Arkell.1956.Jurassic..

57°
MEXICO AND THE GULF REGION
orogeny, and as a thrust fault of Cretaceous date; the formations below
the plane of discontinuity have been described by many observers as a
metamorphic basement complex, consisting of crumpled schists, phyllites
and marbles, whereas the latest writer asserts that they are not metamorphosed
and are normal Upper Cretaceous sediments overridden by a
thrust sheet.
The stratigrapher unable to visit Cuba and see for himself can hold no
opinions, but after reading the accounts by Brown & O'Connell (1922),
Lewis (1932), Schuchert (1935), Dickerson & Butt (1935) and Palmer
(1945), he turns thankfully to the publications of the palaeontologists,
where he is on firmer ground.
From the palaeontological studies it is certain that Cuba has so far
produced two ammonite assemblages, Upper Oxfordian and Middle
Tithonian (the latter equivalent to the Durangites zone of Mexico).
Dickerson & Butt (1935) believed that these two faunas were separated
by a major unconformity and asked us to believe that the concretions
that have yielded hundreds of well-preserved, undeformed, Upper
Oxfordian ammonites came from the metamorphic Cayetano formation of
schists and phyllites beneath the unconformity. Unfortunately Schuchert
(1935, p. 495, and underline of fig. 81) swallowed this. Palmer (1945),
for whom the unconformity is a low-angle thrust plane, derives the
Oxfordian ammonites more happily from above it. He states that the
concretions with ammonites come from 120 m. of 'very thinly bedded
shaly limestone of almost schistose structure', which he names the Jagua
formation, and that this is the lowest formation of the thrust sheet; and
that the Cayetano formation underneath (up to 10,200 m. thick), though
very sparsely fossiliferous, is probably Upper Cretaceous.
Above the Oxfordian Jagua formation, according to Palmer, follow
conformably the beds yielding the second ammonite fauna, which Imlay
(1942) in a detailed monograph proved to be of the age of the Durangites
zone, namely Middle Tithonian (= Upper Portlandian of Imlay). These
beds have always been called Vinales Limestone, but Palmer (1945, p. 7)
seeks to restrict this term to the upper part, which yields mainly aptychi,
and asserts that most of the ammonites come from the lower part, which he
calls the Quemado formation (thickness 1320 m.). The aptychus beds
account for at least another 390 m. Both historically and palaeontologically
it seems preferable, with Imlay (1952, p. 969) to regard the aptychus
beds and Quemado as subdivisions of the Vinales Limestone (total thickness
about 1700 m.). The outcrop cannot be shown on fig. 91 because
all available geological maps include the Vinales Limestone with the
Cretaceous.
The crucial point remains the nature of the junction between the
Oxfordian Jagua and the Middle Tithonian Vinales (Quemado), and on
this Palmer is reticent, though he does not remark on any physical break,
and shows them as in normal, conformable contact in the structural
diagram here reproduced (fig. 92). From the absence of the whole
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