Nigel P. Mountney

Internal stratigraphic relationships in the Etendeka group in the Huab Basin, NW Namibia: understanding the onset of flood volcanism

Abstract The Etendeka Igneous Province in NW Namibia forms the eastern most extent of the Parana–Etendeka Flood Basalt Province and, despite only covering about 5% of the Parana–Etendeka, has been the focus of much interest, due to its extremely well exposed nature. The Huab Basin in NW Namibia forms the focus of this study, and formed a connected basin with the Parana throughout Karoo times (late Palaeozoic) into the Lower Cretaceous. It contains a condensed section of the Karoo deposits, which indicate early periods of extension, and Lower Cretaceous aeolian and volcanic Etendeka deposits, which have their correlatives in the Parana. In the Huab Basin, the volcanic rocks of the Etendeka Group consists of the Awahab and Tafelberg Formations, which are separated by a disconformity. Detailed examination of the Awahab Formation reveals an additional disconformity, which separates olivine-phyric basalts (Tafelkop-type) from basalt/basaltic andesites (Tafelberg-type) marking out a shield volcanic feature which is concentrated in an area to the SE of the Huab River near to the Doros igneous centre. Early volcanism consisted of pahoehoe style flows of limited lateral extent, which spilled out onto aeolian sands of an active aeolian sand sea 133 million years ago. This sand sea is equivalent to the sands making up the Botucatu Formation in the Parana basin. The early expression of flood volcanism was that of laterally discontinuous, limited volume, pahoehoe flows of Tafelkop-type geochemistry, which interleaved with the aeolian sands forming the Tafelkop–Interdune Member basalts. These basalts are on-lapped by more voluminous, laterally extensive, basalt/basaltic andesite flows indicating a step-up in the volume and rate of flood volcanism, leading to the preservation of the shield volcanic feature. These geochemically distinct basalts/basaltic andesites form the Tsuhasis Member, which are interbeded with the Goboboseb and Sprinkbok quartz latite flows higher in the section. The Tsuhasis Member basalts, which form the upper parts of the Awahab Formation, are of Tafelberg-type geochemistry, but are stratigraphically distinct from the Tafelberg lavas, which are found in the Tafelberg Formation above. Thus, the internal stratigraphy of the flood basalt province contains palaeo-volcanic features, such as shield volcanoes, and other disconformities and is not that of a simple layer-cake model. This complex internal architecture indicates that flood volcanism started sporadically, with low volume pahoehoe flows of limited lateral extent, before establishing the more common large volume flows typical of the main lava pile.

Death of a sand sea: an active aeolian erg systematically buried by the Etendeka flood basalts of NW Namibia

Here we report on a ‘fossilized’ sand sea that was progressively engulfed by the basal Etendeka flood basalts in NW Namibia. Preserved relict aeolian landforms include transverse barchanoid dunes and isolated barchan dunes. Present‐day preferential erosion of the lava flows exhumes relict aeolian bedforms preserved in the position in which they were migrating at the time of burial (c. 133 Ma). A passive eruption style of inflated pahoehoe flows has preserved the bedforms without significant deformation. The sediment interlayers record a decrease in sand supply and a change in palaeowind direction, which may have been driven by the ongoing break‐up of west Gondwana, or may be a direct result of the widespread volcanism itself.

Relating eolian bounding-surface geometries to the bed forms that generated them: Etjo Formation, Cretaceous, Namibia

The Cretaceous Etjo Formation is a 200-m-thick eolian and fluvial sandstone succession exposed in the Huab Basin of northwestern Namibia. Eolian sedimentation was terminated abruptly by the emplacement of flood basalts of the Etendeka igneous province across much of the basin at 132 ± 1 Ma. The lavas “drowned” the dunes and, in doing so, preserved bed forms with heights and wavelengths of up to 100 m and 1.3 km, respectively. Subsequent erosion has resulted in the exposure of these bed forms and provides a rare opportunity to accurately reconstruct the three-dimensional geometry of an ancient eolian system and to relate bed-form morphology to bounding-surface geometries. Some bed forms lacked a single active slipface but instead were characterized by a dome form over which smaller transverse dunes migrated. Other bed forms were characterized by an active slipface and a stoss slope over which smaller, superimposed bed forms migrated. One bed form records a transitional evolution between these two end-member types. Preserved bed sets exhibit a positive (but stoss-erosional) angle of downwind bed-form climb of 1°. Bed forms of the Etjo Formation provide an opportunity to directly relate bed-set thickness to original bed-form height via the angle of climb.

