Bayhead Delta Research Articles

Sedimentology and depositional model for the Early Miocene Oficina Formation in the Petrocedeño Field (Orinoco heavy-oil belt, Venezuela)

The Early Miocene Oficina Formation in the Petrocedeño Field (Venezuela) produces extra-heavy oil with an initial gravity of 8.5° API. Reservoir zonation is successfully based on a 3rd-order sequence stratigraphic framework for which a detailed sedimentological understanding is crucial. This paper presents a, primarily core-based, detailed account of the sedimentology and sequence stratigraphy of the Oficina Formation in the Junín (formerly Zuata) Region of the Orinoco heavy-oil belt. In Petrocedeño, the Oficina Formation is typified by the long-term (approximately 7 Ma; 2nd-order sequence) change of a fluvial dominated deltaic system to a tide-dominated lower delta plain and subsequently a tide-dominated, subtidal, upper delta platform. A fluvial braidplain environment with mostly sand-dominated braided and sinuous rivers (Sequences 1 and 2) changed into a fluvially-dominated but notably tidally-influenced delta plain with straight and sinuous channel belts (Sequences 3–6). Subsequently, a mixed-energy (fluvial and tidal) delta front environment with numerous distributaries formed. This delta front drowned and changed into an estuary system with bay-head delta mouth bars (Sequence 7). Maximum transgression was reached in Sequence 8 with the development of fully marine shale. Subsequently, a low-gradient tide-dominated lower delta plain with distributaries and meandering channel belts established (Sequences 9 and 10). These were gradually transgressed to develop into proximal delta platform environments with a limited number of distal distributaries and large protected subtidal areas (Sequence 11). Together, the lower delta plain and upper delta platform facies form one of the few currently known subsurface examples of a tide-dominated delta. The change in depositional style was controlled by a long-term increase of accommodation space in the developing Eastern Venezuelan Basin. The lower part of the formation was primarily controlled by compressional tectonic activity (third-order Sequences 1 through 6). The upper half, however, was primarily controlled by Early Miocene eustatic changes (Sequences 7 through 11).

Spatial and Temporal Distribution of Sand-Containing Basin Fills in the Holocene Rhine-Meuse Delta, the Netherlands

The quantitative significance of coarse-grained deposits in the overbank realm, such as crevasse-splay deposits, has not been studied at the delta scale or at the Holocene timescale. Such knowledge would be beneficial for understanding and explaining sediment distribution in delta plains. This study addresses delta-scale distribution of sand-containing basin fills and their sand-body proportion variability, based on eight valleywide cross sections in the Holocene Rhine-Meuse delta in the Netherlands. We found that sand-containing basin fills form 7.1% of the fluviodeltaic wedge, of which splay deposits are most frequently observed midway between the delta apex and the coast. Organic-clastic lake fills and bay-head delta deposits, in contrast, are limited to the distal delta plain. Over four successive periods (between 9000 and 800 cal yr BP), the largest proportions of splay deposits remain at 50–150 km downstream of the upstream-shifting delta apex. We show that intermediate floodbasin widths (between 3.1 and 3.6 km in the Holocene Rhine-Meuse delta) yield the highest proportions of splay deposits. High rates of base-level rise and wide floodplains both facilitate the creation of accommodation, which in turn provides conditions for peat-forming wetlands in which organic-clastic lake fills can develop. The results show that sand bodies form 26%–30% of sand-containing basin fills. This proportion is shown to be controlled by, among other variables, channel planform and superelevation of the trunk channel and substrate composition. We conclude that potentially large volumes of nonchannel sand bodies exist in distal delta plains. They constitute up to 39% of the reservoir volume in the distal Rhine-Meuse delta and yield relatively high connectedness ratios.

