George Postma

A 300-kyr record of aridity and wind strength in southwestern Africa: inferences from grain-size distributions of sediments on Walvis Ridge, SE Atlantic

Abstract The terrigenous fraction of sediments recovered from Walvis Ridge, SE Atlantic Ocean, reveals a history of southwestern African climate of the last 300 kyr. End-member modelling of a data set of grain-size distributions (n=428) results in three end members. The two coarsest end members are interpreted as eolian dust, the third end member as hemipelagic mud. The ratio of the two eolian end members reflects the eolian grain size and is attributed to the intensity of the SE trade winds. Trade winds were intensified during glacials compared to interglacials. Changes in the ratio of the two eolian end members over the hemipelagic one are interpreted as variations in southwestern African aridity. Late Quaternary southwestern African climate was relatively arid during the interglacial stages and relatively humid during the glacial stages, owing to meridional shifts in the atmospheric circulation system. During glacials the polar front shifted equatorward, resulting in a northward displacement of the zone of westerlies, causing increased rainfall in southwestern Africa. The equatorward shift of the polar front is coupled with an increase of the meridional pressure gradient, leading to enhanced atmospheric circulation and increased trade-wind intensity.

Controls on terrigenous sediment supply to the Arabian Sea during the late Quaternary: the Makran continental slope

The input of terrigenous sediment along the tectonically active Makran continental margin off south-western Pakistan (Gulf of Oman, northern Arabian Sea) is studied on the basis of sediment cores distributed along a transect from the upper slope to the abyssal plain. Spatial and temporal variations in sediment composition, sedimentation rate and turbidite frequency in late Pleistocene–Holocene time (last ∼20 14C ka) will be discussed and related to changes in sea level and climate, and tectonic activity. End-member modelling of the grain-size distributions of the hemipelagic and turbiditic sediments indicate that the sediments are adequately described as mixtures of three end-members, which represent turbidite sand, turbidite silt or eolian dust, and fluvial mud. The geochemical and mineralogical compositions of the hemipelagic sediments indicate that the eolian dust was dominantly supplied from the northern Arabian Peninsula and the Persian Gulf region, and that the fluvial input is from the Makran margin. The ratio of contributions of eolian and fluvial sediment in the hemipelagic intervals is used as an indicator of continental aridity, i.e. summer monsoon intensity. Highest Holocene turbidite frequencies and sedimentation rates are recorded at the deepest coring sites. They are related to the proximity of the deformation front of the accretionary prism. Turbidite sedimentation on the upper continental slope was most frequent during the last glacial period of sea-level lowstand, and continued during the entire deglaciation and sea-level rise. Infrequent turbidite sedimentation occurred during the Holocene highstand of sea level. Turbidite sedimentation during the period of late sea-level rise and the Holocene sea-level highstand is inferred to be due to the strong impact of episodic events (i.e. flash floods and earthquakes) and because of the narrow shelf of the active Makran continental margin.

Controls on terrigenous sediment supply to the Arabian Sea during the late Quaternary: the Indus Fan.

Abstract A previous study on the basis of long-range side-scan sonar data (Kenyon et al., 1995. Geometry of the younger sediment bodies of the Indus Fan. In: Pickering, K.T., Hiscott, R.N., Kenyon, N.H., Ricci Lucchi, F., Smith, R.D.A. (Eds.), Atlas of deep water environments: architectural style in turbidite systems. Chapman and Hall, London, pp. 89–93.) revealed a distributary complex of large channel–levee systems radiating from the mouth of the Indus Canyon and lower-order distributary complexes, each consisting of several smaller channel–levee systems, on the middle Indus Fan. Sediment cores from the Indus Canyon and the middle Indus Fan are analysed in this study in order to reconstruct the timing of turbidite sedimentation on the fan. Sediment cores from the middle fan show that turbidite sedimentation of the last but one switched to the last and youngest channel–levee system at the transition from oxygen-isotope stage 3 to 2 (∼24.8 14 C ka BP) and ceased during the last deglaciation (∼11.5 14 C ka BP). The Indus Fan is subsequently draped by a calcareous ooze of approximately Holocene age. Turbidite sedimentation continued up to (sub)recent times within the main feeder channel and Indus Canyon. The geochemical, mineralogical and grain-size analyses suggest that the sediments deposited on the middle fan during the last glacial period were mainly supplied by the Indus River (fluvial sediments) while during the Holocene they were derived predominantly from the Arabian Peninsula (eolian dust). Although major erosional and depositional cycles on the Indus Fan are strongly controlled by changes in sea level, climate-induced differences in sediment supply and autocyclic mechanisms also influenced fan sedimentation.

Sandy-gravelly mass-flow deposits in an ice-marginal lake (Saalian, Leuvenumsche Beek Valley, Veluwe, the Netherlands), with emphasis on plug-flow deposits☆