AEOLIAN AND ALLUVIAL DEPOSITION WITHIN THE MESOZOIC ETJO SANDSTONE FORMATION, NORTHWEST NAMIBIA

The Cretaceous Etjo Sandstone Formation is a mixed aeolian and fluvial unit exposed over 5000 km2 in the Huab Basin of Damaraland, northwest Namibia. The distribution of fluvial and aeolian facies reveals complex spatial and temporal variations in depositional style that may be related to four distinct phases of sedimentary evolution. The Krone Member forms the base of the formation and comprises coarse clastic material deposited in an and wadi system by flash floods. This is conformably overlain by a mixed aeolian-fluvial unit that is interpreted to represent a transition from ephemeral fluvial to aeolian sandsheet deposition, with the development of small barchan dunes as aeolian sediment supply increased. Repetitive cycles of alluvial plain to dune sedimentation are common and may indicate climate or subsidence controlled variations in the level of the water table relative to the level of the depositional surface. During deposition of the Main Aeolian Unit, sediment supply increased and large transverse draa bedforms migrated across the basin, rapidly filling available accommodation space. Preservation of isolated barchan dunes in the Upper Aeolian Unit indicates a rapid decrease in sediment supply and a return to restricted aeolian activity, following the emplacement across much of the region of Etendeka flood basalts related to continental break-up of West Gondwana at around 132 Ma. Synsedimentary faulting appears to have been a major control on the geometry and distribution of the Etjo Formation. Extrabasinal controls on depositional style include an initially increasing and subsequently decreasing external supply of aeolian sand to the basin, and a gradual transition from semi-arid to hyperacid climatic conditions.

Climatic cyclicity and accommodation space in arid to semi-arid depositional systems: an example from the Rotliegend Group of the UK southern North Sea

Abstract The arid to semi-arid, continental deposits of the Permian Rotliegend Group of the UK southern North Sea (SNS) provide an ideal opportunity to test and further the concepts of sequence stratigraphy in fully non-marine depositional systems. Although these depositional systems are detached from the effects of sea-level change, cyclic fluctuations in the climatic regime result in variations in the vertical facies successions laid down. The effects of increasing or decreasing aridity upon depositional processes are considered. Climatic cycles, bounded at the point of lowest aridity are defined. These climatic cycles have been correlated between different, co-existent depositional environments. This enables depositional systems, such as those present in the Rotliegend, to be considered chronostratigraphically. The application of these concepts to an extensive data set illustrates that, whilst the internal architectures of sedimentary cycles are controlled by the depositional processes and their positions within the basin, the thickness of the cycles directly reflects the rate of accommodation creation.

Climate, sediment supply and tectonics as controls on the deposition and preservation of the aeolian-fluvial Etjo Sandstone Formation, Namibia

Deposition and subsequent preservation of the Jurassic-Cretaceous Etjo Sandstone Formation of Namibia represents a complex interplay between climatic and tectonic factors and related variations in extrabasinal sediment supply. The aeolian and fluvial deposits indicate semi-arid to arid climatic conditions throughout the deposition of four distinct sedimentary units. The succession records either an upward increase in aridity or an upward increase in aeolian sediment supply, represented by a transition from a fluvially dominated basal unit, through a marginal fluvial-aeolian unit to an exclusively aeolian unit. A combination of inherited palaeotopography and syndepositional extensional faulting provided the space necessary for the accumulation of much of the succession. A basinwide unconformity (super surface) divides the succession. This hiatus resulted partly from a lack of available preservation space and partly from a shutdown in aeolian activity related to a regional climatic reorganization. A subsequent shift in the palaeowind direction from northwesterly to southwesterly exploited sand reserves in the Paraná Basin of South America and led to the resumption of aeolian sedimentation across the region. Variations in preserved bedform thickness were directly controlled by differential amounts of tectonic subsidence across the basin. A second major super surface towards the top of the succession resulted from the regional shutdown of large tracts of the aeolian system following the eruption of Etendeka flood basalts across the region.

