Peecook, B. R., Sidor, C. A., Nesbitt, S. J., Smith, R. M. H., Steyer, J. S., and K. D. Angielczyk. 2013. A new silesaurid from the upper Ntawere Formation of Zambia (Middle Triassic) demonstrates the rapid diversification of Silesauridae (Avemetatarsalia, Dinosauriformes) Journal of Vertebrate Paleontology 33:1127-1137 DOI:10.1080/02724634.2013.755991
Abstract - Recent discoveries have shown that non-dinosaurian dinosauromorphs were morphologically diverse, globally distributed, and have a stratigraphic range extending into the Upper Triassic. Silesauridae, the sister group to Dinosauria, contains at least seven species. Here we describe Lutungutali sitwensis, gen. et sp. nov., the first silesaurid from the upper portion of the Ntawere Formation of the Luangwa Basin, Zambia. The upper Ntawere Formation has been correlated with subzone C of the Cynognathus Assemblage Zone of the Karoo Basin in South Africa and the Lifua Member of the Manda beds in the Ruhuhu Basin in Tanzania, both of which are considered Anisian in age and the latter has yielded the silesaurid Asilisaurus kongwe. The results of our phylogenetic analysis, including a new pelvic character, allies Lutungutali with Upper Triassic silesaurids such as Silesaurus, Sacisaurus, and Eucoelophysis rather than with the possibly coeval Asilisaurus. The Zambian silesaurid shares a laterally oriented brevis fossa on the ilium and a transversely thin ischium in cross-section with Upper Triassic forms. Silesaurids were more diverse during their early evolution in the Anisian than previously suspected. Lutungutali and Asilisaurus are the two oldest known members of the bird-line archosaurs represented by body fossils. Together they show that a subclade of bird-line archosaurs was diversifying soon after its origin, building further support for the rapid diversification of Archosauria in the wake of the Permo-Triassic extinction.
Showing posts with label Africa. Show all posts
Showing posts with label Africa. Show all posts
Upper Triassic Sedimentology of Botswana
Bordy, E. M., Segwabe, T., and B. Makuke. 2010. Sedimentology of the Upper Triassic–Lower Jurassic (?) Mosolotsane Formation (Karoo Supergroup), Kalahari Karoo Basin, Botswana. Journal of African Earth Sciences 58:127–140.
Abstract - The Mosolotsane Formation (Lebung Group, Karoo Supergroup) in the Kalahari Karoo Basin of Botswana is a scantly exposed, terrestrial red bed succession which is lithologically correlated with the Late Triassic to Early Jurassic Molteno and Elliot Formations (Karoo Supergroup) in South Africa. New evidence derived from field observations and borehole data via sedimentary facies analysis allowed the assessment of the facies characteristics, distribution and thickness variation as well as palaeo-current directions and sediment composition, and resulted in the palaeo-environmental reconstruction of this poorly known unit. Our results show that the Mosolotsane Formation was deposited in a relatively low-sinuosity meandering river system that drained in a possibly semi-arid environment. Sandstone petrography revealed mainly quartz-rich arenites that were derived from a continental block provenance dominated by metamorphic and/or igneous rocks. Palaeo-flow measurements indicate reasonably strong, unidirectional current patterns with mean flow directions from southeast and east–southeast to northwest and west–northwest. Regional thickness and facies distributions as well as palaeo drainage indicators suggest that the main depocenter of the Mosolotsane Formation was in the central part of the Kalahari Karoo Basin. Separated from this main depocenter by a west northwest – east–southeast trending elevated area, an additional depocenter was situated in the north–northeast part of the basin and probably formed part of the Mid-Zambezi Karoo Basin. In addition, data also suggests that further northeast–southwest trending uplands probably existed in the northwest and east, the latter separating the main Kalahari Karoo depocenter from the Tuli Basin.
Abstract - The Mosolotsane Formation (Lebung Group, Karoo Supergroup) in the Kalahari Karoo Basin of Botswana is a scantly exposed, terrestrial red bed succession which is lithologically correlated with the Late Triassic to Early Jurassic Molteno and Elliot Formations (Karoo Supergroup) in South Africa. New evidence derived from field observations and borehole data via sedimentary facies analysis allowed the assessment of the facies characteristics, distribution and thickness variation as well as palaeo-current directions and sediment composition, and resulted in the palaeo-environmental reconstruction of this poorly known unit. Our results show that the Mosolotsane Formation was deposited in a relatively low-sinuosity meandering river system that drained in a possibly semi-arid environment. Sandstone petrography revealed mainly quartz-rich arenites that were derived from a continental block provenance dominated by metamorphic and/or igneous rocks. Palaeo-flow measurements indicate reasonably strong, unidirectional current patterns with mean flow directions from southeast and east–southeast to northwest and west–northwest. Regional thickness and facies distributions as well as palaeo drainage indicators suggest that the main depocenter of the Mosolotsane Formation was in the central part of the Kalahari Karoo Basin. Separated from this main depocenter by a west northwest – east–southeast trending elevated area, an additional depocenter was situated in the north–northeast part of the basin and probably formed part of the Mid-Zambezi Karoo Basin. In addition, data also suggests that further northeast–southwest trending uplands probably existed in the northwest and east, the latter separating the main Kalahari Karoo depocenter from the Tuli Basin.
