Variations In Subsidence Rates Research Articles

Stratigraphy and fluvial sedimentary facies of the Neocomian lower Strzelecki Group, Gippsland Basin, Victoria

The Strzelecki Group incorporates Berriasian to Albian, fluvial sediments deposited in the Gippsland Basin during initial rifting between Australia and Antarctica. Neocomian strata of the lowermost Strzelecki Group are assigned to the Tyers River Subgroup (exposed in the Tyers area) and the Rhyll Arkose (exposed on Phillip Island and the Mornington Peninsula). The Tyers River Subgroup incorporates two formations: Tyers Conglomerate and Rintoul Creek Formation. The latter is subdivided into the Locmany and Exalt Members. Ten fluvial sedimentary facies are identified in the lowermost Strzelecki Group: two gravelly facies; four sandy facies; and four mudrock facies. Associations of these facies indicate: (i) prevalence of gravelly braided‐river and alluvial‐fan settings during deposition of the Tyers Conglomerate; (ii) more sluggish, sandy braided to meandering fluvial systems during Locmany Member sedimentation; and (iii) a return to active, sandy, braided‐river settings for deposition of the Exalt Member. The Tyers Conglomerate and Rhyll Arkose rest on an irregular erosional surface incised into Palaeozoic rocks of the Lachlan Fold Belt. The overlying Rintoul Creek Formation incorporates more mature sediments where lithofacies associations varied according to base‐level change, variations in subsidence rates, and/or tectonic uplift of the principal sedimentsource terranes to the northwest.

Temporal significance of sequence boundaries

This paper analyses the temporal significance of stratigraphic surfaces bounding the marine portions of the depositional sequence, genetic stratigraphic sequence and transgressive–regressive sequence. These bounding surfaces, known as the `correlative conformity' (c.c.), `maximum flooding surface' (MFS) and `conformable transgressive surface' (CTS), respectively, may either be defined on the basis of stratal stacking patterns (which we call `type A surfaces'), or on the basis of water-depth changes and relative sea-level changes (which we call `type B surfaces'). The type A MFS and CTS are time lines in a depositional-dip section, corresponding to the turnaround points from shoreline transgression to regression and vice versa. They separate prograding (coarsening-upward) from retrograding (fining-upward) geometries, with a timing determined by the interplay between the rates of sedimentation and relative sea-level rise in the shoreline area. The timing of type A MFS and CTS is not affected by the offshore variations in sedimentation and subsidence rates, but it is only controlled by the shoreline movements and the associated facies shifts. The type A c.c. separates rapidly prograding and offlapping forced regressive strata from the overlying lower rate prograding and aggrading normal regressive strata. This surface is diachronous, younger basinward, with the rate of offshore sediment transport. The timing of the type A c.c. in the shoreline area corresponds to the end of relative sea-level fall, but it develops under relative sea-level rise conditions offshore. The timing of the type B MFS and CTS depends on the offshore variations in the sedimentation and subsidence rates. These surfaces, defined on the basis of bathymetric changes, become younger and older seaward, respectively, tending to merge together offshore. The type B c.c. marks the end of relative sea-level fall in any point along a depositional-dip section. It is diachronous, older basinward, as its timing depends on the offshore variations in subsidence rates. The diachroneity of type B surfaces reaches a quarter of the period of the highest frequency variable, whichever that is among the eustasy, tectonics or sedimentation controls. Types A and B surfaces merge together in the shoreline area, but they become temporally divergent offshore. Deepening-upward and shallowing-upward facies should not be confused with transgressive and regressive systems tracts. The latter are strictly controlled by the shoreline movements, which determine the direction of facies shifts and the stratal stacking patterns.

