Variations In Subsidence Rates Research Articles

Monitoring of ground subsidence using PS-InSAR technique in the Southeast Texas (SETX) Region

The southeast Texas (SETX) coastal area, owing to its unique geographical location and geological attributes, is facing a spectrum of geological challenges, such as ground subsidence, flooding, and coastal erosion. This study endeavors to evaluate the recent instances of ground subsidence and their associated rates, focusing on comprehending their implications for flooding within SETX. Employing the Persistence Scatterer Interferometric Synthetic Aperture Radar (PS-InSAR) technique, this study employs Sentinel-1 SAR satellite data with descending orbit observations spanning from January 2020 to March 2023. Our findings indicate that both the northwestern and eastern regions of Houston have been settled with rates up to 2 cm/year. This settlement trend is consistent with data derived from GPS and groundwater level measurements. This investigation explicitly shows the substantial temporal and spatial variations in subsidence rates, predominantly influenced by localized groundwater extraction due to urbanization (e.g., population growth, land development, etc.). By advocating for the integration of InSAR, GPS, and groundwater measurements, this research aspires to make valuable contributions toward the mitigation of subsidence and flood-related hazards in the SETX area.

Sediment Compaction in Experimental Deltas: Toward a Meso‐Scale Understanding of Coastal Subsidence Patterns

AbstractWe present the first investigation of subsidence due to sediment compaction and consolidation in two laboratory‐scale river delta experiments. Spatial and temporal trends in subsidence rates in the experimental setting may elucidate behavior which cannot be directly observed at sufficiently long timescales, except for in reduced scale models such as the ones studied. We compare subsidence between a control experiment using steady boundary conditions, and an otherwise identical experiment which has been treated with a proxy for highly compressible marsh deposits. Both experiments have non‐negligible compactional subsidence rates across the delta‐top, comparable in magnitude to our boundary condition relative sea level rise rate of 250 μm/hr. Subsidence in the control experiment (on average 54 μm/hr) is concentrated in the lowest elevation (<10 mm above sea level) areas near the coast and is likely related to creep induced by a rising water table near the shoreface. The treatment experiment exhibits larger (on average 126 μm/hr) and more spatially variable subsidence rates controlled mostly by compaction of recent marsh deposits within one channel depth (∼10 mm) of the sediment surface. These rates compare favorably with field and modeling based subsidence measurements both in relative magnitude and location. We find that subsidence “hot spots” may be relatively ephemeral on longer timescales, but average subsidence across the entire delta can be variable even at our shortest measurement window. This suggests that subsidence rates over a short time frame may exceed thresholds for marsh platform drowning, even if the long term trend does not.

Litho- and biostratigraphy of the early Eocenelarger benthic foraminifera-dominated carbonates of the central Tethys domain, Zagros Foreland Basin, SW Iran

The Early Paleogene Tethyan domain saw the development of large platform carbonates rich in large benthic foraminifera (LBF), of which the Taleh Zang Formation in Lurestan Basin, SW Iran, represents an excellent example. The LBF assemblage is dominated by alveolinids, together with a variety of foraminiferal genera such as Nummulites, Rotalia, Sackesaria, Assilina, Operculina, and Glomalveolina. Smaller benthic foraminifera, calcareous algae, gastropods, echinoids, and bivalves constitute minor components of the carbonate. Biostratigraphy analysis of the LBF is conducted on two distinct sections of the Taleh Zang Fm. The sections referred to as TZ-1 and TZ-2 have respective thicknesses of 87 m and 132 m. This analysis has constrained the formation age to the early Ypresian within Shallow Benthic Zones (SBZs) 5–7. Facies analysis supports the major accumulation of these successions in a shallow water environment within the euphotic zone. We recognize 9 distinct facies and microfacies representing four main depositional environments including tidal flat, lagoon, bioclastic shoal, and shallow open marine. These environments are distributed along a low-angle homoclinal ramp and form the foundation of a stratigraphic interpretation characterized by intervals displaying shallowing and deepening trends. Three shallowing-upward intervals have been identified within the Taleh Zang succession. The determined intervals cannot be solely attributed to global sea level fluctuations and support a significant impact of the regional structural context resulting from collision-induced uplift and variable subsidence rates. The identified intervals exhibit local and regional correlation and compatibility on the Arabian Plate, providing robust confirmation of the significant roles played by both tectonics and eustasy at a regional scale.