A database approach for constraining stochastic simulations of the sedimentary heterogeneity of fluvial reservoirs

Quantitative databases storing analog data describing the geometry of sedimentologic features are commonly used to derive input for geostatistical simulations of reservoir sedimentary architecture; however, geometrical information alone is inadequate for the detailed characterization of sedimentary heterogeneity. A relational database storing fluvial architecture data has been developed and populated with literature- and field-derived data from modern rivers and ancient successions. The database scheme characterizes fluvial architecture at three different scales of observation—recording style of internal organization, geometries, and spatial relationships of genetic units—classifying data sets according to controlling factors (e.g., climate type) and context-descriptive characteristics (e.g., river pattern). The database can therefore be filtered on both architectural features and boundary conditions to yield outputs tailored on the system being modeled to generate input to object- and pixel-based stochastic simulations of reservoir architecture. When modeling heterogeneity with stochastic simulations, the choice of input parameters quantifying spatial variation is problematic because of the paucity of primary data and the partial characterization of supposed analogs. This database-driven approach permits the definition of various constraints referring to either genetic units (e.g., architectural elements) or material units (i.e., contiguous volumes of sediment characterized by the same value of a given categorical or discretized variable; e.g., same lithofacies type, clay and silt content, and others), which permit the realistic description of fluvial architecture heterogeneity. Applications of this database approach include the computation of relative dimensional parameters and the generation of auto- and cross-variograms and transition-probability matrices, which are necessary to effectively model spatial complexity.

A relational database for the digitization of fluvial architecture concepts and example applications

Depositional (facies) models of fluvial architecture permit straightforward categorization of deposits, but are necessarily simplistic. Here we describe a complementary database methodology which is designed to encapsulate the inherent complexity of fluvial systems and their preserved deposits. The database is implemented as a series of tables (characterizing qualitative and quantitative architectural and geomorphological properties and system attributes) populated with data derived from peer-reviewed studies of both modern rivers and ancient fluvial successions, and from other reliable sources. Architectural properties (geometries, internal organization, spatial distribution and reciprocal relationships of lithosomes) are assigned to three different orders of genetic bodies organized in a hierarchical framework, ultimately belonging to stratigraphic volumes that are homogeneous in terms of their controlling factors and internal parameters. Interrogation of the database generates a varied suite of quantitative information, whose principal applications include: (i) the quantitative comparison of fluvial architecture to evaluate the relative importance of intrinsic and extrinsic controls; (ii) development of quantitatively justified fluvial depositional models through the integration of data from multiple sources; (iii) development of better constraints on the workflows used to infer borehole correlations and to condition stochastic models of subsurface architecture; (iv) identification of appropriate modern and ancient analogues for hydrocarbon reservoirs.

Neogene–Quaternary postrift tectonic reactivation of the Bohai Bay Basin, eastern China

Bohai Bay Basin, located in eastern China, is considered a Cenozoic rifted basin. The basin is atypical in terms of its Neogene–Quaternary postrift subsidence history in that it experienced intensive tectonic reactivation, rather than the relative tectonic quiescence experienced during this stage by most rift basins. This Neogene–Quaternary tectonic reactivation arose principally in response to two tectonic events: (1) activity on a dense array of shallow faults and (2) accelerated tectonic subsidence that occurred during the postrift stage. These two events were neither strictly temporally nor spatially equivalent. The dense array of shallow faults form a northwest–southeast-trending belt in the central part of the basin, with displacement induced by the reactivation of older northeast- and northwest-trending basement faults and an associated substantial component of strike-slip displacement occurring after 5.3 Ma. The intensive reactivation of these faults contributed to the atypically accelerated rate of postrift tectonic subsidence of the basin that commenced ca. . However, this was not the sole cause of this accelerated tectonic subsidence: A combination of geological activity deep within the crust led to the buildup of intraplate stresses, and this, combined with ongoing thermal subsidence, acted as an additional contributory factor that drove unusually high rates of subsidence for this basin. This episode of accelerated postrift tectonic reactivation resulted in conditions favorable for hydrocarbon accumulation.

A meta-study of relationships between fluvial channel-body stacking pattern and aggradation rate: Implications for sequence stratigraphy

A quantitative comparison of 20 literature case studies of fluvial sedimentary successions tests common assumptions made in published models of alluvial architecture concerning (1) inverse proportionality between channel-deposit density and floodplain aggradation rates, and (2) resulting characteristics of channel-body geometries and connectedness. Our results do not support the relationships predicted by established stratigraphy models: the data suggest that channel-body density, geometry, and stacking pattern are not reliable diagnostic indicators of rates of accommodation creation. Hence, these architectural characteristics alone do not permit the definition of accommodation-based “systems tracts” and “settings”, and this calls into question current sequence stratigraphic practice in application to fluvial successions.