Silurian Tectonics of Western Avalonia: Strain-Corrected Subsidence History of the Arisaig Group, Nova Scotia

The Arisaig Group is a Silurian succession of predominantly shallow marine clastic sediments overlying volcanics, exposed in northern mainland Nova Scotia. Sediment accumulation provides a record of the subsidence of western Avalonia during the interval when terranes were being accreted within the Canadian Appalachians. To calculate the amount of subsidence, one must correct the measured thicknesses for the effects of tectonic strain. Deformed fossils on bedding surfaces indicate strain ratios mainly between 1.2 and 1.6. An empirical porosity-depth relationship is used to correct for compaction. The subsidence curves are then adjusted to allow for variations in water depth and eustatic sea level. The resulting curves show significant variations in subsidence rate regardless of which version of the Silurian time-scale is used. An initial episode of rapid subsidence followed eruption of Llandoverian volcanics. Slower subsidence took place in Wenlockian and Ludlovian time, with deeper-water sedimentation during an early Ludlovian eustatic high. This part of the history is consistent with thermal subsidence following an initial extensional event. A rapid increase in subsidence rate occurred during deposition of the Pridolian Stonehouse Formation. This episode was accompanied by a rapid increase in sediment supply, and a change in paleocurrent flow from southwest to northwest. The rapid Pridolian subsidence probably resulted from oblique collision between Avalonia and the Meguma Terrane to the south.

Tectonic control of fan size: the importance of spatially variable subsidence rates

We study the geophysical controls on the size of alluvial fans. Simple relationships between catchment characteristics, sediment yield, subsidence patterns and fan size are developed. As predicting fan size is essentially a conservation of mass problem, our analysis is general, applying to all types of fan landform. The importance of spatially variable subsidence rates has gone largely unrecognized in previous studies of modern fans. Here we stress that the distribution of subsidence rates in the depositional basin is a primary control on relative fan size. Both free coefficients in the oft‐cited power‐law correlation of fan area and catchment area can be shown to be set primarily by the tectonic setting, taken to include source area uplift rate and the subsidence distribution in the depositional basin. In the case of a steady‐state landscape, relative fan size is shown to be independent of both climate and source lithology; only during times of significant departure from steady state can relative fan size be expected to vary with either climate or source lithology. Transients associated with (1) a sudden increase in rock uplift rate, (2) a sudden change in climate and (3) the unroofing of strata with greatly differing erodibilities may produce variation of relative fan areas with both climate and source lithology. Variation of relative fan size with climate or lithology, however, requires that catchment–fan system response to perturbations away from steady state is sensitive to climate and lithology. Neither the strength of transient system responses nor their sensitivity to climate or lithology are known at present. Furthermore, internal feedbacks can significantly dampen any climatic or lithological effect. Thus theoretical considerations of the importance of climatic and lithological variables are inconclusive, but suggest that climatic and lithological effects are probably of secondary importance to tectonic effects. Field data from an unsteady landscape in Owens Valley, California, support and illustrate theoretical predictions regarding tectonic control of fan size. Field data from Owens Valley allow, but do not prove, a secondary dependence on source lithology. In addition, the Owens Valley field data indicate no relationship between relative fan size and climate. Headward catchment growth and enhanced sediment bypassing of fans during times of increased sediment yield (glacial) are put forward as plausible explanations.

Eustatic Curve for the Middle Jurassic--Cretaceous Based on Russian Platform and Siberian Stratigraphy: Zonal Resolution

We have used the stratigraphy of the central part of the Russian platform and surrounding regions to construct a calibrated eustatic curve for the Bajocian through the Santonian. The study area is centrally located in the large Eurasian continental craton, and was covered by shallow seas during much of the Jurassic and Cretaceous. The geographic setting was a very low-gradient ramp that was repeatedly flooded and exposed. Analysis of stratal geometry of the region suggests tectonic stability throughout most of Mesozoic marine deposition. The paleogeography of the region led to extremely low rates of sediment influx. As a result, accommodation potential was limited and is interpreted to have been determined primarily by eustatic variations. The central part of the Russian latform thus provides a useful frame of reference for the quantification of eustatic variations throughout the Jurassic and Cretaceous. The biostratigraphy of the Russian platform provides the basis for reliably determining age and eustatic events. The Mesozoic section of the central part of the Russian platform is characterized by numerous hiatuses. In this study, we filled the sediment gaps left by unconformities in the central part of the Russian platform with data from stratigraphic information from the more continuous stratigraphy of the neighboring subsiding regions, such as northern Siberia. Although these sections reflect subsidence, the time scale of variations in subsidence rate is probably long relative to the duration of the stratigraphic gaps to be filled, so the subsidence rate can be calculated and filtered from the stratigraphic data. We thus have compiled a more complete eustatic curve than would be p ssible on the basis of Russian platform stratigraphy alone. Relative sea level curves were generated by backstripping stratigraphic data from well and outcrop sections distributed throughout the central part of the Russian platform. For determining paleowater depth, we developed a model specifically designed for this region based on paleoecology, sedimentology, geochemistry, and paleogeography. The curve describes a series of high-frequency eustatic events superimposed on longer term trends. Many of the events identified from our study can be correlated to those found by Haq et al. (1988) and other sea level studies from other parts of the world, but there are significant differences in the relative magnitudes of events. Because the eustatic curve resulting from this study is based on a stable reference frame, the curve can be used in sedimentary basin modeling and as a tool for quantifying subsidence history from the stratigraphy of passive margins, basins, and other active regions.