Delta asymmetry: Concepts, characteristics, and depositional models

Delta asymmetry forms in wave-influenced settings where there is strong net longshore drift. Asymmetric deltas typically have two sediment sources and are characterized by morphology and facies asymmetry between the downdrift and the updrift sides. The downdrift, sourced primarily by the feeding river, are commonly influenced by mixed river and wave processes. Deposits on this side are muddy and consist of barrier, bar, lagoon, bay-fill, and bayhead delta facies with variable bioturbation intensity. At or near the river mouth, heterolithic river-dominated successions are more typical. Deposits are overall unburrowed (BI 0–2), but with high burrowing spikes. Ichnogenera are characterized by horizontal, morphologically simple, and facies-crossing structures. The updrift, in contrast, is sourced by a second sediment source and typically consists of laterally continuous sandy beach and shoreface facies. Bioturbation is overall high (BI 3–6) and ichnogenera are healthy and robust, attributable to the Cruziana and Skolithos Ichnofacies. Depending on shoreline trajectory and depositional history, facies characteristics of asymmetric deltas preserved in the ancient record, however, can be different from those predicted by the models.

FEATHERS OF THE INGERSOLL SHALE, EUTAW FORMATION (UPPER CRETACEOUS), EASTERN ALABAMA: THE LARGEST COLLECTION OF FEATHERS FROM NORTH AMERICAN MESOZOIC ROCKS

The Ingersoll shale (Santonian) is a small mudstone lens in eastern Alabama, interpreted as an abandoned tidal-channel fill that accumulated rapidly within the lower reaches of a bayhead delta. The diverse biota found in this fossil Lagerstatte includes 14 individual feather specimens, the largest collection known from the Mesozoic of North America. Occurring separately throughout nearly the entire thickness of the clay lens and with a range of sizes and morphologies, the feathers most likely represent a number of theropod species. Based on known taxa in the region, the largest specimen (16.5 cm) may be a rectrix (tail feather) from a dromaeosaurid dinosaur or from a hesperornithid. Smaller feathers may have belonged to a range of shore birds. The best-preserved specimens were found in the finest grained intervals. SEM examination reveals very well preserved microstructure consisting of carbonized rod-shaped bodies ∼1 µm in length, preserved in three dimensions and solid internally. Although identical in size and shape to modern feather-degrading bacilliform bacteria and displaying some bacteria-like features, their alignment along the axis of feather structures indicates that they are more likely the fossil remains of melanosomes, melanin bodies used for color production during life. No three-dimensional arrays or patterned differences of morphotypes have been seen thus far; almost all elements are elongate (apparently eumelanin). Inferred colors for four of the feathers, based on differences in melanosome morphologies, range from gray and brownish gray to black. Whereas the majority of feather-bearing deposits represent inland lakes, the estuarine setting adds a view of coastal feathered theropods preserved in detail by rapid deposition of fine-grained sediment.

Understanding the fate of iron in a modern temperate estuary: Leirárvogur, Iceland

► Fluvial Fe (aq) and Fe (total) concentrations drop upon mixing with seawater in estuaries. ► The majority of Fe in estuaries is lost in the bay-head delta. ► Our results suggest that the bay-head delta is a key location in Fe-mineral formation. ► Isotopic variation in estuarine waters may play a role in the formation of Fe-minerals. Fluvial dissolved Fe concentrations decrease upon mixing with seawater, resulting in the formation of Fe-floccules. However, a clear understanding of the fate of these floccules has yet to be established. Assessing how tidal processes affect the formation of Fe-colloids in the Leirárvogur estuary, SW Iceland, is an important step in understanding the formation and potential deposition of estuarine Fe-rich minerals within this estuarine system. The Leirárvogur estuary drains predominately Fe-rich basalt, increasing the likelihood of detecting changes in Fe-phases. Fluvial waters and local lake waters that drain into the estuary were compared and the effects of seasonal changes were considered, in an attempt to understand how varying end-members and external factors play a role in Fe-rich mineral formation. Aqueous and colloidal Fe concentrations were found to be greater towards the head of the Leirárvogur estuary, suggesting that potential Fe-rich minerals and complexes are forming at sites of fluvial input. Increasing suspended colloidal Fe towards the estuary mouth suggests that Fe-colloids are readily transported seaward.