Long-term trajectory and temporal dynamics of tropical peat subsidence in relation to plantation management and climate

Disentangling land-use and climatic influences on peat subsidence, and establishing the long-term trajectory of subsidence, are necessary to determine the future economic and environmental sustainability of managed peatland landscapes. While many peatlands in temperate regions such as Europe have been drained for centuries, those of Southeast Asia have mostly been drained for agriculture and forestry practices within the last 30 years. These areas are subsiding rapidly, but few long-term subsidence records exist, and it is unclear whether currently high subsidence rates will be maintained in future. Furthermore, large-scale climate fluctuations associated with the El Niño Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) exert a strong year-to-year influence on rainfall rates, affecting water table depth dynamics in both managed and intact peatlands. In this study, we analysed data collected over more than a decade of subsidence measurements from over 400 plantation and forest plots in Sumatra, Indonesia, including a unique set of 62 sites that have been continuously monitored since 2007. We show that spatial and temporal variations in subsidence rates are primarily determined by water table depth. We also find evidence of declining subsidence rates as a function of time since initial drainage, consistent with previous instrumental records from high-latitude peatlands and recent satellite data from tropical peatlands. Subsidence rates over the study period were strongly affected by the large ENSO/IOD-linked drought event in 2015–16, which caused an acceleration of subsidence across all sites. In plantation areas, we estimate that this climate perturbation caused around 14 % of subsidence observed over a twelve year period. At interior forest sites this rose to 32 %, and we found little evidence of ecosystem recovery to the end of 2018. This raises the possibility that repeated extreme droughts in the region could lead to long-term degradation of peat swamp forest ecosystems.

Sea-level rise impact and future scenarios of inundation risk along the coastal plains in Campania (Italy)

Sea-level rise as a consequence of global warming increases the need to analyze coastal risks to conceive adaptation strategies aimed at coping with marine impacts at both short- and long-term scales. In this context, this study presents future scenarios of inundation risk evaluated along the main alluvial coastal plains of the Campania region (Italy). Due to their geomorphological and stratigraphical setting, the investigated areas are characterized by low topography and relevant but variable subsidence rates. Based on the upgrade of already published data and the new analysis of available datasets derived by multi-temporal interferometric processing of satellite Synthetic Aperture Radar (SAR) images, future scenarios of local sea level for the years 2065 and 2100 have been evaluated coupling global projections with local subsidence trends. Furthermore, aspects related with the distribution of natural and anthropic assets, as well as the local social vulnerability, have been taken into account to calculate the overall risk. The inundation risk maps here proposed can effectively address the request to improve the knowledge of policymakers and local administrators and to raise their awareness about the potential impacts of climate change in coastal areas.

The role of subsidence and accommodation generation in controlling the nature of the aeolian stratigraphic record

Despite a well-documented record of preserved aeolian successions from sedimentary basins characterized by widely variable subsidence rates, the relationship between aeolian architecture and subsidence-driven accommodation generation remains poorly constrained and largely unquantified. Basin subsidence as a control on aeolian sedimentary architecture is examined through analysis of 55 ancient case-studies categorized into settings of ‘slow’ (>1–≤10 m Myr −1 ), ‘moderate’ (>10–≤100 m Myr −1 ) and ‘rapid’ (>100 m Myr −1 ) time-averaged subsidence rates. In rapidly subsiding basins, aeolian successions are thicker and associated with (1) thicker and more laterally extensive dune-sets with increased foreset preservation, (2) greater proportions of wet-type interdunes and surface stabilization features and (3) more extensive interdune migration surfaces, bounding sets that climb more steeply. In slowly subsiding basins, aeolian successions are thinner, and associated with a greater proportion of (1) aeolian sandsheets and (2) supersurfaces indicative of deflation and bypass. Rapid subsidence promotes (1) steeper bedform climb, resulting in increased preservation of the original dune foreset deposits and (2) relatively elevated water tables, leading to sequestration of deposits beneath the erosional baseline and encouraging development of stabilizing agents; both factors promote long-term preservation. Slow subsidence results in (1) lower angles of climb, associated with increased truncation of the original dune forms, and (2) greater post-depositional reworking, where sediment is exposed above the erosional baseline for extended time-periods. Quantitative analysis of sedimentary stratal architecture in relation to rates of basin subsidence helps constrain the mechanisms by which sedimentary successions are accumulated and preserved into the long-term stratigraphic record. Supplementary material: Results of statistical analyses presented here are available at https://doi.org/10.6084/m9.figshare.c.5515695.v1

