Michael S. Steckler

Controls on facies distribution and stratigraphic preservation in the Ganges–Brahmaputra delta sequence

Abundant sediment supply and accommodation space in the Bengal Basin have led to the development of a major Late Quaternary delta sequence. This sequence has formed in a tectonically active setting and represents an important example of a high-energy (marine and fluvial), high-yield continental margin deposit. Recent studies have detailed the delta’s stratigraphy and development, noting that tectonics and sediment supply control the Ganges–Brahmaputra more significantly than in many other delta systems. These ideas are developed here through a discussion of the effects that spatial and temporal variations in tectonics and sediment-supply have had on deltaic processes and sequence character. Unique and differing stratigraphies are found within thedeltasystem,suchthatfine-grainedsedimentpreservationisfavoredinareasofactivetectonicprocessessuchasfolding,block faulting, and subsidence. Coarse-grained deposits dominate the stratigraphy under the control of high-energy fluvial processes, and mixed fine–coarse stratigraphies are found in areas dominantly influenced by eustatic sea-level change. Overlaid upon these spatially varying stratigraphic patterns are temporal patterns related to episodic events (e.g., earthquakes and rivers avulsions) and long-termchangesinclimate andsediment supply.Modelingisalsousedtoinvestigatethe influenceofavariable sedimentsupply on sequence character. Results show that the timing and magnitude of sediment input, relative to sea-level rise, is a significant control on the subaerial extent of the delta and the relative dominance of alluvial and marine facies within the sequence. D 2002 Elsevier Science B.V. All rights reserved.

Reconstruction of Tertiary Progradation and Clinoform Development on the New Jersey Passive Margin by 2-D Backstripping

We have reconstructed the Oligocene to Middle Miocene paleobathymetry and stratigraphy of the New Jersey margin using a modified backstripping technique. By analyzing the geometry of the margin through time, we investigate its response to fluctuating sea level, changing climate, and variable sediment supply during the Tertiary. The reconstructions reveal a change in the margin morphology from a more steeply dipping (1 : 300 to 1 : 500) carbonate ramp in the Eocene to a flatter shelf with a sharp shelf edge at present. This was accomplished by an increase in the terrigenous sediment supply that filled available accommodation and caused progradation across the margin. We link the increase in sediment flux with climatic cooling rather than tectonic processes. The progradation is evidenced by a series of clinoforms whose formation was modulated by sea level and which extend over 100 km across the shelf. The height and dip of the clinoforms increased as they extended onto the deeper parts of the earlier ramp. The Miocene clinoform rollovers at the New Jersey margin had water depths of60‐130 m and are interpreted as the edge of a new continental shelf built over the older ramp. Sea-level fall was probably insufficient to drive the Miocene shorelines past the shelf breaks. Thus, measurements of sea-level amplitude based upon ‘coastal’ onlap over the clinoforms are not reliable.

Clinoform development by advection-diffusion of suspended sediment: Modeling and comparison to natural systems

Clinoforms are the building blocks of prograding stratigraphic sequences. These sig- moid-shaped surfaces can be found forming today on modem deltas. Sedimentation rate profiles over the clinoform surface of these deltas show low rates of sediment accumulation on both topset and bottomset regions, with a maximum accumulation rate on the upper foreset region. We pres- ent a model for the formation of clinoforms that relies on the interpretation of modem clinoform sedimentation as a result of the distribution of shear stresses at the mouth of a river. Model clino- form surfaces are generated using an equation for the conservation of suspended sediment concentration, together with a conservation of fluid equation for simple time-averaged flow velocity fields. In the model, suspended sediment is advected horizontally into a basin, and gravitational settling of sediment particles is counteracted by vertical turbulent diffusion. In shallow water, shear stresses are too large to allow deposition, and sediment bypasses the topset region. With increasing water depth, near-bed shear stresses decrease, and sediment is allowed to deposit at the foreset region, with gradually decreasing rates toward deeper water. This sedimentation pattern leads to progradation of the clinoform surfaces through time. The clinoform surfaces produced by the model capture the fundamental morphological characteristics of natural clinoforms. These include the gradual slope rollover at the topset and bottomset, steeper foreset slopes with increased grain size, and an increase in foreset slope through time as clinoforms prograde into deeper water. Because the parameters controlling the model clinoforms have a direct relation to physical quantities that can be measured in natural systems, the model is an important step toward unraveling the physical processes associated with these deposits.

Fission Track Evidence on the Initial Rifting of the Red Sea: Two Pulses, No Propagation

Fission track analyses indicate that the Red Sea initially opened simultaneously along its entire length. Two distinct pulses of uplift and erosion characterized the early stages of rifting in the Red Sea throughout Egypt and in southwestern Saudi Arabia. The first pulse began at ∼34 million years ago (Ma). The second pulse began in the early Miocene (21 to 25 Ma) and marked the start of the main phase of extension. These data support a rigid plate model for continental extension. These results also indicate that the initiation of rift flank uplift, and therefore rifting, and volcanism occurred nearly simultaneously. This conflicts with classical models of active and passive extension that predict sequential development of these features.