Contrasting sedimentation rates in Lake Illawarra and St Georges Basin, two large barrier estuaries on the southeast coast of Australia

Sedimentation rates over the last 100 years within two lagoons on the southeast coast of Australia, Lake Illawarra and St Georges Basin, have been quantified to determine the effects of catchment land use change and native vegetation clearance on infill rates, and spatial variations in the rate at which the estuaries have filled. Both catchments have similar lake and catchment area but have experience different degrees of modification due to land clearing for agriculture practices, urbanisation and industrialisation. Results indicate that in the heavily modified catchment of Lake Illawarra sedimentation rates close to fluvial deltas can be in excess of 16 mm/year, and between 2 and 4 mm/year in the adjacent central basin. This is approximately an order of magnitude greater than the pre-European rates. In contrast, at St Georges Basin, where the catchment has experienced much less modification, sedimentation rates in the central basin appear to have remained close to those prior to European settlement. However, sedimentation rates in the urbanized margin of St Georges Basin are relatively high (up to 4.4 mm/year). This rapid modern sedimentation in the margin of the estuarine embayments has been detected in several other estuaries in the region. However the degree of sedimentation within the bay-head deltas, and more significantly in the central basin appears proportional to the degree clearance of native vegetation (forest) in the catchment, urban expansion and development of heavy industry in the respective catchment areas.

Re-interpreting Great Lakes shorelines as components of wave-influenced deltas: An example from the Portage River delta (Lake Erie)

Deltas deliver both sediment (bedload sands and suspended load muds) and water to a coastal environment. In the past, deltaic models have emphasized the sandy bedload component constructing a depositional feature that protrudes from the coastline. In contrast, wave-influenced deltas form where river discharge effectively blocks the prevailing longshore drift. The resulting delta is asymmetric, with an extensive strandplain of multiple beach ridges updrift, and fewer beach ridges with wetlands, ponds, and subsidiary bay-head deltas downdrift. In Lake Erie, an analysis of 28 vibracores from the Portage River delta demonstrates significant updrift–downdrift sedimentological differences. Updrift of the delta consists of > 3 m thick gravel-rich sands overlying glaciolacustrine sediment. The deposits are organized into coarsening-upward, progradational shoreline sequences showing facies transitions from lower shoreface to upper shoreface to beachface to backbeach. A 1939 aerial photograph suggests > 15 prograding shoreline sequences were accreted during present lake levels (highstand systems tract), resulting in re-attachment of a bedrock high (Catawba Island) to the mainland. Downdrift of the delta consists of a one progradational shoreline sequence < 1 m thick that overlies peats and glaciolacustrine sediment. The peats have 14C ages between 1616 and 2025 cal YBP, and are interpreted as wetlands that formed during an earlier phase of rising lake levels (lowstand and transgressive systems tracts). The overlying beach ridge was accreted during present lake levels (highstand system tract). The coastal features in this portion of Lake Erie are best understood as components of an evolving wave-influenced delta, the first recognized in the Great Lakes.

Bay-head deltas across the northern Gulf of Mexico back step in response to the 8.2 ka cooling event

Estuaries along the northern Gulf of Mexico show rapid landward shifts in estuarine environments (back-stepping) around 8200 cal yr BP. Paleogeographic maps of the estuaries based on seismic data and cores, and constrained chronologically by radiocarbon dates, depict bay-head delta retreat in excess of 25 m/yr. During these events the sub-aerial delta plain flooded to create sub-tidal upper- and middle-bay environments. Abrupt sea-level rise associated with the 8200 cal yr BP climate-change event caused these estuaries and others around the world to reorganize abruptly. In the northern Gulf of Mexico, the sea-level rise associated with the event was only ∼0.1 m over a sub-decadal time period. This rise was superimposed on a background decelerating rate of sea-level rise of ∼1.0–0.5 m/century. The widespread bay-head delta back-stepping, documented here, may be an appropriate analog for possible abrupt future climate change in response to global warming. It also shows that estuaries fringed by low-gradient shorelines are highly sensitive to low-amplitude and sudden sea-level rise.

