Arkell.1956.Jurassic..

334 EAST AFRICA
Janensch, 1914; Parkinson, 1930a.) Some, including Brachiosaurus,
occur also in the Morrison formation of Colorado, Wyoming and Utah.
The geological age of these beds has given rise to much discussion and
many conflicting opinions, unfortunately not untouched by nationalistic
feeling. It is now, however, settled beyond reasonable doubt on the basis of
ammonites by the work of Dietrich (1927, 1933, 1933a) and Spath (1933).
The chief cause of difficulty is the fact that Tendaguru lies near the
Mesozoic shoreline, with the result that the 73 5 m. of marine Upper Jurassic
formations just described (unbottomed) in the Kiswere hinterland have
partly thinned and partly wedged out until at Tendaguru they are represented
by only 120 m. of beds, resting directly on gneiss of the basement
complex. In addition, a large part of the strata have become fluvio-marine
or 'estuarine', with disappearance of the marine fossils.
Above the Jurassic dinosaur beds there is an inferred (but seldom
perceptible) disconformity, on which follow Lower Cretaceous Trigonia
schwarzi beds (Hauterivian-Barremian), which pass up into Aptian
(Urgonian).
The Jurassic beds, to which the name Tendagura Beds is best restricted,
form a mainly sandy series of alternating or cyclic coarse to pebbly sandstones,
fine sandstones, silts, and sandy clays, often well laminated and
occasionally greyish-green or reddish in colour. There is much variation
also along the strike. The bedding-planes change direction frequently,
as if material had been deposited from a slowly-moving quicksand,
and there are clay pellets of 'desiccation breccia' type. Elsewhere the
silts are well laminated, as if deposited in still water. The Tendaguru
Beds, in fact, are products of a large river subject to tropical floods and
desiccation. The sea was never far off, perhaps kept out by a lagoon bar;
occasionally it broke through and drifted in ammonites and other marine
shells, which thus alternate in succession with products of the river and
land, including the dinosaur bones (Parkinson, 1930).
The German expeditions established the following detailed sequence,
which, owing to facies changes and local variations, is admittedly schematic
(Dietrich, 1933, 1933a):—
[Gap above: then Trigonia schwartzi Sandstone]
Upper Saurian bed
Smeei bed: sandstone packed with Trigonia smeei
Littoral bed with Cyrena and Mytilus
Middle Saurian bed
Littoral bed with Cyrena and Mytilus
Nerinella bed: sandstone with Trigonia dietrichi
Lower Saurian bed
[Gneiss below]
There is no significant difference between the vertebrate faunas of
the three saurian beds: which suggests that the whole series did not take
a geologically long time to form.
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