Climatic and tectonic control on the Cenozoic evolution of the West African margin

We reconstruct the stratigraphy and paleowater depths along two seismic profiles across the Congo and Angolan margins from the Eocene to the present. The results indicate that in the Eocene, both margins had deep-water carbonate shelves with shelf breaks lying at about 500 m water depth. At the Eocene‐Oligocene transition, submarine erosion removed as much as 1000 m of sediments from the upper slope and shelf break of the margin. In the Miocene, the rate of terrigenous sediment supply increased dramatically and progradation of clinoform-shaped deposits resulted in the shallowing to the water depth of modern margin. We also found that there was a tectonic reactivation of the West African margin in the Miocene. This caused the uplift of the margin by at least 500 m and consequent erosion of the shelf. The evolution of the West African margin postrift stratigraphy and paleowater depth do not result only from the interaction between thermal subsidence, absolute sea-level change and sediment supply. They are also controlled by global oceanographic and climatic change related to the Tertiary greenhouse to icehouse transition and to the Miocene epierogenic uplift of Africa. Global cooling at the Eocene‐Oligocene transition affected the sedimentation of the margin by replacing the carbonate sedimentation of the Eocene by terrigenous siliciclastic sedimentation in the Oligocene and Miocene. The related onset of intermediate depth Antarctic oceanic currents may have triggered the submarine erosion and mass wasting at the Eocene‐ Oligocene transition. The increase in sedimentation off Angola seems to be dominated by the effect of global cooling in the Oligocene and in the Miocene. A major pulse of sedimentation on the Angolan margin correlates with the middle Miocene d 18 O shift marking the beginning of permanent glaciation of Antarctica. However, the sedimentation in Congo contains no such pulse. Rather, it correlates with the gradual Miocene tectonic uplift of Africa. Miocene sedimentation on the Angolan profile may be more sensitive to global climatic change since it receives the discharge of the large Congo River drainage system. Sedimentation in the Congo profile is less affected by the Congo River. The sedimentation here may reflect the supply from the regional rivers that erode the edge of the uplifted margin and, therefore, the local erosion caused by the Miocene uplift of the margin. However, we note that large-scale shifts in the depocenter of the Congo River may confound this interpretation. q 2001 Elsevier Science B.V. All rights reserved.

The role of the sediment load in sequence stratigraphy: The influence of flexural isostasy and compaction

Simplified, physically based models are utilized in order to examine the effects of sea level, sedimentation, tectonic subsidence, isostatic compensation, and compaction on development of unconformities. Unconformities divide volumes of relatively conformable sediments defining the time and spatial extent of sedimentary sequences. The model parameters are varied both individually and in concert in order to isolate their contributions to sequence architecture. These models reveal the importance of sedimentation and subsidence rates, as well as sea level amplitude and rate, in determining the type and extent of sequence boundary formed. Sedimentation and subsidence rate can vary significantly within and between margins, producing different sequence boundaries given an identical eustatic sea level fall. It is shown that sea level amplitude plays an important role in determining which systems tracts are present and sequence boundary timing for type 1 sequences and that sea level rate controls the same attributes in type 2 sequences. Isostatic compensation and compaction, which have been described as secondary effects, are shown to have considerable influence on sequence architecture. These two processes, due to isostatic response to sediment loading and sediment self-loading, initiate feedback by enhancing and partitioning accommodation space. These additional effects invalidate application of the equilibrium point as a guide to where and how all accommodation space is created. The manner in which flexure distributes accommodation space is a function of lithospheric rigidities: higher rigidities partition space laterally, producing wide shelves which favor type 1 sequence boundaries, whereas lower rigidities partition space vertically forming narrow shelves which favor type 2 sequence boundaries. Compaction generates a net landward shift of the shelf edge and favors type 2 sequence development Both isostatic compensation and compaction introduce time delays in sequence development. One consequence of compaction is to rotate sedimentary packages, generating growth and leakage of anticlinal structures as fluids are driven from the underlying section. This process has important implications for hydrocarbon migration in compaction-dominated systems, such as the Gulf of Mexico margin.

Fission-track analysis of basement apatites at the western margin of the Gulf of Suez rift, Egypt: evidence for synchroneity of uplift and subsidence