The importance of sea-level and climate change, including changing wind energy, on the evolution of a coastal estuary: Copano Bay, Texas

123km of high-resolution 2-D seismic lines and 6 sediment cores up to 16m in length were collected from Copano Bay, Texas. These data were used to examine the role of climate and sea-level changes in the evolution of the bay. Paleogeographic maps of the bay were constructed to show how the bay evolved through time. The overall transgression was punctuated by five environmental changes within Copano Bay over the past 9.6ky. The largest environmental change occurred at 8.2ka when the bay-head delta back-stepped approximately 7.5km as a result of a 0.4–1.2m rapid rise in sea level. With a well constrained model of how the bay changed through time, textural data from the most continuous record of middle-bay deposits was used to produce a proxy for wind strength over the last 8.2ky. Our wind strength proxy reveals that Copano Bay experienced relatively tranquil winds during the early Holocene followed by its windiest conditions from 5.2 to 4.1ka and a return to more tranquil winds during the late Holocene. The wind strength proxy was compared with other published paleo-climatic records for the region and a reasonable correlation was found between periods of higher wind energy and drier climate. This study illustrates that despite their complex depositional systems and many sediment and water sources, a system-wide approach to studying an estuary can still be used to extract paleoclimate archives from estuarine deposits.

Preliminary assessment of ichnofacies in the Gething Formation of NE British Columbia

Abstract The Gething Formation in the Kahntah River–Ring/Border region of northeastern British Columbia is a variably thick, heterolithic succession of facies deposited in continental and marginal-marine depositional settings. This unit was deposited on the southwestern flank of the Keg River Highlands in the northern Western Canada Foreland Basin (approximately 58°N, 120°W). Although Gething Formation sandstone bodies are exploration targets further south, it is not considered to be a primary exploration target in the study area due largely to its complex and laterally discontinuous nature. However, in the study interval several of the lithofacies possess good reservoir properties, and the overlaying Wilrich Shale provides a potential stratigraphic seal. Hydrocarbon sources are available in both the Wilrich Shale and the Triassic Montney Formation into which the Gething Formation incises. An understanding of the overall palaeogeography and lithofacies distribution of the Gething Formation in the study area is necessary for efficiently exploiting these hydrocarbon resources and will facilitate characterization of other Gething Formation reservoirs that may flank the Keg River Highland to the east and west. Subsurface mapping in the area of Ring and Kahntah River gas fields shows that the Gething Formation in the study area occurs within three sinuous incised valleys that debouch southward into a large structural basin. In their northernmost (upstream) reaches, Gething Formation strata are dominated by unbioturbated, high-angle cross-bedded and current-ripple laminated upper-fine-grained sandstones, rich in organic detritus (primarily coal flakes). These are interpreted as fluvial channel and bayhead delta deposits. Lithofacies deposited in continental settings grade southwards into bioturbated, low- and high-angle cross-bedded, lower to upper-fine-grained sandstones with thin, bioturbated siltstone and mudstone intervals. Rhythmic bedding, mud flasers and mud/sand couplets characterize these lithofacies. The trace fossils Planolites, Skolithos, Cylindrichnus, Arenicolites, Teichichnus, and fugichnia are common. These strata are interpreted to represent brackish-water bay deposits. South of this, the Gething Formation is dominated by interstratified, sporadically bioturbated sand, silt and mud, sedimentologically similar to the aforementioned brackish channel deposits, but with more diverse and robust ichnofauna (the previous assemblage as well as large Thalassinoides, rare Helminthopsis and Phycosiphon). These deposits are ascribed to tidal channels in a larger, more marine bay.

Coastal dynamics and cultural occupations on Choctawhatchee bay, Florida, U.S.A.