Regional viscosity variations, small‐scale convection and slope of the depth‐age1/2 curve

Cooling plate and cooling half‐space models are commonly employed to describe the depth‐age1/2 curve of seafloor. These conductive models are not able to reproduce the large range of subsidence rates that are observed with reasonable changes in the physical parameters. We examine the influence of small‐scale convection in the mantle on subsidence rates with a one‐dimensional, parameterized‐convection model. The parameterization is based on a recent experimental study of thermal convection in a fluid with temperature‐dependent viscosity. In our model, convection in the asthenosphere begins at the ridge axis, enhancing heat transfer out of the asthenosphere and increasing the subsidence rate above that for simple conductive cooling. Variations in deformation mechanism, temperature and water content of the mantle cause changes in viscosity that affect the vigor of convection and may be responsible for large variations in subsidence rates. Average subsidence rates can range from ∼250–500 m/Ma1/2 when small‐scale convection is considered.

Long‐lived mid‐ocean ridge segmentation: Constraints on models

Long‐lived, large scale segmentation of the mid‐ocean ridge, as defined by variations in subsidence rate of the ocean crust, zero‐age depth, and geoid rate, has been documented along three mid‐ocean ridge systems: the South Atlantic, the Pacific‐Antarctic, and the Southeast Indian. Many of the observations in these three areas are inconsistent with standard thermal models for the generation of oceanic crust and lithosphere. Three models for subsidence rate and geoid rate and two models for zero‐age depth are examined. Sensitivity tests are done to determine how much the thermal and physical “constants” in these models must vary in order to explain the observations and whether or not such values are plausible. Only the zero‐age depth variations can be adequately explained using existing models. An internally consistent, qualitative model is presented which can explain the data; it involves regional and local temperature changes in the mantle both along and across strike of the mid‐ocean ridge.

Subsidence of the seafloor in the Atlantic and Pacific Oceans: Regional and large-scale variations

We present a deteiled survey of seafloor subsidence over the entire Atlantic Ocean and in the Pacific Ocean. We examine the evolution of depth as age increases within narrow (200 km wide) tectonic corridors elongate in the direction of spreading. Over the part of the young seafloor where the depth-age 1 2 relationship is valid, we estimate the subsidence rate by a least-squares adjustment. In the three oceanic regions (West Atlantic, East Atlantic and Pacific) the subsidence rate shows regional (1000–2000 km) and basin-scale fluctuations of the order of 100–150 m/m.y. 1 2 . We also note that in some regions subsidence rate and axial depth are correlated, with shallow ridges showing fast subsidence and deep ridges slow subsidence. On the basis of published models, we infer that regional variations in subsidence rate reflect temperature changes in the subaxial mantle of ∼ 100°C. Asymmetrical subsidence is observed in the Atlantic, especially in the South Atlantic. The age of departure from the depth-age 1 2 linear trend is highly variable, with values larger than 100 m.y. reported in several instances but very low values (20 m.y. or less) characterizing the central Pacific. Flattening predicted by the plate model occurs in some areas. In a number of cases however, quite distinct behaviour is observed, with a sudden depth increase where the depth-age 1 2 relationship breaks down, and a slow rise of the seafloor at older ages. Our results suggest that the subsidence behaviour of the seafloor is much more complex than global plate or half-space cooling models predict.

