Dion Brandt

Impact Origin of the Chesapeake Bay Structure and the Source of the North American Tektites

Seismic profiles, drill core samples, and gravity data suggest that a complex impact crater ∼35.5 million years old and 90 kilometers in diameter is buried beneath the lower Chesapeake Bay. The breccia that fills the structure contains evidence of shock metamorphism, including impact melt breccias and multiple sets of planar deformation features (shock lamellae) in quartz and feldspar. The age of the crater and the composition of some breccia clasts are consistent with the Chesapeake Bay impact structure being the source of the North American tektites.

Detailed structural analysis of the rim of a large, complex impact crater: Bosumtwi Crater, Ghana

The 1 Ma Bosumtwi Crater in Ghana is an 11-km-diameter, presumably complex, well-preserved impact structure that is associated with the Ivory Coast tektite strewnfield. Detailed structural geologic studies along a complete traverse through the northwestern rim section indicated four zones characterized by distinct deformation styles from just outside of the crater rim to near the crater floor. Zone 1 is dominated by thick deposits of lithic impact breccia, intercalated in places with products of local mass wasting. Zone 2 contains inward-dipping thrust planes, conjugate radial fractures, isoclinal folding, and overturned stratigraphic sequences. Zone 3 represents a megabreccia zone, in which block size decreases upward and outward toward the rim crest. The innermost zone 4 is dominated by intense thrust faulting of multiple orientations, resulting in complex duplex- and lens-shaped bodies. These deformation styles generally correspond to those previously reported from the rims of simple bowl-shaped meteorite-impact craters and appear to be characteristic of impact structures in general.

Late Jurassic age for the Morokweng impact structure, southern Africa

Abstract A roughly 70 km diameter circular feature buried beneath the Kalahari sands in South Africa is revealed on regional aeromagnetic maps. Boreholes drilled into the centre of the structure intercept a ∼ 250 m thick sheet of quartz norite, interpreted as an impact melt, which overlies brecciated and shock metamorphosed basement granite. Zircons recovered from the quartz norite, yield U-Pb ages of 145 ± 0.8 Ma, and biotites provide Ar-Ar ages of 144 ± 4 Ma. These data provide strong evidence for the occurrence of a Late Jurassic impact crater (the Morokweng impact structure) ∼ 100 m beneath the surface.

Morokweng impact structure, Northwest Province, South Africa: geophysical imaging and shock petrographic studies

Abstract The existence of a large, near-circular, structure centred on the area around Morokweng (23°32′E/26°31′S) in the Northwest Province of South Africa, and possibly extending into southern Botswana, is inferred from aeromagnetic and gravity data. Results of surface geological studies support earlier suggestions that this feature could represent an impact structure. Samples from an autochthonous ironstone breccia exposure about 47 km to the west of the centre of this structure, and several samples of allochthonous quartzite from south of the central part of the structure, contain shock metamorphosed quartz with impact diagnostic planar deformation features (PDFs). The geophysical evidence is suggestive of an original size of this impact structure of at least 70 km, but several annular anomalies outside of the study region could be interpreted as indicating a diameter of 300–340 km. The age of this structure is only loosely constrained, but is believed to be younger than the 2.25–2.5 Ga Transvaal Supergroup.

THE GEOMORPHOLOGY OF THE NYL RIVER AND FLOODPLAIN IN THE SEMI-ARID NORTHERN PROVINCE, SOUTH AFRICA

ABSTRACT The Nylsvlei wetland is located on the 240 km2 floodplain of the lower Nyl River in the semi-arid Northern Province. The wetland is supplied seasonally with water and sediment by the Nyl and other small tributaries but has been little studied geomorphologically. The Nyl floodplain has formed within a basinal to synclinal structure on the downthrown side of a fault and the variable stratigraphy influences both ground and surface water flows, and the resultant fluvial landforms and ecology. Owing to downstream decreases in discharge and channel-bed gradient, the Nyl and several other tributaries presently decrease in size downstream and disappear at the margins of the floodplain, such that summer flooding occurs primarily as sheetflow. Thin clay layers deposited by these sheetflows effectively seal the floodplain surface, prolonging inundation and limiting groundwater recharge. Some groundwater recharge, however, occurs by floodwater infiltration through the channel beds of gravelly sand at the floodplain margins. Groundwater supply may contribute to growth of the numerous elevated, circular, floodplain islands which are characterised by woody fringes but sparsely-vegetated interiors. At the downstream end of the floodplain, bedrock outcrop induces convergence of the rare floodwaters and, together with a steepening of gradient, results in a well-defined channel again forming (Mogalakwena River). Correct identification of the factors giving rise to such wetlands, and the hydrogeomorphological processes governing their development, provides an essential context for long-term ecological studies as well as information for the design of effective management guidelines for these fragile habitats.