A multidisciplinary project on an archaeological site on theMitchell River, which feeds into Choctawhatchee Bay on the Floridapanhandle, was designed to understand human adaptations to a dynamic hydrological environment during the Middle and Late Archaic period (ca. 8000–3000 B.P.). Now in a freshwater environment, on a sandy terrace above the Mitchell River floodplain, the Mitchell River 1 archaeological site contains an oyster‐shell midden and other features indicating human exploitation of an estuarine environment. Estuarine exploitation at the site occurred over a long span of time, from around 7300 to 3400 2cal B.P., although the site was abandoned two or three times over the millennia. The site was more permanently abandoned after 3400 B.P. Because estuarine shellfish, such as oysters, are low trophic level species, they have been considered marginal resources, and archaeologists modeling collector strategies assume that people will not travel far to obtain them. Under an optimal foraging model, estuarine resources should have been closer to the site than at present. A multidisciplinary team was assembled to address whether a mid‐Holocene sea‐level highstand had produced estuarine conditions in the Mitchell River floodplain during the Archaic. Using microfossils and stratigraphy from a dated core taken in the floodplain due south of the site, the project members attempted to correlate the changing paleoenvironment with human occupation and abandonment of the area. Results indicate that, at ca. 7300 cal B.P., when the Mitchell River 1 site was first inhabited, the floodplain was a shallow, open, sedge marsh. The site inhabitants must have traveled some distance to gather the oysters and other estuarine species that were discarded on the site. The earliest occupation was brief, but the site was reoccupied between 5900 and 5300 cal B.P., when the floodplain had become a Taxodium/Nyssa swamp. Site deposits indicate intensive exploitation of oyster and to a lesser extent Rangia, which may have been closer to the site than at 7300 B .P. but still would have required some travel. At some point, the mouth of the Mitchell River was forced eastward, and the bayhead delta, recognized as a 2‐meter‐deep wedge of sand in the core, was located adjacent to the site. By 4700 cal B.P., brackish water conditions prevailed, although direct evidence of oyster beds in the immediate area is lacking. Unfortunately, scouring of the core sediments sometime after 4700 cal B.P. destroyed the paleoenvironmental record for the lastpart of the Archaic occupation of the site. However, some evidence in Core 1, along with research elsewhere on the Florida panhandle, suggests that catastrophic storms may have played a part in the more permanent abandonment of the site after 3400 B.P.

The incised valley of Baffin Bay, Texas: a tale of two climates

Baffin Bay, Texas is the flooded Last Glacial Maximum incised valley of the Los Olmos, San Fernando and Petronila Creeks along the north-western Gulf of Mexico. Cores up to 17 m in length and high-resolution seismic profiles were used to study the history of Baffin Bay over the last 10 kyr and to document the unusual depositional environments within the valley fill. The deposits of the Baffin Bay incised valley record two major and two minor events. Around 8·0 ka, the estuarine environments backstepped more than 15 km in response to an increase in the rate of sea-level rise. Around 5·5 ka, these estuarine environments changed from environments similar to other estuaries of the northern Gulf of Mexico to the unusual suite of environments found today. Another minor flooding event occurred around 4·8 ka in which several internal spits were flooded. Some time after 4·0 ka, the upper-bay mud-flats experienced a progradational event. Because of its semi-arid climate and isolation from the Gulf of Mexico, five depositional environments not found in the other incised-valley fills of the northern Gulf of Mexico are found today within Baffin Bay. These deposits include well-laminated carbonate and siliciclastic open-bay muds, ooid beaches, shelly internal spits and barrier islands, serpulid worm-tube reefs and prograding upper-bay mud-flats. Based on these unusual deposits, and other characteristics of Baffin Bay, five criteria are suggested to help identify incised valleys that filled in arid and semi-arid climates. These criteria include the presence of: (i) hypersaline-tolerant fauna; (ii) aeolian deposits; and (iii) carbonate and/or evaporite deposits; and the absence of: (iv) peat or other organic-rich deposits in the upper bay and bay-margin areas; and (v) well-developed fluvially dominated bayhead deltas.