Tectonic corridors in the south Atlantic: Evidence for long‐lived mid‐ocean ridge segmentation

The South Atlantic is investigated for evidence of long‐lived (tens of millions of years), tectonic segmentation of the mid‐ocean ridge by examining a variety of geophysical parameters. These parameters include: subsidence rate of the oceanic crust (east flank, west flank, average, asymmetry), zero‐age depth, geoid height decrease with age (geoid rate), and the residual geoid field. The variability in these parameters along‐strike of the mid‐ocean ridge is typically systematic. The ridge can be subdivided into segments (flowline corridors in plan view) within which characteristics are uniform. Eight tectonic corridors have been determined in the area between the Ascension Fracture Zone and the Falkland‐Agulhas Fracture Zone, which serve to define a large scale ridge segmentation on the order of hundreds of kilometers. This segmentation is in addition to the well‐known small‐scale, near‐axis segmentation. Near‐axis geochemical data show boundaries that are generally consistent with the tectonic segmentation. Many of the primary geophysical observations are found to be internally inconsistent with simple thermal conduction models for oceanic crust. Such observations include asymmetric crustal subsidence, large along‐strike variations in subsidence rate and geoid rate, the lack of a systematic correlation between subsidence rate and geoid rate, and the abrupt nature of some of the inferred tectonic boundaries. Thus, the data indicate that in addition to uniform lithospheric cooling, other factors such as variable asthenospheric temperatures are important contributors to the creation and subsequent modification of the oceanic crust and possibly the oceanic lithosphere.

The large‐scale dynamics of grain‐size variation in alluvial basins, 1: Theory

ABSTRACTWe study the interplay of various factors causing vertical grain‐size changes in alluvial basins using a simple coupled model for sediment transport and downstream partitioning of grain sizes. The sediment‐transport model is based on the linear diffusion equation; by deriving this from first principles we show that the main controls on the diffusivity are water discharge and stream type (braided or single‐thread). The grain‐size partitioning model is based on the assumption that the deposit is dominated by gravel until all gravel in transport has been exhausted, at which point deposition of the finer fractions begins.We then examine the response of an alluvial basin to sinusoidal variation in each of four basic governing variables: input sediment flux, subsidence rate, supplied gravel fraction, and diffusivity (controlled mainly by water flux). We find that, except in the case of variable gravel fraction, the form of the basin response depends strongly on the time‐scale over which the variation occurs. There is a natural time‐scale for any basin, which we call the ‘equilibrium time’, defined as the square of basin length divided by the diffusivity. We define ‘slow’ variations in imposed independent variables as those whose period is long compared with the equilibrium time. We find that slow variation in subsidence produces smoothly cyclic gravel‐front migration, with progradation during times of low sedimentation rate, while slow variation in sediment flux produces gravel progradation during times of high sedimentation rate. Slow variation in diffusivity produces no effect. Conversely, we define ‘rapid’ variations as those whose period is short compared with the equilibrium time. Our model results suggest that basins respond strongly to rapid variation in either sediment flux or diffusivity; in both cases, deep proximal unconformities are associated with abrupt gravel progradation. This progradation occurs during times of either low sediment flux or high diffusivity. On the other hand, basin response to variation in subsidence rate gradually diminishes as the time scale becomes short relative to the equilibrium time. Each of the four variables we have considered ‐ input sediment flux, subsidence, gravel fraction, and diffusivity ‐ is associated with a characteristic response pattern. In addition, the time scale of imposed variations relative to the equilibrium time acts in its own right as a fundamental control on the form of the basin response.

Fluvial, coastal, nearshore, and shelf deposition in the Upper Proterozoic (?) to Lower Cambrian Addy Quartzite, northeastern Washington

Quartzose and lesser argillaceous strata of the approximately 1300 m thick Upper Proterozoic (?) to Lower Cambrian Addy Quartzite of northeastern Washington record shelf, coastal, and continental sedimentation on an actively subsiding, relatively young passive margin. The lower 400 to 500 m of the Addy Quartzite was deposited immediately following the initiation of subsidence in a wave-dominated coastal and eventually storm-dominated shelf environment. This sequence records relative sea level rise that culminated in high-stand conditions. These sediments grade upwards into approximately 150 m of fluvial strata that record progradation of terrestrial environments during a period of relative sea level fall and a regional low-stand. The upper 700 m of the Addy Quartzite fines upwards from nearshore- to shallow-marine-dominated sediments into shelf-dominated deposits. This sequence marks the reestablishment of net sea level rise. The overall fining-upwards trend displayed by the Addy Quartzite indicates that regional subsidence of the newly formed passive margin was one of the most important factors controlling Addy sedimentation. However, variation seen in this trend throughout the Addy Quartzite indicates that rates of sea level rise associated with subsidence fluctuated and occasionally even reversed. Causes for the variation are inferred to be related to variations in subsidence rates and sediment supply. Textural and compositional data suggest Addy detritus was derived from relatively proximal recycled sedimentary source terranes and more distal continental crystalline terranes.