Measuring, modelling and projecting coastal land subsidence

Coastal subsidence contributes to relative sea-level rise and exacerbates flooding hazards, with the at-risk population expected to triple by 2070. Natural processes of vertical land motion, such as tectonics, glacial isostatic adjustment and sediment compaction, as well as anthropogenic processes, such as fluid extraction, lead to globally variable subsidence rates. In this Review, we discuss the key physical processes driving vertical land motion in coastal areas. Use of space-borne and land-based techniques and the associated uncertainties for monitoring subsidence are examined, as are physics-based models used to explain contemporary subsidence rates and to obtain future projections. Steady and comparatively low rates of subsidence and uplift owing to tectonic processes and glacial isostatic adjustment can be assumed for the twenty-first century. By contrast, much higher and variable subsidence rates occur owing to compaction associated with sediment loading and fluid extraction, as well as large earthquakes. These rates can be up to two orders of magnitude higher than the present-day rate of global sea-level rise. Multi-objective predictive models are required to account for the underlying physical processes and socio-economic factors that drive subsidence.

Synsedimentary tectonics vs paleoclimatic changes across the Aptian-Albian boundary along the Southern Tethyan margin: The panormide carbonate platform case history (NW Sicily)

The Aptian-Albian boundary is a deeply investigated time interval on the basis of the deep oceanographic, environmental and climatic changes, globally recorded along the sedimentary rock successions.A regional latest Aptian extensional pulse in the Southern Tethyan margin is here envisaged as being responsible for different events of sedimentation and erosion in the Panormide carbonate platform as highlighted by the outcropping sections studied from the NW Sicilian fold and thrust belt.Integrating facies and stratal pattern analysis with backstripping methods has permitted to define the interaction between tectonics and sedimentary factors of the Lower Cretaceous shallow-water carbonates.Based on geometric configuration, lateral facies relationships, and thicknesses variations of the Lower Cretaceous Requienid limestone, is here envisaged a paleophysiography characterised by banks separated by deep channels. The margins of the isolated carbonate platform were oriented towards both the present-day west, where originally the Cala Rossa intraplatform basin developed driven by transtensional movements, and to the present-day east, where the continental slope was spreading.Including subsidence analysis and significance of the top-unconformity of the Requienid limestone a tectonic event at the end of the Aptian was detected as responsible for the fragmentation of the platform in various rotating fault-blocks and reworking of shallow-water carbonates towards the slope. The variable subsidence rates correspond to the different responses, both emersion and drowning, of the various platform-blocks to the extensional tectonics.Paleoceanographic changes, detected around the world during the Aptian-Albian, are well recorded also from the Sicilian sections, where continental clays – as the product of weathering of crystalline rocks – metallic crusts (hardgrounds) and condensed phosphatized pelagic deposits – as the product of sediment starvation, condensation and drowning – cover the Lower Cretaceous shallow-water carbonates respectively in the different faulted blocks of the segmented carbonate platform. Chemical and mineralogical analyses confirm for these deposits an influence in their formation caused by variations in oceanographic and environmental conditions, as increasing of metal ions in the surficial sea-water and high rates of groundwater precipitation. Paleoclimate evaluation reveals changes from the greenhouse arid conditions, developed during the lower Cretaceous and permitting the sedimentation of the shallow-water carbonates, to warm-humid climate condition during the latest Aptian-lower Albian boundary.