Depositional architecture of a low‐gradient ramp shelf in an epicontinental sea: the lower Cretaceous of Svalbard

ABSTRACTFacies belts have been recorded using conventional field methods in the Lower Cretaceous Helvetiafjellet Formation in Spitsbergen, Svalbard. The formation was deposited upon a regional subaerial unconformity (SU) that developed downdip from an incised valley complex to the W, NW and possibly north of Spitsbergen during fall in relative sea level and incision into the marine Rurikfjellet Formation. The unconformity likely resulted from crustal tectonics related to the formation of the proto‐Amerasian basin. The unconformity was initially covered by braided stream facies sand and locally by small bay head delta deposits. Further rise in relative sea level resulted in transgression, and the initial braidplain turned into a low‐sloping shelf ramp of coastal plain and paralic to marine environments. Balance between increase in accommodation space and rate of sediment supply gave rise to aggradational stacking architecture of the middle and upper part of the Helvetiafjellet Formation, until further rise in sea level turned the shelf again into an open marine setting with deposition of the overlying Carolinefjellet Formation. Architectural pattern of fluvial distributary channel sandstone bodies in the aggradational succession is explained within the framework of the Boreal basin, not only the Spitsbergen outcrop domain as in previous models. The revised depositional model for the Helvetiafjellet Formation implies that fluvial systems in similar emerged epicontinental basins may have large potential to carry sand several hundreds of kilometres out into the basin with formation of fluvial sandstone intervals of large regional extent, bounded by semi‐parallel stratigraphic surfaces transecting time‐lines at very low angles. Sandstone bodies with clinoform geometry are in this setting restricted to local small‐scale paralic deltas and shoreface deposits.

Late Quaternary valley-fill succession of the Lower Tagus Valley, Portugal

The Lower Tagus Valley in Portugal contains a well-developed valley-fill succession covering the complete Late Pleistocene and Holocene periods. As large-scale stratigraphic and chronologic frameworks of the Lower Tagus Valley are not yet available, this paper describes facies, facies distribution, and sedimentary architecture of the late Quaternary valley fill. Twenty four radiocarbon ages provide a detailed chronological framework. Local factors affected the nature and architecture of the incised valley-fill succession. The valley is confined by pre-Holocene deposits and is connected with a narrow continental shelf. This configuration facilitated deep incision, which prevented large-scale marine flooding and erosion. Consequently a thick lowstand systems tract has been preserved. The unusually thick lowstand systems tract was probably formed in a previously (30,000–20,000 cal BP) incised narrow valley, when relative sea-level fall was maximal. The lowstand deposits were preserved due to subsequent rapid early Holocene relative sea-level rise and transgression, when tidal and marine environments migrated inland (transgressive systems tract). A constant sea level in the middle to late Holocene, and continuous fluvial sediment supply, caused rapid bayhead delta progradation (highstand systems tract). This study shows that the late Quaternary evolution of the Lower Tagus Valley is determined by a narrow continental shelf and deep glacial incision, rapid post-glacial relative sea-level rise, a wave-protected setting, and large fluvial sediment supply.

BRACKISH-WATER ICHNOLOGICAL TRENDS IN A MICROTIDAL BARRIER ISLAND-EMBAYMENT SYSTEM, KOUCHIBOUGUAC NATIONAL PARK, NEW BRUNSWICK, CANADA

