Palaeoenvironment studies in the Miocene (Libya, Australia)
The environments of deposition and geological history of lower
and middle Miocene rocks belonging to the Marada group have been considered.
The rocks, cropping out in east-west trending escarpments in
the area of the Zelten oilfield in central Cyrenaica, Libya, are flat
lying and consist of rapid alternations of various lithelogical types.
The basis of the study has been a detailed examination of the
fossils and bioturtation from an ecological point of view. The fossils
have been grouped into biofacies, 20 of which have been recognised,
while the bioturbation has been both described and grouped into 10 ichno -
fades. The environments represented by the rocks have been deduced
from their internal structures and fossil content.
Two phases of deposition have been recognised:
(i) An earlier elastic phase, comprising fluviatile sands and
shales together with intertidal and subtidal oalcareous sands, shales
and interlaminated beds, found in the lower parts of the southern and
western sections. The fluviatile sediments contain a rich biota of
vertebrates and silicified wood indicating a tropical savannah climate,
while the intertidal sediments are characterised by heavy bioturbation
by Ophiamorpha Lundgren and Caegichnus nom.nov. amongst others. This
phase is thought to be late Aquitanian and early Burdigalian.
(ii) A later phase of carbonate deposition, thought to be Burdigalian
and Helvetian characterises the upper parts of the northern an
eastern sections. It comprises richly fossiliferous shallow water lime
stones and estuarine calcareous sandstones and shales. The former were
deposited in water depths of 0 - 50 metres and were nearly always affected
by influxes of brackish water, only rarely being wholly eh-motorised by
fully marine genera such as Echinolampas, Idmonea and Licgta. In most units
large numbers of euryhaline genera like Crassostrea and Placenta are
present, and they also characterise the estuarine calcareous sands and