Arkell.1956.Jurassic..

578
THE ANDES OF SOUTH AMERICA
trough from the open sea, converting it into a chain of evaporating-basins.
In them were deposited gypsum and anhydrite, which reach a maximum
thickness of about 300 m. in the central Andes. In the Caracoles area of
north Chile the gypsum follows upon fossiliferous marine beds with
typical Upper Oxfordian ammonites, and in the Neuquen area of Argentina
it is overlain by and passes laterally into shales with Lower Kimeridgian
ammonites. From this it may be inferred that the gypsum is of uppermost
Oxfordian date (Bimammatum Zone), but it is not necessarily contemporary
in all parts of the Andes.
In parts of west-central Argentina the main gypsum formation is overlain
by up to 1000 m. of lavas, crystalline tuffs and explosion-pyroclastics
(the 'porphyrite formation'). These rocks first attracted the attention
of Darwin on the voyage of the 'Beagle'. They contain peculiar transitions
from lava to breccia and conglomeifete, the last formed, apparently, by
contemporaneous erosion of volcanic rocks at sea-level. Towards the
east they pass laterally into conglomerates, arkoses and other coarse
clastic sediments. The relation of these rocks to the marine Lower
Kimeridgian of Neuquen is unknown, but it seems probable that they are
equivalent to continental sandstones and red clays which overlie the
Lower Kimeridgian and that the volcanic episode they record coincides
with the major stratigraphical break everywhere apparent in South, as in
Central, America below the transgressive Tithonian.
The Tithonian transgression carried the sea farther east and south than
ever before in the Mesozoic: even in northern Peru and far south into
Patagonia. The new marine series begins usually, but not always, with a
basal conglomerate, and consists of an immensely thick shale formation
with subordinate limestones. Over 7 degrees of latitude in Argentina, from
Neuquen to the vicinity of Aconcagua, there are at or near the base a few
metres of limestone crowded with Virgatosphinctes spp. of the Lower
Tithonian. Upon this follow shales and limestones, altogether well over
1000 m. thick, with distinct Middle and Upper Tithonian, Berriasian
and Valanginian ammonite faunas. In Patagonia volcanic activity continued,
for the Tithonian shales are underlain, overlain and interfingered
with porphyritic lavas and contain thick beds of tuff.
In the northern Andes (NE. Peru, Equador, Colombia, Venezuela)
absence of marine Jurassic faunas between Sinemurian and Tithonian
(and usually absence of those also) makes it impossible to decipher the
history so clearly. In this region the Upper Jurassic is believed to be
represented by the La Quinta formation and its equivalents (formerly
classed as 'Giron formation', which included Trias and Permian)
(= Chapiza formation of east Equador). It consists largely or wholly
of red beds: sandstones, dark red shales, with basal polygenetic conglomerate
locally, and often containing extensive pyroclastic rocks, lavas
and intrusives, especially towards the top. These beds overlap the
marine Sinemurian discordantly in east Peru and transgress on to the
Carboniferous there and in eastern Equador, and on to older crystallines in
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