A complex variety of marginal-marine microtidal environments from Kouchibouguac Bay, New Brunswick, Canada, present an opportunity to ichnologically and sedimentologically characterize microtidal settings in a high-latitude, temperate subarctic climate. Variations in bioturbate fabrics and distribution of infauna, analysis of the distributions of sediments and physical sedimentary structures, and the distribution of total organic carbon (TOC) can be associated with characteristic depositional processes. From these data typical sedimentary facies associations are produced. In outer estuary tidal inlets and areas of the flood-tidal deltas, strong currents and wave action eradicate the ichnological signature, resulting in variably laminated and bedded sand. In the central estuary, infauna activity coupled with generally low hydraulic energy levels lead to an absence of primary sedimentary structures. The inner estuary near bay-head deltas experiences riverine currents and freshwater influence. As a consequence, primary sedimentary structures are preserved. Mapping of infauna, sediment texture, TOC, and salinity reveals strong links between animal distribution and these three physicochemical parameters. Consequently, the distribution and type of bioturbation observed is at least passively related to grain size, TOC, and salinity. In outer estuaries and lower-central estuaries, salinity is near marine levels and fluctuates minimally. The distribution of infauna in these areas corresponds directly to sediment texture and TOC. Further up the estuaries, lower and fluctuating salinities—in addition to sediment texture and TOC content—control the distribution and diversity of infauna. Mapping of diversity and infaunal size up-estuary reveals two significant trends attributable to salinity stresses: (1) vermiform diminution, and (2) a significant decrease in infaunal diversity.

Late Quaternary geomorphology of the Manawatu coastal plain, North Island, New Zealand

This review examines the geomorphological evolution of the Manawatu coastal plain from the Last Glacial Maximum through the Holocene post-glacial marine transgression (c. 7500 cal yr BP) until the present, providing a context within which to examine the extent to which the Holocene evolution of the lower Manawatu valley follows tripartite evolutionary models of incised-valley infill. During the last glaciation the Manawatu River incised a broad, deep valley, while tributaries within the catchment dissected relict marine terraces. This occurred concurrently with the formation of a parabolic dune field at Koputaroa, with sediment sourced from the bars of the Manawatu River. During the post-glacial marine transgression rising sea-levels inundated the incised-valley. At the culmination of the Holocene marine transgression, the Himatangi anticline and Poroutawhao High restricted oceanic influences in the proto-Manawatu estuary, which was partially infilled with estuarine sediment. Estuarine deposition was quickly succeeded by fluvial deposition and floodplain due to high volumes of fluvial sediment introduced by the Manawatu River. A lack of characteristic bay-head delta development may be explained by rapid infill and consequent sediment bypass.

The Palaeo-Environmental Contexts of Three Possible Phoenician Anchorages in Portugal

The 2002 Joint Expedition of the Institute of Nautical Archaeology and the Centro Nacional de Arqueologia Nautica e Subaquatica examined Phoenician maritime involvement in Portugal, combining archaeological, geological and geophysical surveys of former coastal sites—Santa Olaia, Abul and Castro Marim—where previous excavations have uncovered Phoenician remains. Significant geomorphic evolution of bay-head delta and estuarine settings masks the fact that Phoenician sites were situated at the head of marine embayments or on estuary margins, had easy access to the sea, and immediate access to at least one natural anchorage. All sites exhibited topographical criteria familiar from Phoenician trading-stations and merchant outposts in the Mediterranean. © 2009 The Authors

From river valley to estuary: the evolution of the Rhine mouth in the early to middle Holocene (western Netherlands, Rhine-Meuse delta)

AbstractThe aim of this paper is to reconstruct the evolution of the early to middle Holocene Rhine-Meuse river mouths in the western Netherlands and to understand the observed spatial and temporal changes in facies. This is achieved by constructing three delta wide cross-sections using a newly accumulated database with thousands of core descriptions and cone penetration test results, together with a large set of pollen/diatom analyses and OSL/14C-dates. Most of the studied deposits accumulated in the fluvial-to-marine transition zone, a highly complex area due to the interaction of terrestrial and marine processes. Understanding how the facies change within this zone, is necessary to make correct palaeogeographic interpretations.We find a well preserved early to middle Holocene coastal prism resting on lowstand valley floors. Aggradation started after 9 ka cal BP as a result of rapid sea-level rise. Around 8 ka most parts of the study area were permanently flooded and under tidal influence. After 8 ka a bay-head delta was formed near Delft, meaning that little sand could reach the North Sea. Several subsequent avulsions resulted in a shift from the constantly retreating Rhine river mouth to the north. When after 6.5 ka the most northerly river course was formed (Oude Rijn), the central part of the palaeovalley was quickly transgressed and transformed into a large tidal basin. Shortly before 6 ka retrogradation of the coastline halted and tidal inlets began to close, marking the end of the early-middle Holocene transgression.This paper describes the transition from a fluvial valley to an estuary in unprecedented detail and enables more precise palaeo-reconstructions, evaluation of relative importance of fluvial and coastal processes in rapid transgressed river mouths, and more accurate sediment-budget calculations. The described and well illustrated (changes in) facies are coupled to lithogenetic units. This will aid detailed palaeogeographic interpretations from sedimentary successions, not only in the Netherlands, but also in other estuarine and deltaic regions.

