Christian Betzler

Normal vs. strike-slip faulting during rift development in East Africa: The Malawi rift

Kinematic analysis of Neogene and Quaternary faults demonstrates that the direction of extension in the Malawi rift rotated from east-northeast to southeast. Rift development commenced with the formation of half-grabens bounded by northwest-, north-, and northeast-striking normal faults. Owing to slightly oblique rifting, the northwest-striking faults in the northernmost rift segment show a small dextral oblique-slip component, whereas north- and northeast-oriented faults in the central part of the rift display a sinistral oblique-slip component. This first event resulted in block faulting and basin subsidence, which is largely responsible for the present-day basin morphology of Lake Malawi. A major change in fault kinematics occurred because of rotation of the extension direction and permutation of the principal compressive and intermediate axes. The structural pattern inherited from the first rifting phase was no longer suitably oriented to accommodate extensional deformation, and strike-slip faulting assumed a major role. The strike-slip regime amplified uplift of basement ridges within the rift in regions of local transpression, but it also created alluvial basins because of local transtension. This new kinematic style is compatible with the recent seismicity. Older faults that show mainly the first deformational increment are restricted to the outermost parts of the rift. Toward the center, the faults depict an increase in strike-slip components of movement, suggesting deformation propagation toward the rift center, which results in a narrowing of the active rift environments with time.

Controls on modern carbonate sedimentation on warm-temperate to arctic coasts, shelves and seamounts in the Northern Hemisphere: Implications for fossil counterparts

SummaryIn contrast to the well studied tropical carbonate environments, interest in non-tropical carbonate deposition was rather low until the basic ideas of theForamol-concept were outlined byLees & Buller (1972). In the following two decades studies on non-tropical carbonate settings evolved as a new and exciting branch of carbonate sedimentology (seeNelson 1988). This is archieved in a great number of publications dealing on temperate carbonate deposits from numerous coastal and open shelf settings on both hemispheres. The existence of wide extended carbonate depositional systems and even reefal frameworks in Subarctic and Arctic seas which are in focus by our research group made it possible to study modern non-tropical carbonate settings along a latitudinal transect from the warm-temperate Mediterranean Sea to the cold Nordic Seas. Because of increasing seasonality in environmental conditions towards high latitudes, the major controls in biogenic carbonate production can be more clearly addressed in these areas. After the initiation of the priority program “Global and regional controlling processes of biogenic sedimentation-evolution of reefs” by the German Science Foundation four years ago, a set of modern case studies were comparatively analysed specifically with regard to their principle controlling processes:(1)Modern and Holocene coralline algal reefs and rhodolith pavements formed in wave-protected shallow waters along the coast of the Brittany and northern Norway. Their finetuned interaction with herbivores resulted in the development of widespread but low-diverse, slowly growing coralline algal frameworks with high competitive value against the rapid-growing phaeophytic communities.(2)The MediterraneanCladocora caespitosa-banks provide an instructive example of non-tropical hermatypic coral framework construction out of the subtropical-tropical coral reef belt.(3)The geometry and environmental controls of several kilometer long coral reefs formed by the azooxanthellateLophelia pertusa andMadrepora oculata are studied in more than 250 m water depth in mid and northern Norway.(4)ModernBryomol-sediments are widely distributed on non-tropical deeper shelf settings. The formational processes converting bryozoan-thickets into huge piles of sand and gravel dunes are recently studied on the outer shelves off northern Brittany and off northern Norway.(5)Arctic sponge-bryozoan buildups on the seamount Vesterisbank in the Greenland Sea and(6)balanid-dominated open shelf carbonates on the Spitsbergen Bank form the Arctic endmembers of modernForamol-deposits. Seasonalice-edge phytoplankton blooms and efficient mechanisms of pelagic-benthic food transfer characterize these depositional settings. Fossil counterparts of each of these modern case studies are discussed in context with their paleoceanographic and environmental settings.

Monsoon-induced partial carbonate platform drowning (Maldives, Indian Ocean)

Multibeam maps and high-resolution seismic images from the Maldives reveal that a late Miocene to early Pliocene partial drowning of the platform was linked to strong sea-bottom currents. In the upper Miocene to Holocene, currents shaped the drowned banks, the current moats along the bank edges, and the submarine dune fields. Bottom currents in the Maldives are driven by the monsoon. It is proposed that the onset and the intensification of the monsoon during the Neogene provoked platform drowning through injection of nutrients into surface waters. Since the late Miocene, topographically triggered nutrient upwelling and vigorous currents switched the Maldives atolls into an aggradational to backstepping mode, which is a growth pattern usually attributed to episodes of rising sea level.