Preliminary results of land subsidence monitoring in the Ca Mau Province

Abstract. A previous study of the Ca Mau province in Vietnam (Karlsrud et al., 2017a) suggested that ongoing groundwater pumping, which by 2012 had caused a drawdown of the water level in aquifers of up to 20 m, caused subsidence of the order 2–4 cm yr−1, and could have reached over 40 cm already. Earlier InSAR studies also suggested ongoing subsidence rates of that order. If the groundwater pumping continues, the total subsidence could reach well over 1 m within the next few decades. The predicted climate driven sea level rise, to be of the order of 60 cm by 2100, will further add to the severe effect of the subsidence. As most of the Ca Mau province lies only 0.5 to 1.5 m a.s.l. (above sea level), the consequences would rapidly become very serious for the livelihood of people in the region. Increased saltwater intrusion into canals and tributaries in the province, and beginning salination of some of the aquifers from which groundwater is pumped, is already observed. In 2017, for the first time, a physical system for subsidence monitoring was installed at three selected locations in the Ca Mau province. At each location a deep benchmark to a depth of 100 m was installed, each with 3 piezometers at depths ranging from 15 to 60 m. An InSAR corner reflector was also installed at each site. The paper presents data collected from these new monitoring stations up until the middle 2019. When including estimated subsidence stemming from the soil levels deeper than 100 m, the total present rate of subsidence at the three new monitoring stations range from 17 to 44 mm yr−1. New and previous data show an almost linear decrease in water level within the aquifers from which groundwater is pumped. The data show some seasonal variations in subsidence rates, which is also reflected in variations in pore pressures in the sediments. Such variations are probably related to seasonal variations in levels of groundwater pumping. It is feared that many of the other provinces south of Ho Chi Minh city, face similar subsidence problems. The monitoring program should be extended to verify that. Measures to reduce groundwater and subsidence are urgently needed.

Aptian-Albian transition in central Tunisia: Tectonosedimentary and paleogeographic records

The Aptian-Albian series are characterized, in Tunisia, by several variations in subsidence rates and records discrete unconformity between Albian and Aptian in central and southern Tunisia. In central Tunisia Upper Aptian regressions were sustained until the beginning of Albian with a low sea level stand. In this paper, wells data gathered from central Tunisia exposes well expressed Aptian-Albian hiatus associated to several partial sedimentary gaps, erosion surface and unconformities and correlated with the “Aptian Crisis” highlighted at the Tethyan scale. This latter is coeval with an extensional regime in concurrence with the African rifting responsible for halokinesis movements and magmatism and leaded to the establishment of paleoreliefs as much as tilted blocks and half grabens in central Tunisia. Deposition shows regional hiatus and lateral transition from platform facies to south to, open marine facies to the north. The structural configuration of this domain within normal faulting, horsts and grabens leaded to the establishment of several distinct basins with different subsidence rates. Discordances occur specially around and upward uplifted blocks and emerged land and are especially associated with Aptian reef, karsts and shallow marine facies. They are unconformably overlaid by the transgressive open marine facies of Albian-Cenomanian series acknowledged as source rock in Central Tunisia. In addition, the reefal Aptian facies is considered a potential reservoir rocks in central Tunisia and it is frequently associated to mineralization and Hydrocarbure reservoirs.

Qualitative and quantitative evaluation of the influence of anthropic pressure on subsidence in a sedimentary basin near Rome

This paper presents the results of analyses measuring variations in anthropic pressure and the influence of anthropic pressure on the variations in subsidence rates from 1954 to 2005 in the Tivoli-Guidonia area, located to the East of Rome, Italy. In the last 60 years, this area underwent exceptional demographic growth, as well as an increase in the quarrying and hydrothermal industries. The calculation of the increase in anthropic pressure was based on two factors: the expansion of built-up areas and the increase in quarrying. Photogrammetric techniques permitted a multi-temporal analysis of aerial photos taken in 1954, 1985, 1993, 1998 and 2005. The aim of this analysis was to estimate the increase in both urban surface area and travertine quarrying. Non-conventional techniques, such as the reconstruction of photograms using photo retouching software, allowed for the use of aerial photos from 1954. These photos were fundamental for establishing a medium-term evaluation of anthropic activity in the area. The results of the photogrammetric analysis were compared to the phases of acceleration and stabilization in subsidence phenomena, deduced from a RADAR interferometry study published by the Italian Ministry of the Environment. This comparison showed a close relationship between the rise and temporary halt of anthropic development and the rise and fall of subsidence rates. Furthermore, data on the distribution of private wells in the study area, variations in the groundwater table level and the precipitation regime were analyzed, but, although helpful in defining the geological context and considerable transformations of the territory, they were considered as qualitative data due to the lack of systematic measures or an unclear relationship to the variations in subsidence rate. Despite the difficulties involved in evaluating both the amount of anthropic pressure and the role of different human activities on environmental change, it was possible to estimate the variations in anthropic pressure and their influence on potentially dangerous geological processes.

