Middle Jurassic – earliest Late Cretaceous palynofloras, coastal Tanzania - Part Two
Samwel Msaky, Emma
published: Nov 11, 2011
This Part Two of the current monograph (Msaky 2011) completes the systematic palaeontology and discusses the biostratigraphic and palaeogeographic significance of the palynofloras together with the results of palynofacies analyses. In the systematic section, two new species are established: Pseudoceratium redactum and Sentusidinium tanzaniensis. Six informal, stratigraphically successive, dinoflagellate assemblages are distinguished, as follows: Assemblage I (Bajocian–Bathonian); Assemblage II (Callovian); Assemblage III (Oxfordian); Assemblage IV (Kimmeridgian–Tithonian); Assemblage V (Berriasian–Barremian); and Assemblage VI (Aptian–Cenomanian). Coastal Tanzania appears referable to the Austral dinoflagellate realm due to the presence of, inter alia, Broomea ramosa, Komewuia glabra, Wanaea clathrata, and Wanaea digitata. Some minor biostratigraphic anomalies are noted, including the introduction of Glossodinium dimorphum; this is datable as early Oxfordian in Tanzania whereas in Australasia the species appears earlier (in the Callovian). A few noteworthy quantitative phenomena are evident; for instance, the abundance of Lithodinia jurassica in the vicinity of the Callovian–Oxfordian boundary and the marked increase of Wanaea spp. in the Oxfordian, specifically in the Ruvu Basin of the northern coastal belt. These events are of chronostratigraphic significance and could be applicable to regional correlation. Middle–Late Jurassic assemblages show a conspicuous degree of cosmopolitanism, and include some species previously considered to be restricted to certain regions in the low and high latitudes. The Early Cretaceous (Berriasian–Barremian) assemblages show close affinity to the Tethyan Realm. The latest Early Cretaceous (Albian) to earliest Late Cretaceous (Cenomanian) strata are allied to the Albian–Cenomanian Elaterates province. The presence of elater-bearing pollen in offshore and onshore sections of Tanzania indicates that the Elaterates had a wider geographic distribution than previously reported: i. e., extending beyond the northern part of the African and South American continents. The lower part of the Kipatimu Formation, which contains Assemblage IV, yielded Komewuia glabra, Broomea ramosa, and Rigaudella aemula and their presence precludes the Early Cretaceous age previously ascribed to this part of the formation. Likewise the Makonde and Mikindani Beds have yielded typically Early Cretaceous pollen grains including Classopollis braziliensis, Retitricolpites vulgaris, and Elaterocolpites castelainii. The Jurassic/Cretaceous boundary is coincident with the upper limit of Assemblage IV, based on the last appearances of, in particular, Komewuia glabra and Rigaudella aemula. Within the Kipatimu Formation (as represented in wells SS-5 and SS-7), the systemic boundary appears conformable. But elsewhere in the coastal belt, the boundary is clearly unconformable. On the basis of palynostratigraphic data, significant biostratigraphic events, such as introductions and exits of certain dinoflagellate species commonly match sequence boundaries. From palynofacies analyses and palynostratigraphic data, the Kipatimu Formation is dominated by phytoclasts indicating high terrestrial inputs during its deposition. Abundance of black wood (of high buoyancy) in the majority of samples implies high energy settings and abundance of brown wood (less buoyant) is an indication of a sudden terrestrial input into the marine system. The organic composition of sedimentary rocks is an important means whereby the depositional history of the enclosing sediments can be understood. Different environmental settings clearly influence the character of palynofacies. For example, abundant chorate cysts (e. g., Systematophora spp.) in samples from the upper sections of SS-5 suggest open marine settings. Therefore, palynofacies analyses can be viewed as providing useful pointers to interpreting the environmental circumstances in which the rocks, in this case the Kipatimu Formation, were deposited.