Role of climate in partial drowning of the Queensland Plateau carbonate platform (northeastern Australia)

Abstract Ocean Drilling Program Leg 133 core data, samples, and geophysical data (logs and seismics) were analyzed to document late Miocene-early Pliocene partial drowning of the Queensland Plateau carbonate platform off Northeast Australia. The modern plateau consists of a mosaic of pinnacle reefs and larger (10 × 50 km) reefs representing relicts of early to middle Miocene buildups. Late Miocene-early Pliocene floatstones, packstones, and mudstones, rich in the larger benthic foraminifers Lepidocyclina and Cycloclypeus, drilled in a transect of sites across the drowned margin of a middle Miocene buildup show that the late Pliocene partial drowning of the platform was preceded by 4 Myr of neritic carbonate deposition without any reefs. The carbonate factory was unable to aggrade to sea level during this period as indicated by the lack of any shallowing trend in the succession. Monitoring of the Miocene to Recent neritic shedding pattern on the windward and leeward side of a pinnacle reef on the Queensland Plateau supports this interpretation. Shedding on the leeward side of the reef, which records periods of active reef growth, played a minor role during the late Miocene, increased during the Pliocene, and reached very high values during the late Pliocene and Pleistocene. Shedding on the windward side of the reef, which was active during lowstands of relative sea level, occurred during the late middle-early late Miocene and during the Pliocene-Quaternary. The data presented here, when combined with paleoceanographic data points to low surface water temperatures (17°–19°C) as a major factor which suppressed reef growth during the late Miocene-early Pliocene period.

Periplatform drift: The combined result of contour current and off-bank transport along carbonate platforms

Hydroacoustic and sedimentological data from the western leeward flank of the Great Bahama Bank document the interplay of off-bank sediment export, along-slope transport, and erosion, which together shape facies and thickness distribution of slope carbonates. The integrated data set depicts the combined product of these processes and allows formulation of a comprehensive model of a periplatform drift that significantly amends established models of carbonate platform slope facies distribution and geometry. The basinward-thinning wedge of the periplatform drift at the foot of the bank escarpment displays along-slope and downslope variations in sedimentary architecture. Sediments are muddy carbonate sands that coarsen basinward. The drift wedge has a pervasive cover of cyclic steps. In zones of lower contour current speed, depth-related facies belts develop, whereas strike-discontinuous sediment lobes, scarps, and gullies characterize areas with higher current speed. This understanding of the impact of currents on carbonate-slope sedimentation has wider implications for seismic and sequence stratigraphic interpretation of carbonate platforms and for applied aspects such as hydrocarbon exploration.

Models of temperate carbonate deposition in Neogene basins in SE Spain: a synthesis

Abstract Upper Miocene (Tortonian-Messinian) to Lower Pliocene (Zanclean) temperate bioclastic limestones occur in the Betic intermontane basins mixed with diverse proportions of siliciclastics. Components are mostly originally calcitic skeletons of invertebrates (especially bryozoans and bivalves) and coralline algae. Carbonate mud content is usually low and cementation is generally weak. These temperate carbonates formed in ramps. The depositional surface profile and local hydrodynamic conditions in each example controlled the occurrence of diverse facies at similar positions within the ramp. Shallow-water facies are well represented and formed in beaches and backshore lagoons, spits, rocky shores and submarine cliffs. Shoals developed seawards of shore deposits; the relatively quiet environments basinwards of the shaosl were the areas of maximal carbonate production (factory facies). The lack of early lithification favoured mobilization of skeletal particles. Waves and currents during storms transported carbonate grains landwards from the factory areas to shoals, spits and beaches. Skeletal grains were also transported downslope along the ramp. Re-deposited carbonates occur within basinal marls in submarine lobes and channels fed by channels cross-cutting and excavating the platform sediments. The absence of hermatypic corals and calcareous green algae in shallow-water deposits suggests cool surface water temperatures during carbonate formation. Large benthic foraminifers and oxygen stable isotope values indicate winter surface water temperatures of 16–17°C.