Abstract In three pits in the Leuvenumsche Beek Valley (near Ermelo and Elspeet), massive and diffusely banded clean gravelly sands, found in association with sandy sediments showing some similarities to the Bouma sequence, were interpreted as originating from sediment gravity flows. The flows, which came to rest on a ca. 6° paleoslope, probably originated from slumping of parts of the ice-pushed ridges at either side of an ice-marginal lake. Major characteristic features in the three sand pits studied are U-shaped channel forms with a maximum depth and width of 10 × 25 rmm. The fills consist of sand and gravels, locally containing giant sand-clasts. The sediments just below the base of these channel-forms are commonly distorted and folded. Especially the massive more sandy fills are surrounded by a finer-grained diffusely bedded and inversely graded zone. These sedimentary features suggest that “rigid” sediment plugs sheared downslope, generating a finer-grained shear zone around the plug scouring a slide scar till its present semi-cylindrical form and preserving the steep sides (exceeding the angle of repose) of the channel-form by “freezing”. The U-shaped channel forms (plugged troughs) are locally overlain by shallower gently sided trough structures, with mainly diffusely banded infillings. Very shallow and often smaller troughs are found again on top of these massive and diffusely banded infillings. The sediments in these troughs consist of repetitions of two types of microsequences: (1) thickly bedded sequences of normally graded massive sands, near-horizontal stratifications, plane beds and cross-bedded sets; and (2) thinly bedded sequences of relatively thick units TA and thin units TC and TD of the Bouma sequence. The repetitions of the microsequences are interpreted as flow pulsations. Upwards thinning of the sequence, together with a finer grain size may point to waning flow conditions. Their association with plugged troughs is suggestive of retrogressive flow-slides.

Effects of climate, sea level, and tectonics unraveled for last deglaciation turbidite records of the Arabian Sea

Turbidite sedimentation in a slope basin of the active margin of southwestern Pakistan (Makran) and on the abyssal plain of the Arabian Sea (Indus Fan) off the passive margin of southeastern Pakistan (Sindh) was most frequent during the last sea-level lowstand. In both regions turbidite sedimentation rates decreased during early sea-level rise. It stopped on the midfan of the Indus system after 11500 14C yr B.P., while turbidite sedimentation in the slope basin of the Makran continued, although at slower rates. After ca. 7000 14C yr B.P. sedimentation rates in the Makran decreased further as a consequence of increasing continental aridity, but turbidite sedimentation did not stop. The climate signal is identified in the hemipelagic intervals by conspicuous trends in grain size and chemical composition. The continued sedimentation on the Makran can be attributed to the narrow shelf width, which is a characteristic of active plate margins.

Martian stepped-delta formation by rapid water release

Deltas and alluvial fans preserved on the surface of Mars provide an important record of surface water flow. Understanding how surface water flow could have produced the observed morphology is fundamental to understanding the history of water on Mars. To date, morphological studies have provided only minimum time estimates for the longevity of martian hydrologic events, which range from decades to millions of years. Here we use sand flume studies to show that the distinct morphology of martian stepped (terraced) deltas could only have originated from a single basin-filling event on a timescale of tens of years. Stepped deltas therefore provide a minimum and maximum constraint on the duration and magnitude of some surface flows on Mars. We estimate that the amount of water required to fill the basin and deposit the delta is comparable to the amount of water discharged by large terrestrial rivers, such as the Mississippi. The massive discharge, short timescale, and the associated short canyon lengths favour the hypothesis that stepped fans are terraced delta deposits draped over an alluvial fan and formed by water released suddenly from subsurface storage.

Sea-level-related architectural trends in coarse-grained delta complexes

Distinct stratigraphical styles of delta development can be related to basically different patterns of relative sea-level (base-level) change: (1) an overall rise with periods of standstill and without significant fall (Rs); (2) an overall rise with superimposed falls (Rf); (3) an overall fall with superimposed rises (Fr); and (4) an overall fall with periods of standstill and no significant rises (Fs). The related stratigraphical models contain either preserved elements of prototype deltas or show characteristic trends in development from one prototype delta into another. Each stratigraphical model is characterized by lateral and vertical lithofacies features, which can be reasonably well established from outcrop. In coarse-clastic basin margins, where age control is poor, sedimentological and stratigraphical delta models as presented here may help refining reconstructions of basin fill patterns and basin subsidence kinematics.

Basin-Floor Fans in the North Sea: Sequence Stratigraphic Models vs. Sedimentary Facies: Discussion

Original article : Shanmugan, G., R. B. Bloch, S. M. Mitchell, G. W. J. Beamish, R. J. Hodgkinson, J. E. Damuth, T. Straume, S. E. Syvertsen, and K. E. Shields, 1995, AAPG Bulletin, v. 79, p. 477-512.

Supercritical and subcritical turbidity currents and their deposits - a synthesis

Common facies models of turbidite deposits are based on idealized sequences of turbidite units, which are assumed to reflect the depositional processes of a decelerating turbidity current. We show how suites of turbidite units, i.e., distinct turbidite facies associations that are easily described from core and outcrop, may characterize the entire range of large-scale dynamics of turbidity currents, enabling estimates of their densimetric Froude number (Fr; subcritical versus supercritical) and suspension fall-out rate (stratified versus nonstratified flows). The linking of facies associations with large-scale flow dynamics resolves process-facies links that were hitherto unresolved by the common turbidite facies models.

ASTRONOMICALLY AND TECTONICALLY LINKED VARIATIONS IN GAMMA-RAY INTENSITY IN LATE MIOCENE HEMIPELAGIC SUCCESSIONS OF THE EASTERN MEDITERRANEAN BASIN

Abstract Amplitude variations of a 3 My gamma-ray record of Late Miocene hemipelagic successions on Gavdos (Crete, Greece) show remarkable correlation with the amplitude variations of the theoretical insolation curve of Laskar et al. (1993) [Astron. Astrophys. 270, 522–533], with peaks for precession, 100 ka and 400 ka eccentricity cycles. Overall increases in gamma-ray intensities around 8.3 Ma, 7.9 and from 7.16 Ma till the end of our record coincide with tectonic events that can be related to the convergence of the African and the Eurasian plates. These events may have caused important changes in water circulation related to the progressive closure of the seaways between the Mediterranean Basin and the Atlantic Ocean.