict bayhead delta of the Mississippi Delta using the mouth bar to overbank stratigraphic boundary that formed near sea level, temporally constrained by optically stimulated luminescence dating. Vertical displacement rates at this boundary, averaged over 750–1,500 years, are on the order of a few mm/yr. Total subsidence scales to ∼50% of the thickness of overlying deposits, significantly greater than the 28%–35% loss estimated for inland localities underlain by peat, indicating that bay muds in the study area are more compaction‐prone than terrestrial organic‐rich deposits. Modeling shows a modest reduction of ∼13% in deltaic land‐area gain under a realistic compaction scenario for 1,000 years of simulated delta progradation, compared to a no‐compaction scenario. Our findings indicate that load‐driven compaction does not majorly hinder land‐area gain and may in fact promote long‐term growth at engineered sediment diversions through channel maintenance driven by compaction, thereby adding further support to this restoration strategy.

The interplay of environmental constraints and bioturbation on matground development along the marine depositional profile during the Ordovician Radiation.

This study documents the distribution of matgrounds in a wide variety of environments recorded in the Ordovician Lashkerak and Ghelli Formations in the Alborz Mountains of northern Iran in order to evaluate controls on their distribution along the marine depositional profile. Detailed facies analysis allowed differentiating three groups of facies associations in the Lower to Upper Ordovician deposits of the Lashkerak formation: (i) estuarine system; (ii) wave-dominated shoreface-offshore complex; and (iii) mixed river- and wave-influenced deltaic system. The Middle to Upper Ordovician deposits of the Ghelli formation are divided into two groups of facies associations: (i) tide-influenced deltaic succession and (ii) deep-water fan system. Microbially induced sedimentary structures (MISS) are present in deposits formed in the central estuarine basin (Lashkerak formation) and in proximal lobes and lobe fringes of deep-water turbidite fans (Ghelli formation). On the contrary, MISS are absent in deposits from the wave-dominated shoreface-offshore complex, river- and tide-dominated deltas, and various subenvironments of the incised wave-dominated estuary (i.e., bayhead delta and estuary mouth) and the deep-marine turbidite fan system (i.e., turbidite channel, slope, and outer lobe). The lack of evidence of mat-building microorganisms in the deltaic systems may have resulted from two factors: (1) high physico-chemical stressors caused by river-induced processes, and (2) increase in degree of sediment disturbance, biodiffusion, and bioirrigation by burrowing organisms. Formation of microbial mats in the wave-dominated shoreface-offshore complex was inhibited by the activity of an abundant and diverse infauna capable of reworking the sediment. Our analysis shows that the spatial distribution of microbial mats was controlled by an interplay of environmental factors and innovations in animal-substrate interactions, mostly expressed by secular changes in bioturbation. This study supports the notion that the agronomic revolution was diachronic, with marginal-marine and deep-sea ecosystems lagging behind shallow-marine settings.

Bayhead delta evolution in the context of late Quaternary and Holocene sea-level change, Richards Bay, South Africa.