Paleochannel hydraulics, geometries, and associated alluvial architecture of Early Cretaceous rivers, Sevier Foreland Basin, Wyoming, USA

Early Cretaceous, retro-foreland basin fluvial deposits throughout Wyoming record interactions between orogenesis, subsidence, sediment accumulation, basin physiography, and syndepositional structural deformation associated with the early stages of the Sevier Orogeny. Quantitative paleochannel reconstructions presented here are important for understanding these interactions, evaluating controls on alluvial architecture, and can be applied to basin-modeling studies. Most paleochannel sandstones and conglomerates represent point bars associated with meandering rivers, although some rivers may have been braided. Paleoflow of earliest Cretaceous Cloverly A-interval paleochannels (forebulge depozone, central WY) was generally to the north, northeast, and east, which suggests that most are deposits of basin-axial rivers. Discharges of overlying B-interval paleochannels are less than most of those of the A interval, possibly reflecting a temporal decrease in water supply related to the eastward expansion through time of an orographic rain shadow caused by progressive rising of the Sevier Orogen to the west. The Bechler (western WY), Cloverly B (central WY), and Lakota L2 (eastern WY) intervals are correlative and record deposition throughout the basin in the foredeep, forebulge, and backbulge depozones, respectively. Paleocurrents suggest that Bechler paleochannels are deposits of basin-transverse rivers that flowed to the east, whereas B and L2 paleochannels are deposits of basin-axial rivers that flowed dominantly to the north and northeast. The scales and discharges of most L2 paleochannels are much greater than those of the Bechler and B-interval. This eastward increase in discharge may reflect an eastward increase in precipitation related to the spatially decreasing effects of an orographic rain shadow caused by the Sevier Orogen to the west. Additionally, or alternatively, the higher discharges of most L2 rivers may indicate that they represent a more distal part of a tributary fluvial system than B-interval rivers (consistent with some lower slopes of L2 paleochannels).The alluvial architecture of thick foredeep deposits contrasts markedly with that of stratigraphically equivalent, much thinner deposits farther east that were associated with the forebulge and backbulge depozones. Foredeep deposits are dominated by overbank and lacustrine mudstones, and channel deposits tend to be isolated with limited lateral extents typically on the order of 10's of meters. Forebulge and backbulge channel deposits tend to be laterally and vertically connected forming sandstones and conglomerates with lateral extents on the order of 10's of km to >100 km. Long-term compacted sediment accumulation rates for the foredeep (generally 10−2 mm year−1) are an order of magnitude greater than those for the forebulge and backbulge depozones (10−3 mm year−1). Quantitative simulations of channel-deposit proportions indicate that basin-wide differences in alluvial architecture are attributable to differences in sediment accumulation rates, which, in turn, reflect variable subsidence rates of the different depozones. Additionally, in some areas of the fore- and backbulge depozones, alluvial architecture was controlled by local syndepositional structures. However, the alluvial architecture in areas influenced by syndepositional structures is broadly similar to that in areas where such structures were absent, both reflecting the same general tectonic setting that experienced limited regional subsidence. Hence, the two cases are not easily distinguished solely on the basis of alluvial architecture.

Migration of dynamic subsidence across the Late Cretaceous United States Western Interior Basin in response to Farallon plate subduction

Backstripped cross sections of the Late Cretaceous succession across central Utah, Colorado, and southern Wyoming in the Western Interior Basin, United States, reveal a component of continuously evolving long-wavelength residual subsidence, in addition to subsidence driven by the Sevier thrust belt and associated sediment loads. The loci of maximum rates of this residual subsidence moved eastward from ca. 98 to 74 Ma in phase with the west to east passage of the Farallon slab, as reconstructed from tomography based on quantitative inverse models. These new subsidence data allow testing of existing subduction models and confi rm the dynamic subsidence origin of the Western Interior Basin. Furthermore, regional variations in subsidence rates suggest a possible defi cit of negative buoyancy (mantle loading) inside the slab beneath Colorado, supporting the hypothesis that the thickened slab represents a subducted oceanic plateau. This paper documents how the Cretaceous stratigraphy records the timing, patterns, and position of underlying mantle processes during Farallon slab subduction. The new data also reveal patterns indicative of the commencement of the Laramide orogeny in the western United States.