Fifty-six apatite fission-track ages and 52 horizontal confined track-length measurements are reported from Precambrian crystalline rocks along the western margin of the Gulf of Suez, Egypt. Ages fall in the range of ca. 11–385 m.y. and older ages often occur within very close geographic proximity to younger ones, indicating non-uniform uplift. The wide range in ages is accompanied by a systematic variation in the distribution of horizontal confined fission track lengths. On the basis of apatite fission track ages and their length distributions, data fall into three distinct groups. Group I: ages ranging from 43 to 385 m.y. Length distributions are all positively skewed and with decreasing age become progressively broader with shorter mean track length. Group II: ages ranging from 23 to 31 m.y. Length distributions are negatively skewed with either a distinct tail or a small peak of short tracks. Group III: ages ranging from 11 to 20.5 m.y. Length distributions are al unimodal, narrow, negatively skewed and have the longest mean lengths among samples studied. Apatite ages from groups I and II are interpreted as “mixed ages” as a result of cooling during uplift from different levels within the apatite partial track annealing zone. Ages from Group III are interpreted as “cooling ages” due to uplift from the apatite total track annealing zone with minor partial annealing. Correcting the ages of the two oldest samples in this group for track-length reduction yields ages of21 ± 2.2and23 ± 1.5m.y. It is proposed that the onset of rift-flank uplift in the Gulf of Suez—northern Red Sea area occurred between 21 and 23 m.y. ago. Fission-track analysis in combination with subsidence data from the Gulf of Suez basin, indicate that commencement of basement uplift postdate the start of rifting and is interpreted as evidence for passive rifting at the Gulf of Suez. Furthermore, this uplift is contemporaneous with, and is directly related to, the process of extension and subsidence at the Gulf of Suez.

Contributions of floodplain stratigraphy and evolution to the spatial patterns of groundwater arsenic in Araihazar, Bangladesh

Extreme spatial heterogeneity has emerged as a salient characteristic of groundwater arsenic in many complex fluviodeltaic environments. Here we examine patterns of arsenic heterogeneity in the shallow ( 1 −10 3 m in the 25 km 2 area indicate that the concentration of arsenic in shallow groundwater largely varies with the grain size, thickness, and distribution of fine-grained ( < 63 µm) sediments that overlie buried aquifer sands. The overall pattern shows that lower arsenic concentrations are typically found where aquifer sands outcrop at or near the surface, whereas higher arsenic levels typically underlie, or are adjacent to thicker, fine-grained deposits. Furthermore, chronostratigraphic reconstructions of aquifer sediments indicate that sediment distribution, and consequently the patterning of dissolved arsenic, is readily explained in the context of local river history and flood-plain development within the past 1000 yr. An important implication is that complex patterns of groundwater arsenic in afflicted fluviodeltaic settings can be better understood through reconstructions of local aquifer history. This finding is especially relevant because the village and tube-well locations are closely linked with surface landforms such as former levees and bars. An additional and worrisome finding is that the artificial filling of villages to protect from flooding can mimic the natural fine-grained stratigraphy commonly associated with high concentrations of arsenic.

The effect of sedimentary cover on the flexural strength of continental lithosphere

The factors that control the flexural rigidity — or effective elastic thickness (EET) — of continental lithosphere have been extensively studied over the past two decades. Using EET estimates derived from the analysis of topography, basin structures and gravity anomalies, several authors have shown that crustal thickness, geothermal gradient, strain rate, rheology and plate curvature all affect the flexural strength of continents. Recognition that certain combinations of these parameters result in a significant reduction of flexural strength caused by decoupling of the crust and the upper mantle, has been a critical step in understanding why many continental areas have estimated EETs that are thin compared with the total mechanical thickness of the continental lithosphere. Here we develop a semi-analytical model of the EET through a parametrization of the yield stress envelope, that includes the effects of crust–mantle decoupling. We perform a detailed comparison of EET estimates at foreland basins and mountain belts to values predicted by our model and find that, to predict the EET estimates successfully, we need to take into account the effect of the sediment cover and to use a strong plagioclase-controlled rheology. The effect of sediment cover is to weaken the lithosphere because of the lower density of sediments relative to crystalline crust,, and by thermally insulating the lower crust,,.

Offshore sedimentary effects of the 12 January 2010 Haiti earthquake

Although the 12 January 2010 Haiti earthquake was one of the deadliest earthquakes in history, it left no clear geological evidence of rupture on land. As a tectonic event, the earthquake was complex; even the faults involved remain unclear. Using geophysical and coring data, we document direct evidence of the sedimentation generated by the catastrophic 12 January 2010 earthquake offshore. These studies document submarine paleoseismology methods that can be used for assessing seismic risk in this and other tectonic settings such as the California San Andreas fault, where deeper buried blind thrusts may exist. Shaking by the 12 January main shock triggered sediment failures and turbidity currents from coastal sources to deep-water sinks. An ~0.05 km 3 turbidite was deposited in the Canal du Sud basin (1750 m water depth) over 50 km 2 . Almost 2 months after the main shock, a 600-m-thick sediment plume was still present in the lowermost water column at this location. The turbidite was time correlated to the 12 January earthquake by the excess 234 Th in the sediments. With a half-life of 24 days, its presence documents an infl ux of terrigenous sediment mixing with marine sources derived from the basin slopes. This turbidite, and older ones observed beneath it, displays complex cross-bedded and fi ning-upward stratigraphy indicative of long waves and seiche oscillations that are consistent with locally reported tsunamis. This 12 January sedimentary record highlights the potential for submarine paleoseismology to unravel the seismic history of continental transform boundaries such as the Enriquillo‐Plantain Garden fault in the Dominican Republic, Haiti, and Jamaica, as well as other tectonic settings where no clear land-based evidence for a rupture exists.