Richards Bay is part of a back-barrier lagoon fronted by high coastal dunes on the NE, Indian Ocean coast of South Africa. In the early 1970s, a berm was constructed, dividing the original Mhlathuze Estuary into two separate systems; the Richards Bay Harbour and the new Mhlathuze Estuary. This study investigates the stratigraphic evolution of the incised valley system and bayhead delta in the Richards Bay Harbour segment. Seven seismic units (Units 1–7) were imaged. A single regionally developed sequence boundary (SB) along with two tidal ravinement surfaces (tRS1 and tRS2) were identified. Surface SB is associated with the LGM lowstand which developed when sea levels were ~ 130 m below present, until ~18,000 year BP. Cretaceous age siltstones (Unit 1) form the basement. Transgressive material overlying SB (Unit 2) reflects the filling of an incised valley located in the middle segment of a wave-dominated back-barrier system. It is overlain by a bayhead delta (Unit 3), the geometry and seismic signature of which indicate alternating periods of aggradation/progradation and backstepping. The behaviour is attributed to episodic jumps in sea-level, and is tentatively (on the basis of elevations in relation to the regional sea-level curve) linked to periods of rapidly rising sea-level (8.2 ka event and Meltwater Pulse (MWP)-1d). These intervals of rapidly rising sea-level, combined with relatively low gradient settings facilitated backstepping of the delta. Fills (Unit 4) occur within minor incisions along the delta top. These are interpreted as distributary channels that fed sediment to the seaward edge of the bayhead delta system. Elongated mounds on the seafloor (Unit 5) are interpreted as spoil from contemporary port dredging. Slump deposits (Unit 6) along the delta front are attributed to a combination of oversteepening of the delta by dredging, as well as deposition of modern sediments brought into the system by tidal currents. The system is capped by fine-grained, tidally redistributed and deposited sediments (Unit 7) which were possibly sourced from older organic material of an indeterminate source. This site is especially sensitive to episodic rates of sea-level change due to the relatively small Glacial Isostatic Adjustments (GIA) during the postglacial transgression and the flat antecedent gradients of both the subaerial unconformity and the overlying tidal ravinement.

The late Holocene Erin deflected and asymmetric wave-dominated delta – Puerto Grande Bay, Trinidad

The Erin modern marginal-marine depositional system sits along the Trinidad southwestern coastline. It is among the many unstudied and undocumented modern, dynamic coastal depositional systems around Trinidad. Our study defines the Erin system as being a deflected and asymmetric wave-dominated delta, and in so doing, elucidates the dynamics of the Erin system. This was done using a compendium of satellite images, temporal cross-shore elevation profiles, modern shallow sediment cores, and sediment samples.Geomorphologically, the Erin system comprises a vegetated delta plain and the unvegetated modern deposits. The delta plain has four amalgamated sand bars with intercalated lagoonal deposits. The unvegetated modern deposits is defined as the fifth bar of the system. These five bars were loosely correlated to the five 6th order eustatic cycles of the Holocene highstand. The modern, unvegetated deposit of the fifth bar is divided into an updrift segment and a downdrift segment relative to the inlet mouth. The updrift segment comprises a spit and beach depositional elements, while the downdrift segment comprises a barrier spit, back-barrier lagoon, rip-channels, and sometimes a bayhead delta at the Erin River mouth. The dynamic, annual evolution of the spit reflects a seasonally controlled, possibly cyclical evolution wherein the delta transitions from being asymmetric to deflected-asymmetric. This phenomenon is postulated to be brought on by seasonal changes in the wave regime, and not, as one might expect, seasonally controlled river discharge. That is, during the summer wave season, when net-deposition occurs, the system morphology is deflected as a consequence of increase sedimentation in the nearshore. During the winter wave season, when net-erosion is expected, the system morphology reflects that of a typical asymmetric delta, as a consequence of reduced sedimentation in the nearshore. This cyclical movement of sediment is also observed in cross-shore elevation profiles.Sedimentologically, six depositional elements were sampled using sediment cores: foreshore, updrift spit, channel bars, mouth bar, barrier spit (downdrift), and back-barrier lagoon. Physical sedimentary structures reflected wave energy dominance across the system with an increase in tidal signatures within the lagoon deposits and lower foreshores. Biogenic sedimentary structures were limited across much of the system, but the traces recorded were mostly isolated in the downdrift back-barrier lagoon deposits, and few in the upper barrier spit. Grainsize distributions showed an average grainsize of fine sand across the system with coarse-grained, poorly sorted, bio-clastic sands present as channel lag deposits. An increase in mud content was recorded in the downdrift segment of the delta. The geomorphological, sedimentological, and ichnological data described above provided sufficient evidence to define the Erin system as an asymmetric wave-dominated delta with seasonally controlled deflected morphology.