Geomorphic evolution of the Piedmont Zone of the Ganga Plain, India: a study based on remote sensing, GIS and field investigation

The geomorphic evolution of the Piedmont Zone of the Ganga Plain is not well understood, since the identification of various morpho-tectonic features in the field is very difficult because of the thick forest cover, extensive anthropogenic modifications in the landscape and easily reworkable gravels. Interpretation of satellite imagery, Digital Terrain Models (DTMs) and field data has helped in identification and mapping of various morpho-tectonic features in a hitherto unstudied part of the Piedmont Zone and the adjoining areas in north-western Ganga Plain, India. The Piedmont Zone has formed as a result of laterally coalescing alluvial fans of variable dimensions and talus deposits, and is traversed by active faults and thrusts. The variable subsidence rates along the basin margin (i.e. the Himalayan Frontal Thrust) and climatic events have controlled the accommodation space and basinward progradation of alluvial fans. The landscape of the Piedmont Zone is presently being continuously modified by the ongoing tectonic movements and seasonal climatic variations.

Experimental Measurement of the Relative Importance of Controls on Shoreline Migration

Abstract Shoreline position in sedimentary rocks is a sensitive recorder of the interplay of several controlling factors. The most important of these are thought to be the stratigraphic trinity: eustatic sea level, subsidence, and sediment supply. In ancient rock sequences, it is generally difficult to disentangle the effects of these variables. Here we analyze the relative influence of sea level, subsidence, sediment supply, and other controlling variables on shoreline migration in an experimental basin equipped with a subsiding floor. The experiment used a linear-hinge type subsidence profile for which the rate was kept constant in time, constant overall sediment supply, and base-level variation on two time scales that were first applied separately and then superimposed. Although base level was the only controlling variable that was externally varied in time, the base-level changes induced changes in other variables indirectly (e.g., by changing the partitioning of sediment between the fluvial and offshore segments of the system). We examine the relative importance of measured direct and indirect changes in all the governing variables through the use of a moving-boundary equation for shoreline migration. When measured values are used for all the variables in the equation, shoreline migration rate throughout the run can be predicted with a maximum R2 of > 0.92. Starting with this optimal prediction of the observed shoreline behavior, we successively replace variables in the equation with their run-averaged values, degrading the prediction. The relative loss of prediction accuracy as each variable is replaced is a measure of the importance of that variable in accounting for the observed shoreline migration. By this measure, base level is the most important variable, followed in turn by sediment supply to the foreset, geometry of the foreset, and the average subsidence rate over the foreset. From the shoreline migration equation, we also derive a quantitative version of the often applied in sequence stratigraphy. The new formulation reduces to a form comparable to the traditional descriptive A/S ratio if changes in foreset slope, gain or loss of sediment to the fluvial system, and spatial variation in subsidence rate are all negligible.

Experimental Test of Tectonic Controls on Three-Dimensional Alluvial Facies Architecture

Abstract In an extended series of papers by M. Leeder, J.R.L. Allen, J.S. Bridge, J. Alexander, and others, the depositional stacking patterns of channel-belt deposits were related to tectonic and other external controls (we refer to these models collectively as LAB models). These models established a basic connection between subsidence, channel avulsion, and the mesoscale (channel-belt) architecture of an alluvial deposit. Two fundamental predictions resulted from this work: (1) channel-belt stacking density and hence connectedness is inversely correlated to temporal (vertical) changes in sedimentation rate; and (2) channel-belt stacking density and hence connectedness is directly correlated to lateral changes in sedimentation rate. In this paper we make use of the Experimental EarthScape (XES) Facility at the St. Anthony Falls Laboratory to examine the effects of temporally and laterally variable subsidence rates on mesoscale alluvial architecture. We then compare the experimental results with the main predictions of the early LAB models. Regarding the predictions of the LAB models, under conditions of high sediment supply and a highly active alluvial system, lateral and downstream variation in subsidence geometry and rate have little effect on the details of alluvial architecture. We found that the principal architectural signature of changes in subsidence geometry (i.e., laterally variable subsidence rate) is stratal tilting and that channel belts are not attracted to the subsidence maximum. We hypothesize that the effect of variable subsidence geometry is felt by the fluvial system only if subsidence proceeds faster than the river can adjust to the formation of a topographic low via the deposition of overbank material in the form of splays and sheet sands. Our results lend impetus to the development of more complete three-dimensional numerical models of mesoscale alluvial architecture that couple architecture to broader, allogenic forcing mechanisms.