Regional variations in subsidence rate of lithospheric plates: implication for thermal cooling models

Although simple thermal models of lithospheric cooling predict to first order the general behaviour of observed seafloor depth with increasing age, important regional variations in seafloor subsidence, in the range 250–400 m Ma 1 2 , are reported for several lithospheric plates. Such variations cannot be accounted for by classical cooling models unless implausible variations in asthenospheric temperature of ∼550°C are assumed. Here we present an alternative cooling model, which assumes that at the ridge axis the temperature may deviate from the mean asthenospheric temperature. Such a model satisfactorily explains the data provided that the temperature deviation is ±100°C only.

Regional variations in subsidence rate of oceanic plates: a global analysis

A global analysis of the dependence of seafloor depth on crustal age has been performed for each large oceanic plate independently. Each plate has been divided into regions bounded by major tectonic features. A total of 32 regions have been considered. Bathymetry corrected for sediment loading has been plotted as a function of crustal age for each region. Except for three regions (western Pacific between 10°S and 40°N, North American plate between 24°N and 38°N and African plate between 10°N and 24°N) where ocean floor older than 80–100 Ma flattens with age, in other regions, depth increases linearly with age 1/2. Subsidence rate over each region has been computed by linear regression. Large variations (up to a factor 2) are reported from one region to another. Asymmetrical subsidence is also observed in a number of areas. However, the largest asymmetries are less than the regional variations in subsidence rate occurring on individual plates. Regional variations in subsidence rate appear linearly related to regional variations in ridge crest topography, shallow ridge subsiding quickly and deep ridge subsiding slowly. Hence seafloor depth subsides at a rate modulated by the value of the initial (axial) depth. This observation suggests that regional variations in subsidence rate reflect the thermal regime of the mantle beneath mid-ocean ridges rather than lateral temperature variations inside the asthenosphere involving small-scale convection.

Intra-arc sedimentation and tectonism: Late Paleozoic evolution of the eastern Klamath terrane, California

Rocks of the eastern Klamath terrane (Redding section) in northern California record deposition, magmatism, and tectonism in an island-arc setting. Volcanism and sedimentation persisted for more than 220 m.y., spanning Devonian to Middle Jurassic time, and yielding thick successions of water-laid volcanic debris and lavas, with interstratified carbonate and clastic sequences. This study focuses on the upper Paleozoic rocks within the section. The oldest rocks studied, within the Upper Devonian(?) and Mississippian Brag-don Formation, contain epiclastic debris that was deposited in a submarine-fan setting. Sandstone compositions indicate that quartzose sedimentary and metasedimentary rocks formed the source area for epiclastic debris and that volcanism persisted throughout sedimentation. Transitional shallow-water facies in the upper part of the Bragdon interfinger with the overlying Baird Formation, which contains abundant volcanic debris. Volcanogenic shelf and strandline deposits characterize the lower part of the formation, and volcaniclastic turbidites characterize the upper part. The Baird Formation is locally interfingered with the overlying Lower Permian McCloud Limestone, which records two stages of Early Permian carbonate platform development: early Wolfcampian and late Wolfcampian to early Leonardian. Only the younger of the two platforms is present within the study area, and it contains more distal facies than those preserved to the south. The strata contain abundant fusulinids and corals which provide age constraints for carbonate-platform development and form the basis for regional biogeographic correlation. The time-transgressive Upper Permian Bollibokka Group (Nosoni and Dekkas Formations) succeeds the McCloud Limestone, comprising thick sequences of volcanogenic debris, intercalated lava, and, in the upper part, lenses of shallow-water limestone. The Upper Permian strata reflect volcanism and rapid subsidence within the are. All upper Paleozoic units were intruded by small gabbroic bodies and diabase dikes which may be partly cogenetic with Upper Permian basalts in the upper Dekkas Formation. The upper Paleozoic succession records local pulses of magmatism and variable subsidence rates within the eastern Klamath arc. Mississippian epiclastic rocks reflect uplift of the volcanic-arc basement, which contained rocks similar to those now exposed in the adjacent Yreka terrane or Shoo Fly Complex. Pulses in volcanism were restricted to mid-Carboniferous and Late Permian time. Diminished volcanism during the Early Permian allowed development of carbonate platforms. Rapid fluctuations of sedimentation and sub-sidence rates; pronounced lateral facies and age variations; accumulation of thick, volcanic-debris aprons during episodic volcanism; and sporadic persistence of volcanism during apparent lulls characterized deposition within this succession and are common processes in modern volcanic arcs. The stratigraphic record is composed predominantly of volcanogenic rocks and lesser carbonates, reflecting events and processes within the arc itself. Evidence for nearby basins or landmasses is sparse. The Mississippian epiclastic rocks, however, which were derived from the arc basement, reflect an ultimate sialic crustal source. Comparison of volcanic-arc evolution with the late Paleozoic evolution of continental margin and basinal rocks to the east indicates possible paleogeo-graphic ties between the eastern Klamath volcanic arc and rocks of the Golconda allochthon or the North American continent.