Anthropogenic Impacts on Sedimentary Sources and Processes in a Small Urbanized Subtropical Estuary, Florida

The Loxahatchee River, a 500 km2 watershed on the southeast Florida coast, is a barrier-impounded drowned river valley experiencing rapid urbanization during the past 50 years. The estuary is currently composed of a sandy central basin with bifurcating forks accumulating organic-rich muddy sands. The Northwest Fork drains natural forest, residential, and agricultural catchments and has a much larger bayhead delta than the channelized Southwest Fork, which is only 50 years old and diverts much of the flow from the Northwest Fork. Sediment accumulation rates within muddy sand deposits are about 2–3 mm a−1, commensurate within error of the current rate of local sea-level rise. Previously established Holocene accumulation rates are close to relative sea-level rise, implying that sediment accumulation is in equilibrium with the creation of accommodation space. The main anthropogenic influences have been changes in surface sediment texture corresponding to dredging-induced modification of tidal currents and a localized >threefold increase in sediment accumulation rate supplied by sources local to the estuary. Late Holocene sedimentation has been episodic, and lithofacies vary from bioturbated transgressive sands at depth to peat and laminated fine-grained sediments then back to mottled muddy sands in the uppermost strata. This facies transition is attributed to a change of water column stratification conditions caused by decreased tidal flushing in the estuary possibly brought about by variation in sea level or episodic inlet closure by littoral sand transport. The magnitude of the anthropogenic changes in sedimentation is roughly equal with natural changes in lithofacies formation.

Climate change signature of small-scale parasequences from Lateglacial–Holocene transgressive deposits of the Arno valley fill

Despite recent report of short-term cyclicity from Lateglacial–Holocene deposits of several coastal plains worldwide, no precise documentation of the key factors controlling cyclic facies architecture has been made available by previous work. Detailed sedimentological analysis of a continuously-cored borehole, around the town of Pisa, in western Tuscany, provides evidence for the occurrence of three high-frequency, transgressive–regressive cycles within the post-Last Glacial Maximum (LGM) transgressive succession (13–8 cal. kyr BP) of the Arno incised-valley fill. These cycles, which are bounded by lateral equivalents of marine flooding surfaces, are 8–12 m thick and correspond to small-scale parasequences. Micropalaeontological (foraminifers and ostracods) investigations based upon differentiation of eight microfossil associations, allow to refine the stratigraphic framework, emphasizing subtle changes in palaeosalinity across parasequence boundaries. Diagnostic changes in vegetation patterns, driven by opposite climate conditions, enable precise documentation of parasequence development as a function of climate change. Pollen spectra invariably show expansions of broad-leaved forests at parasequence boundaries, suggesting that rapid shifts to warmer climate conditions accompanied episodes of rapid sea-level rise. In contrast, stillstand phases saw the development of cold-temperate communities (upper parts of parasequences), suggesting transition to temporary colder climate conditions. Reconstruction of parasequence architecture on the basis of adjacent stratigraphic data, combined with palaeoclimate characterization and radiometric dating enable identification, within the transgressive Arno valley body, of three major “regressive” pulsations that are tentatively correlated with the most important cooling events of the post-LGM period. The sedimentary response to these short-term phases of climatic cooling is clearly documented by episodes of widespread coastal-plain and bay-head delta progradation, leading to partial estuary infilling and temporary establishment of continental environments in the proximal and central sectors of the valley.