Expression and modelling of stratigraphic sequence distortion

This study describes the lateral variability of stratigraphic sequences under changing conditions of subsidence and/or sediment supply. Changes in sediment supply, subsidence and absolute sea-level result in at least three modes of distortion of a depositional sequence. Our study of a case history from offshore West Africa, together with numerical analysis, provides insights as to how changes in subsidence and sedimentation may be extracted from the stratigraphic record. We analysed a listric fault/raft system, located on the Congo–Cabonda margin, resulting from gravity-induced extension of a mixed siliciclastic/carbonate platform sequence. Within this system, quantitative analysis was carried out on high-resolution stratigraphic sequences of five wells, as well as on their geometry, on the relative duration of prograding compared to retrograding half-cycles, and on the timing of the inversion of these half-cycle trends. Parameters were defined to quantify the distortion of depositional sequences that results from either (i)spatial variations in subsidence and sedimentation rates (“spatial distortion” D), or (ii) superposition of two frequencies of stratigraphic cyclicity (“cycle superposition distortion” D′). Using a numerical model, we investigate the distortion of depositional sequences showing two superposed scales of cyclicity, in response to spatial and temporal variations of subsidence and sedimentation rate. We show that: (i) Spatial variations in subsidence rate can result in modification of the timing of trend inversion: the onset of progradation may be delayed and the onset of retrogradation may occur earlier in the more rapidly subsiding areas. (ii) The sedimentation rate can modulate the amount of distortion related to subsidence by amplifying, limiting, compensating or even inverting the temporal offset. (iii) Spatial variations in sedimentation rate alone may also induce changes in the timing of trend inversion: the onset of progradation may take place sooner and the onset of retrogradation may be delayed in the area showing the fastest sedimentation rate. (iv) The amount of distortion depends not only on the sedimentation and subsidence rates, but also on the maximum rate of sea-level change causing the depositional sequence, that is, it depends on the period and amplitude of the sequence. The faster the sea-level change (i.e. the higher the frequency and the larger the amplitude), the weaker is the distortion produced. (v) In terms of resulting time lag, numerical analysis shows that the distortion of high frequency depositional sequences (genetic units) is negligible. These sequences can therefore be safely used to correlate time lines across the studied area, whereas use of lower frequency sequences with significant spatial distortion could lead to significant errors in correlation. We use these results to interpret the distortion observed in the case study, in terms of temporal and spatial variations of subsidence and sedimentation rates. In this case, complex temporal and spatial variations in subsidence and sedimentation rates lead to variations of the distortion of stacks of genetic unit. This distortion produced difference in timing of the onset and duration of the inversion of trend within prograding and retrograding half-cycles.

Interlinked barrier chain and delta lobe development, northern Gulf of Mexico

A wealth of new data provides a well-constrained chronology of mid- to late Holocene coastal development in the Louisiana–Mississippi borderland that may also be utilized in a globally applicable sedimentation model. Barrier sand and deltaic mud sequences illustrate a process by which potential ground water and hydrocarbon reservoir rocks accumulated unusually rapidly and were preserved. Set against decelerating Holocene sea-level rise and locally variable subsidence rates, the study provides an example of the interplay between an emerging, prograding, and partially stranded barrier island chain, a sizable estuary, and several extensive delta lobes. Utilizing microfossil fauna-based depositional facies information and archaeological data, absolute dates helped to reconstruct the history of the Alabama–Louisiana barrier chain and deltas between ca. 5.7 and ca. 1.5 14C ka BP. Protected by the substantial dune cover that prevented island submersion, the regional eustastic transgression paradoxically was synchronous with significant progradational barrier and deltaic regression. The earliest barrier islands emerged ca. 4.6–4.4 14C ka BP (ca. 5.7–5.0 cal years) when sea level stood at ca. −1.0 to −1.5 m. These islands isolated Mississippi Sound from the greater Gulf of Mexico. The absence of a lagoonal-inshore sediment interval beneath the islands and the 3–15 m thick basal nearshore marine muddy–sandy unit that blankets the Pleistocene surface refutes the transgressive history of barrier initiation. The islands aggraded on a 3.0–16.5 m thick fine sandy shallow-marine regressive platform lithosome that, in turn, overlies a transgressive muddy–sandy nearshore marine lower sediment interval. Avulsion of the Mississippi River had abruptly reduced nearshore salinities by ca. 3.9–3.7 ka 14C BP. Renewed Mississippi delta growth induced rapid aggradation and progradation on the opposite Pearl River delta-mainland shore as well. Gulf influences have rapidly diminished in areas so affected. A new absolute chronology of the Mississippi–St. Bernard delta lobes constrains these events. Delta growth and mainland progradation first isolated, then severely constricted the Lake Borgne embayment. Accompanied by ongoing subsidence, the western barriers were stranded, then buried. Shoaling, related to St. Bernard delta progradation, interfered with westward littoral drift to maintain Cat Island. Archaeology provided important supplementary data for dating environmental changes. Refuting earlier suggestions that Native colonization rapidly followed delta complex formation, the earliest known Indian sites postdated the start of the associated St. Bernard delta lobe by 1.9–3.1 14C ka.