Origin of Late Pleistocene Bryozoan Reef Mounds; Great Australian Bight

Bryozoan-rich biogenic mounds grew periodically on the prograding carbonate slope of the central Great Australian Bight throughout Pliocene-Pleistocene time. Cores from three ODP Leg 182 drill sites provide a record of mound growth during the last 300,000 years over a stratigraphic thickness of similar to 150 m. These mounds, the first such structures described from the modern ocean, grew between paleodepths of 100 and 240 m; we infer that the upper limit of growth was established by swell wave base, and the lower boundary was fixed by an oligotrophic water mass. Detailed chronostratigraphy, based on radiometric and U-series dating, benthic foraminifer stable-isotope stratigraphy, and planktonic foraminifer abundance ratios, confirms that buildups flourished during glacial lowstands (even-numbered marine isotope stages) but were largely moribund during interglacial highstands and are not extant today. Mound floatstones are compositionally a mixture of in situ bryozoans comprising 96 genera and characterized by fenestrate, flat robust branching, encrusting, nodular-arborescent, and delicate branching growth forms. The packstone matrix comprises autochthonous and allochthonous sand-size bryozoans, benthic and planktonic foraminifers, serpulids, coralline algae, sponge spicules, peloids, and variable glauconite and quartz grains, together with mud-size ostracods, tunicate spicules, bioeroded sponge chips, and coccoliths. Intermound, allochthonous packstone and local grainstone contain similar particles, but they are conspicuously worn, abraded, blackened, and bioeroded. An integrated model of mound accretion during sea-level lowstands begins with delicate branching bryozoan floatstone that increases in bryozoan abundance and diversity upward over a thickness of 5-10 m, culminating in thin intervals of grainstone characterized by reduced diversity and locally abraded fossils. Mound accumulation was relatively rapid (30-67 cm/ky) and locally punctuated by rudstones and firmgrounds. Intermound highstand deposition was comparatively slow (17-25 cm/ky) and typified by meter-scale, fining-upward packages of packstone and grainstone or burrowed packstone, with local firmgrounds overlain by characteristically abraded particles. Mound growth during glacial periods is interpreted to have resulted from increased nutrient supply and enhanced primary productivity. Such elevated trophic resources were both regional and local, and thought to be focused in this area by cessation of Leeuwin Current flow, together with northward movement of the subtropical convergence and related dynamic mixing.

Synchroneity of major late Neogene sea level fluctuations and paleoceanographically controlled changes as recorded by two carbonate platforms

Shallow-water carbonate systems are reliable recorders of sea level fluctuations and changes in ambient seawater conditions. Drilling results from Ocean Drilling Program (ODP) Legs 133 and 166 indicate that the timing of late Neogene sedimentary breaks triggered by sea level lowerings is synchronous in the sedimentary successions of the Queensland Plateau and the Great Bahama Bank. This synchrony indicates that these sea level changes were eustatic in origin. The carbonate platforms were also affected by contemporary, paleoceanographically controlled fluctuations in carbonate production. Paleoceanographic changes are recorded at 10.7, 3.6, and 1.7–2.0 Ma. At the Queensland Plateau, sea surface temperature shifts are documented by shifts from tropical to temperate carbonates (10.7 Ma) and vice versa (3.6 Ma); the modern tropical platform was established at 2.0–1.8 Ma. At Great Bahama Bank, changes were registered in compositional variations of platform-derived sediment, such as major occurrence of peloids (3.6 Ma) and higher rates of neritic carbonate input (1.7 Ma). The synchroneity of these changes attests to the far-field effects of modifications in the oceanographic circulation on shallow-water, low-latitude carbonate production.

Sub-Milankovitch cycles in periplatform carbonates from the early Pliocene Great Bahama Bank

High-resolution bulk sediment (magnetic susceptibility and aragonite content) and δ18O records from two different planktonic foraminifera species were analyzed in an early Pliocene core interval from the Straits of Florida (Ocean Drilling Program site 1006). The δ18O record of the shallow-dwelling foraminifera G. sacculifer and the aragonite content are dominated by sub-Milankovitch variability. In contrast, magnetic susceptibility and the δ18O record of the deeper-dwelling foraminifera G. menardii show precession cycles. The relationship between the aragonite and the paleoproxy data suggests that the export of sediment from the adjacent Great Bahama Bank was triggered directly by atmospheric processes rather than by sea level change. We propose a climate mechanism that bears similarities with the semiannual cycle component of eastern equatorial Pacific sea surface temperatures under present-day conditions.

Ecological controls on geometries of carbonate platforms: Miocene/Pliocene shallow-water microfaunas and carbonate biofacies from the Queensland Plateau (NE Australia)

SummaryThe Miocene and Pliocene of three ODP Leg 133 sites (812, 813, 814) record the biofacies evolution prior and during the partial drowning of the Queensland Plateau carbonate platform. Four major skeletal assemblages occur in the succession. The first, middle Miocene assemblage consists of a tropical chlorozoan association. The second assemblage, which records warm-temperate depositional conditions, lacks aragonitic skeletal elements. It is dominated by foraminifera and bryozoans. The third skeletal association (uppermost Lower Pliocene) contains green algae, foraminifera, and bryozoans. The last skeletal association is pelagic (ooze) and mainly consists of planktonic foraminifera and calcareous nannoplankton.The middle Miocene depositional geometry in the analysed transect of drill sites is that of a carbonate bank with a well-defined rim and a flank. During the late Miocene and early Pliocene carbonate ramps formed. Upper Miocene and lower Pliocene deposits in the drill holes are rich in large benthic foraminifera. Combination of micropaleontological with seismic data allows the reconstruction of a curve of relative sea level for the Tortonian and Messinian. The long term trend of relative sea level is characterised by a rise punctuated by four short term falls.Lepidocyclina (Nephrolepidina) rutteni is described from the Australian faunal province for the first time.