Computer models of carbonate platform cycles driven by subsidence and eustasy

Six computer simulations of cycles developed on carbonate platforms were generated using MAPS, a BASIC program. Four simulations employed a flat platform and sinusoidal sea-level oscillations with heights ranging from 0.02 to 20 m, but a constant 25 ka period superimposed over a constant relative sea-level rise of 0.05 m/ka. The lower height oscillations might reasonably model variable subsidence rates, whereas the higher oscillations model glacioeustatic sea-level changes. Metre-thick cycles were generated in all cases. Two simulations involved variations in periods of sinusoidal sea-level variations and subsidence rates on a tilting platform. The thicknesses of cycles generated are a function of both period of sea-level oscillation and subsidence rate. In geologic applications, either the period of sea-level oscillations or the subsidence rate needs to be known to determine the other. We suggest that two end-member models need to be considered for interpreting carbonate platform cycles: (1) constant subsidence with variable sea level; (2) constant sea level with variable subsidence. Combinations of these two end members are likely.

Variations in subsidence rates along intermediate and fast spreading mid-ocean ridges

The nature of subsidence near the ridge crest of the intermediate and fast spreading mid-ocean ridges of the Indian and Pacific Oceans is investigated using surface—ship bathymetry and magnetics profiles. The ridge can be divided into discrete sections, apparently bounded by distinct structural features such as major fracture zones, in which bathymetry plotted against crustal age forms a well-defined envelope with a width roughly the amplitude of the local bathymetry. The averaged bathymetry in all of the regions studied follows closely a square root of age subsidence curve which in most regions has a subsidence coefficient, C1, in the range of 340–390 m Myr−1/2. The best fitting subsidence curve, however, never reproduces the amplitude of the axial topographic high. The most notable region displaying unusual behaviour is the East Pacific Rise between 9°S and 22°S. In this region, the western flank of the ridge is subsiding at 200–225 m Myr−1/2 while the eastern flank is subsiding at ‘normal’ rates of 350–400 m Myr−1/2. Other anomalous areas include the region between the Easter Island hot spot and the Chile Rise triple junction in which the ridge crest is shallow and which is subsiding at rates of about 290 m Myr−1/2, and the region east of the Australia-Antarctic Discordance in which the northern flank is subsiding at 440 m Myr−1/2. This area may also be subsiding asymmetrically although there is not much data from the southern flank. The asymmetric subsidence in the 9°S-22°S region of the East Pacific Rise begins immediately at the ridge crest and the low subsidence rates on the west flank continue to at least 12 Myr old crust. Oligocene-aged crust on the western flank is subsiding at more normal rates, but is 500 m shallow with respect both to the world-wide average and to the conjugate crust on the eastern flank. The simplest model to explain these observations is that the western flank is underlain by a hotter mantle, perhaps as the result of upwelling resulting from the large-scale return circulation from the trenches. Depending on the depth of compensation, the observed asymmetry could result from a lateral temperature gradient of 0.05–0.10°C km−1 and a total lateral temperature variation of under 100°C.