ABSTRACT: Origin of Petroleum in the Mexican Gulf Coast Basin

Abstract Several generation models for petroleum generation have been proposed for the Mexican Gulf Coast Basin. Various oil/oil and source rock/oil correlations have been postulated relying on bulk oil features as well as isotopic and biomarker compositions. The tectonic evolution of this area has been a main control of the source rocks and the physico-chemical variations of their related oils. The best Mexican source rocks are related to the rift tectonic stage. Individual structural blocks developed during this stage had different suites of sedimentary facies because of variations in subsidence rates. In the grabens, deposition occurred in organic-rich anoxic marine carbonate environments from Oxfordian until Tithonian times. Variations in salinity, oxygen depletion levels, and clay and carbonate content can be identified by the molecular features of their related oils. Tithonian oils have a widespread distribution throughout the Mexican Gulf Coast Basin from the Tampico-Misantla Basin to Campeche Sound. Marine carbonate platforms were developed during the sag stage from Neocomian until Middle Cretaceous times. During this stage, thick carbonate sequences were deposited in the Tampico-Misantla, Veracruz, Campeche Sound, Chiapas-Tabasco, Sierra de Chiapas, and Yucatan areas. Cretaceous carbonate rocks may contain fine stringers of oil-prone organic matter, probably the remains of bacterial mats, and as a result they could be source rocks for produced oils. The oils related to these environments are present in the Veracruz, the Chiapas-Tabasco, and the Sierra de Chiapas areas. As elsewhere in the Gulf of Mexico Basin, the Tertiary in the Mexican Gulf Coast was a time of deltaic sedimentation. Tertiary source rocks were capable of sourcing gas and condensate, and perhaps some light oils. Tertiary oils are mainly produced in the Burgos and Macuspana basins. Their molecular features suggest a source from reworked terrestrial higher plants deposited in siliciclastic anaerobic depositional environments. This diversity of generative subsystems in the Mexican Gulf Coast Basin can explain the huge petroleum potential of this area. End_of_Record - Last_Page 661-------

Subsidence History of the Rukwa Rift in South West Tanzania Analysed from Ivuna Well.

The variation in subsidence rate during rift basin development is a good indication for the Geodynamic history of a sedimentary basin. The sedimentary section of Ivuna Well is herein used to explain the structural evolution of Rukwa Basin within the Western Rift of the East African Rift System. The sedimentary record of Ivuna Well is extracted from published information. The effects of sedimentary load, corrected for compaction and variation in water depth, and lake-level have been removed to obtain the "tectonic subsidence." Curves show two phases of accelerated subsidence related to the fault controlled rifting phases: The Karoo rifting and the Late Cenozoic rifting. Though several phases of rifting are proposed within Karoo time in eastern and southern Africa, it is difficult, with the present information from Ivuna well, to infer them. But the change of gradient of the Geohistory plots within the Karoo section does suggest at least variations of sedimentation rates. The Karoo rifting phase is followed by a steady subsidence which resulted from thermal contraction of the lithosphere thinned during Karoo crustal and lithospheric stretching, while Late Cenozoic rifting is still young at its initial phase of rifting (t = 0). Tanzania Journal of Science Volume 26 (2000), pp. 1-14