Bosumtwi Impact Crater, Ghana: A Remote Sensing Investigation

The Bosumtwi crater in Ghana is a ~1.05 million year old, very well-preserved complex meteorite impact structure of approximately 10.5 km rim-to-rim diameter. The interior of the structure is largely filled by Lake Bosumtwi, and the crater rim and the environs of the crater are covered by dense tropical rainforest. Thus, direct geological study of this structure is not easy; but remote sensing and geophysical data can be used to better constrain the surface and subsurface geology. Landsat Thematic Mapper (TM) data reveal detailed structure of the crater rim, including arrays of radial, concentric, and tangential or oblique fractures. In all data sets, the presence of an apparent ring feature at ~10–11 km from the crater center is visible. Digitised topographic information and, to a lesser extent, ERS-1/2 radar data provide an improved picture of the crater morphology. A digital elevation model (DEM) for the wider Bosumtwi crater area also reveals the presence of a subdued elevation feature at about 10–11 km radial distance from the lake center, which, in the western area, coincides with the distribution pattern of vegetation. Perspective views of the DEM and other remote sensing data suggest the existence of a further structural feature at 17–19 km distance from the crater center, and which is mainly visible in the southeast sector. An “outer rim“ topography is also known from other impact structures, for example, the Elgygytgyn crater in Russia; the origin of this feature is still debated, but the existence of two apparent ring features may suggest that they represent ring faults caused by the impact compression.

The late Mesozoic palaeosoils and Cenozoic fluvial deposits at Vaalputs, Namaqualand, South Africa: possible depositional mechanisms and their bearing on the evolution of the continental margin

Late Mesozoic palaeosoils and Cenozoic deposits in the Northern Province, South Africa, centered on Vaalputs, Namaqualand are exposed in radioactive waste disposal trenches and buried beneath the surficial sands that cover much of the Bushmanland region. The sediments are preserved in a depression, in the “Vaalputs Basin”, inland of the Great Escarpment. They overly a kaolinized and silicified basement of Late Mesozoic arenites (Dasdap beds) and by high grade rocks of the Namaqualand Metamorphic Complex. The Vaalputs sediments are interpreted to be fluvial in origin, possibly deposited as unchannelized floodouts from a provenance area in the west. Extensive bioturbation and pedogenesis accompanied deposition. The age of the sediments is poorly constrained but post-dates ~67 Ma volcanic pipes of the region. The Vaalputs sediments are influenced by north-northwesterly trending faults which also displace the underlying kaolinized and silicified basement. To the east of Vaalputs faulting in this orientation has produced a local depression (the Santab-se-Vloer). The distribution and apparent ages of the kaolinized and silicified surface, the Dasdap and Vaalputs sequences, and the position of the Santab-se-Vloer depression suggest to us that the continental margin has been characterized by an easterly migrating arch of uplift since the opening of the Atlantic, accompanied by increasing aridification of the region.

Sequence stratigraphic analysis of the Early Cretaceous Maconde Formation (Rovuma basin), northern Mozambique

Abstract The Cretaceous (Aptian–Lower Albian) Maconde Formation of the Rovuma Basin in northern Mozambique is documented and its sequence development is interpreted. New evidence for the age of the formation is presented, and a basinal marker horizon is identified in the upper part of the formation that may represent tectonic uplift of the underlying highly kaolinised? Jurassic clays. Two episodes of regional transgression are documented, which with further palaeontological refinement may in future be tied to regional and global transgressive episodes.

Mesozoic fluvial deposits on a rifted continental margin near Vaalputs, Namaqualand, South Africa

Scattered outcrops of faulted, silicified and kaolinitised (saprolitised) sediments comprising conglomerate and cross-bedded sandstone, form prominent mesas and inliers near Banke, south of Vaalputs, the national radioactive waste disposal site in Namaqualand. Although their geomorphologic expression is similar, these sediments are lithologically and texturally distinct from the equally silicified soils, diatreme infills and hillside screes of the palaeo-weathering profile type commonly occurring between the farm Banke and the Vaalputs site. Sedimentary and structural features identified in this study show that the Dasdap sediments may represent an alluvial fan sequence with a source area in the west, and deposited on a continental margin marked by relatively steep topographic gradients. The Dasdap beds could not be dated conclusively, but there is evidence their deposition was likely coeval with the emplacement of melilititic and kimberlitic diatremes in the Late Cretaceous. The robust fluvial system inferred by the Dasdap sediments is compatible with the humid climate responsible for the siliceous saprolites widely associated to the erosional African surface across southern Africa. Subsequent to these events, widespread faulting and jointing of the sediments provide striking evidence for tectonic reactivation of the continental margin of southwestern Africa in the Cenozoic. On the basis of these distinguishing features, the Dasdap sediments may deserve the formation status.

The origin and development of the Nyl River floodplain wetland, Limpopo Province, South Africa: trunk-tributary river interactions in a dryland setting

The Nyl River floodplain wetland, one of South Africas largest floodplain wetlands and a Ramsar site of international conservation importance, is located in an area of long-term and still active valley sediment accumulation. Creation of accommodation space for sedimentation has previously been attributed to tectonic controls, but new investigations reveal that a more likely cause is progradation of coarse-grained tributary fans across the narrow river valley downstream of the main area of floodplain wetland. Obstruction of trunk river flow and sediment transfer by these tributary fans has led to backponding and upstream gradient reduction and to accumulation of valley fills up to ∼35 m thick. Chronological control for the timing and rate of sediment accumulation is limited, but we hypothesise that a semi-arid to arid climate, characterised by asynchronous trunk–tributary activity that results in marked discontinuities in downstream water and sediment transfer, is likely to have been a key control. These interpretations are supported by other studies of dryland rivers globally and the findings add to our growing understanding of the controls on the origin and development of southern African wetlands, particularly by demonstrating how the combination of a particular physiography and a dryland climate can impart some distinctive geomorphological characteristics.