Lithology and subsidence in the North Sea

The North Sea sedimentary basin has developed on the northwestern margin of the European tectonic plate and contains an almost continuous record of epeirogenic marine and deltaic sedimentation from Carboniferous to Recent times. The subsidence required to accommodate the pile of sediment deposited, which in places exceeds 12 km, has been brought about at various times and in various places by differing geodynamical processes. As a result the types of sedimentary rocks deposited vary widely both in time and space, but the nature of the mechanism is reflected in the sedimentary type deposited. The following broad generalizations can be made. The late Carboniferous was a period of deltaic sedimentation during which eustatic changes in sea level or local variations in subsidence rates are reflected in the typical Coal Measures swamp deposits. Late Carboniferous - early Permian times saw the silting up of this basin, and in an arid climate aeolian sands were deposited grading laterally to sabkha shales and evaporites. The Permian culminated in a series of widespread marine incursions during which repetitive evaporites were deposited. Triassic times were marked by a period of major rifting and the deposition of thick sequences of continental elastics in the north, while widespread marine sedimentation persisted in southern areas. Jurassic times saw the re-establishment of marine to deltaic deposition in a series of basins possibly controlled in their distribution by the Triassic fault systems. Late Jurassic deposits were laid down in a sea whose bathymetry reflected the structure of the underlying horsts and grabens inherited from Triassic times, and towards the close of the Jurassic the bottom waters at least of this sea become increasingly stagnant. Sands deposited during the late Jurassic were deposited as near-shore marine bars, beach sands, and proximal and distal submarine fans. Triassic to early Cretaceous deposition was concentrated in the areas now occupied by the main grabens of the North Sea, i.e. the Viking, Central and Moray - Witch Ground grabens. Subsequent deposition in late Cretaceous to Tertiary times took place in a more widely subsiding area, resulting in progressive onlap onto the surrounding basin margins. Deposition within this broadly subsiding and relatively unfaulted basin is characterized by chalky limestones in southern areas, giving way laterally to shales and minor sands to the north. During early Tertiary times a large delta was formed in the area beneath the present Moray Firth, and from this delta a supply of sand was fed into submarine fans to the northeast and southeast of the delta front. Late Tertiary deposition is largely represented by a monotonous sequence of marine shales.

A review of Middle Ordovician sedimentation in the St. Lawrence Lowland, eastern Canada

AbstractMiddle Ordovician sediments of the St. Lawrence Lowland, eastern Canada, and its northeastward extension to St‐Siméon, are subdivided into the numerous formations of the Chazy, Black River and Trenton Groups. Details of each formation and interpretation of environments of deposition are presented and a coherent model for the development of the upper Middle Ordovician Trenton Group throughout the region is presented. In the southwest, around Montreal, a complete and continuous Middle Ordovician sequence is present and Trenton Group sediments overlie well‐developed tidal flat and lagoonal (Black River Group) and mixed shallow subtidal (Chazy Group) sediments. This sequence was deposited on a slowly subsiding, essentially flat, broad shelf environment. Northeastward from Montreal, toward the Montmorency Promontory of the Quebec City area, basal Middle Ordovician sediments become younger and the extent of the shelf area narrowed significantly. The latter resulted in skeletal shoal sediments (lower Trenton Group) developing closer to shore and concomitant less well‐developed clastic‐rich lagoonal sediments (Black River Group and basal Trenton Group). At Montmorency Promontory the shoal sediments (basal Trenton Group) accumulated along an irregular and rugged coastline. Northeast of the Promontory a steep onshore to offshore profile and rapidly deposited basal inshore clastics (Black River Group) precluded the deposition of skeletal shoals and rapid submergence promoted the early development of deeper shelf (middle and upper Trenton Group) and slope and basin (top Trenton Group, Saint Irénée Formation) sediments. In contrast, corresponding offshore sediments (middle and upper Trenton Group) in the southwest reflect a lower depositional gradient and more gradual subsidence. These patterns of deposition were determined by the interaction of the changing nature of the Ordovician coastline southwest, at, and northeast of the Montmorency Promontory and the variable subsidence rates influenced by the eastward evolving Taconic Orogen.