Sequence Stratigraphy of the Upper Triassic Succession in Algeria
Bourquin, S., Eschard, R., Hamouche, B. High-resolution sequence stratigraphy of Upper Triassic succession (Carnian - Rhaetian) of the Zarzaitine outcrops (Algeria): a model of fluvio-lacustrine deposits, African Earth Sciences (2010), doi: 10.1016/j.jafrearsci.2010.04.003
Abstract - The detailed facies analysis of the Zarzaitine outcrops (Illizi Basin in the In Amenas area of Algeria) allows the depositional environment of the Upper Triassic succession to be defined: braided rivers within an arid and humid alluvial plain, low sinuosity rivers within a humid alluvial plain, lake deposits and marginal sabkha. The description of the outcrops helps to define three types of genetic units from a proximal to a distal depositional environment: fluvial, fluvial-lacustrine and lacustrine. The spatial and temporal evolutions of the genetic unit were characterised by five specific stages from dry to humid climate conditions, inducing sediment supply and lake-level variations. During the first two stages (stage 1 and 2) under a dry climate, the lake level was low and sediments mostly by-passed and poorly preserved. During stages 3 and 4, an increase in humidity and rainfall induced a rise in the lake-level and the development of vegetation, as well as a decrease in the sediment supply, although the sediment preservation were then at its maximum. The last stage (stage 5) marked the beginning of a decrease in the humidity, the minimum of sediment supply and the maximum of the lake level. Therefore, the recognition and the description of genetic units within this fluviallacustrine environment help to demonstrate the interaction between climate, sediment supply and lake-level variation, at the scale of these units. Six stratigraphic cycles have been recognised in the vertically stacked genetic units and can be grouped in three megacycles (denoted as I, II and III). The first megacycle, attributed to Carnian to early Norian, corresponds to the base of braided river systems with some ephemeral channels developed in an arid environment where some aeolian deposits were preserved. It evolved trough time to humid conditions favouring the development of extensive floodplain, associated with hydromorphic soils, and perennial lake environments. During the Carnian times and Norian, the Zarzaitine area was not connected to the Berkine basin northward, a Hercynian unconformity palaeorelief forming a drainage divide. According to the directions of the palaeocurrents, the sediment provenances were mostly from the southwest and the north. The connection with the Berkine basin only occurred during the upper part of the megacycle II deposition, characterised by fluvio-lacustrine environments. The maximum flooding of the megacycle I, Norian in age, could be correlated with the early Norian maximum flooding observed on the Saharan platform. The late Norian-Rhaetian second megacycle was mostly a lacustrine environment associated with extensive floodplain, with the development of a hydromorphic palaeosol with root imprints attributed to a warm and humid climate. The maximum flooding episode of this second megacycle could be
attributed to the major marine trangression recorded in Algeria, during the Rhaetian, and
could be correlated with a relative sea-level rise. A dolomitic level, attributed to sabkha
environments, marks the beginning of the retrogradational trend of the third cycle,
attributed to the Rhaetian-Liassic. The palaeocurrent of the upper part of megacycle II
and of the megacycle III were always oriented toward the northeast, attesting that the
relief was located to the southwest.
Abstract - The detailed facies analysis of the Zarzaitine outcrops (Illizi Basin in the In Amenas area of Algeria) allows the depositional environment of the Upper Triassic succession to be defined: braided rivers within an arid and humid alluvial plain, low sinuosity rivers within a humid alluvial plain, lake deposits and marginal sabkha. The description of the outcrops helps to define three types of genetic units from a proximal to a distal depositional environment: fluvial, fluvial-lacustrine and lacustrine. The spatial and temporal evolutions of the genetic unit were characterised by five specific stages from dry to humid climate conditions, inducing sediment supply and lake-level variations. During the first two stages (stage 1 and 2) under a dry climate, the lake level was low and sediments mostly by-passed and poorly preserved. During stages 3 and 4, an increase in humidity and rainfall induced a rise in the lake-level and the development of vegetation, as well as a decrease in the sediment supply, although the sediment preservation were then at its maximum. The last stage (stage 5) marked the beginning of a decrease in the humidity, the minimum of sediment supply and the maximum of the lake level. Therefore, the recognition and the description of genetic units within this fluviallacustrine environment help to demonstrate the interaction between climate, sediment supply and lake-level variation, at the scale of these units. Six stratigraphic cycles have been recognised in the vertically stacked genetic units and can be grouped in three megacycles (denoted as I, II and III). The first megacycle, attributed to Carnian to early Norian, corresponds to the base of braided river systems with some ephemeral channels developed in an arid environment where some aeolian deposits were preserved. It evolved trough time to humid conditions favouring the development of extensive floodplain, associated with hydromorphic soils, and perennial lake environments. During the Carnian times and Norian, the Zarzaitine area was not connected to the Berkine basin northward, a Hercynian unconformity palaeorelief forming a drainage divide. According to the directions of the palaeocurrents, the sediment provenances were mostly from the southwest and the north. The connection with the Berkine basin only occurred during the upper part of the megacycle II deposition, characterised by fluvio-lacustrine environments. The maximum flooding of the megacycle I, Norian in age, could be correlated with the early Norian maximum flooding observed on the Saharan platform. The late Norian-Rhaetian second megacycle was mostly a lacustrine environment associated with extensive floodplain, with the development of a hydromorphic palaeosol with root imprints attributed to a warm and humid climate. The maximum flooding episode of this second megacycle could be
attributed to the major marine trangression recorded in Algeria, during the Rhaetian, and
could be correlated with a relative sea-level rise. A dolomitic level, attributed to sabkha
environments, marks the beginning of the retrogradational trend of the third cycle,
attributed to the Rhaetian-Liassic. The palaeocurrent of the upper part of megacycle II
and of the megacycle III were always oriented toward the northeast, attesting that the
relief was located to the southwest.
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