Intertrappean

The extensive outpouring of the Oligocene Trap basalts over eastern Africa and western Arabia was interrupted by a period of quiescence marked by the deposition of terrestrial sediments. These so-called intertrappean beds are often lignitiferous and yield recurrent floras and faunas, sometimes represented by endemic mammals. We intended to highlight the peculiar features of these sedimentary intercalations using a large-scale approach including eastern Africa and the western Arabian peninsula. Starting from a new mapping in the Eritrean highland, the intertrappean beds resulted a continuous level that was a few tens of meters thick and traceable for some tens of kilometers. They consist of fluvial red, green and gray mudstones and siltstones with subordinate channelized pebbly sandstones, and lignite seams. Two new 40Ar–39Ar datings constraint the age of the intertrappean beds between 29.0 Ma and 23.6 Ma. The outcrops near Mendefera have yielded the remains of two proboscidean families, the Deinotheriidae and the Gomphoteriidae. The morphological grade of the two Mendefera proboscideans would suggest a more derived stage than that of representatives of the same families from other Oligocene African sites (e.g., Chilga, Ethiopia). An Oligocene age could be inferred for them. The occurrence of the genus Prodeinotherium at Mai Gobro possibly represents the first occurrence of this taxon, while the Gomphotheirum sp. might represent the oldest occurrence of this taxon in Africa before its dispersal towards Asia and Europe. Proboscideans have also been found in the lowland intertrappean beds of Dogali near Massawa. These sediments were contiguous with the Eritrean highland intertrappean beds during the Oligocene, but are now tectonically displaced from them by two thousand meters of vertical topographical distance. Dogali is also known for the occurrence of possible Deinotheriidae remains and the primitive elephantoid Eritreum. Entering the Ethiopian highland, an inspection of the Agere Selam (Mekele) intertrappean beds revealed the occurrence of lacustrine limestones and diatomites, which were contrastingly quite subordinate with respect to the fine clastic sediments found in the nearby Amba Alaji area. Further south, the intertrappean section in the Jema valley (100 km north of Addis Ababa and close to the Blue Nile gorge) is 120 m thick with predominant clastic sediments and a few diatomite's at the top. Literature information from 35 additional sites, including northern Kenya, Yemen, Sudan and Saudi Arabia sections, confirms the fluvial and lacustrine depositional environment of the intertrappean beds, underlines the interest in their mammal fauna (Chilga, Losodok), and reports exploitable coal seams for some of them. As for the vegetal landscape in which the intertrappean beds were deposited, pollen and plant analysis results indicative of a tropical wet forest, similar to that of present-day western Africa. Another common feature of the intertrappean beds is their relatively limited thickness, averaging a few tens of meters, but reaching a few hundred meters in graben-related basins, such as Delbi Moye in southern Ethiopia. In most cases only thin, lens-shaped successions were deposited above the hummocky topography of their volcanic substratum, commonly unaffected by significant faulting. An average duration of the intertrappean beds is from one to three million years. This time interval is commonly matched by a few tens (or more rarely, hundreds) of meters of sediments left over after erosive episodes or depositional starvation. As to the lateral continuity of the intertrappean beds, the present-day outcrops show large differences: from some tens of kilometers in the Mendefera area, to a few tens of kilometres in the Jema valley, and to a few hundreds meters in the Agere Selam (Mekele) area. Even if it is difficult to quantify the original size of the sedimentation areas, it nevertheless proves that the intertrappean basins exceed thousands of square kilometers in only a single case (Mendefera), but were quite restricted in most cases. Their most likely endorheic and local character, together with a regional ill-defined fluvial network, was the effect of a water-course rerouting caused by the progressive rising of the eastern African and Arabian plateaux. Chronological constraints for the intertrappean beds can be inferred from the age of the hosting Trap succession and by the stratigraphical position that they occupy. Intervolcanic sedimentary episodes are typically found in the basaltic and subordinately rhyolitic successions that followed the 31–29 Ma old basaltic widespread paroxysm. With due caveats deriving from the discontinuous availability of datings specifically dedicated to this issue, we regard the age of the intertrappean beds as mostly encompassed in the interval from 29 to 27 Ma at the transition between the Early and Late Oligocene in the Ethiopia/Yemen Trap core. In marginal areas, such as SW Arabia, Eritrea and Kenya, the volcanic activity above the intertrappean beds resumed later, and its quiescence allowed a more prolonged period of sedimentation. The intertrappean beds fall in the second cooling event of the Oligocene climatic deterioration. During the contemporaneous apparent drop in the global sea-level and closure of the Tethyan Ocean between Arabia and southwestern Asia, connections were established between the African and the Eurasian continents. At that time, southwestern Asia was experiencing severe aridity with faunal exchanges toward the luxuriously vegetated eastern Africa.

Late Cretaceous Lameta Formation of Nand-Dongargaon basin of Maharashtra occupies an area of about 700 km sq. in Nagpur and Chandrapur districts. The sediments are believed to be deposited in fluvio-lacustrine environment in semiarid-arid climate with strong seasonality (Mohabey and Udhoji, 1996; Mohabey et al., 1993). The study area of Pisdura lies in Chandrapur district of Maharashtra and lithologically comprises of red clays, green clays, marls and sandstones deposited in overbank and palustrine environments. The sediments have yielded fossils of vertebrates (including dinosaurs, turtles, snakes, mammals and fishes), invertebrates (molluscs and ostracods), charophytes, plant megafossils and plant-bearing coprolites. The Polgaon locality lies in Nagpur District and lithologically comprises of red and green clays with thin bands of fossiliferous marls deposited in palustrine environment (Mohabey et al, 1993). The marls and clays contain a rich and diverse assemblage of gastropods, ostracods, plant fossils (angiosperm seeds and fruits, monocot and dicot leaf impressions, Araucarites, Isoetes and charophytes) associated with freshwater fishes and turtles (Mohabey and Udhoji, 1996).Palynological study of the sediments of Pisdura and Polgaon localities and sauropod coprolites from Pisdura yielded very good assemblage of palynomorphs such as Classopollis, Cycadopites, Compositoipollenites, Cretacaeiporites, Graminidites spp., Longapertites, Palmaepollenites, Periporopollenites, Retimonosulcites, and a few tri- and tetracolporate grains. The sediments also show presence of well preserved phytoliths of grasses and dicots, silicified cuticles with stomata and trichomes, fungal spores and Chlamydospores of mycorrhizal fungi Glomus and algal remains. Study indicates that the overall preservation of palynomorphs in the coprolites is better in comparision to that of the host sediments possibly owing to their concentrated mass in the animal gut which prevented their direct aerial exposure and biodegradation. The record of mega and microflora from the sediments indicates that both gymnosperm and angiosperm plants were present during Lameta times and angiosperms plants were quite diverse.

References:

Mohabey, D. M., and Udhoji, S. G., 1996, Fauna and flora from Late Cretaceous (Maastrichtian) non marine Lameta sediments associated with Deccan volcanic episodes, Maharashtra: its relevance to the K/T boundary problem, palaeoenvironment and palaeoclimate: Gondwana Geological Magazine, Special Volume 2, p. 349–364. Mohabey, D. M., Udhoji, S. G., and Verma, K. K., 1993, Palaeontological and sedimentological observations on non-marine Lameta Formation (Upper Cretaceous) of Maharashtra, India: their palaeoecological and palaeoenvironmental significance: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 105, p. 83– 94.