Arkell.1956.Jurassic..

MEXICO 565
Burckhardt (pi. xii, figs. 1-4) which is a Normannites and can only be
Bajocian. Some doubtful forms on pi. xi (figs. 5, n, 12) could also be
Bajocian (Leptosphinctinae ?). A Garantiana ? fragment found at another
locality (pi. xvi, figs. 10, n, 16) could be Upper Bajocian or Bathonian.
It appears therefore that further search of the lower layers of nodules
in these beds may reveal a larger Bajocian fauna. Further researches would
be of great interest, for the Bajocian should be less meagrely represented
in Mexico, and it is important to establish the age of the beds with nodules
which overstep on to ancient mica-schists in some places in Oaxaca
(Burckhardt, 1930, p. 25).
TOARCIAN has not been proved in Mexico but may be represented in some
parts by continental deposits with plant remains.
PLIENSBACHIAN
The Upper Pliensbachian is believed to be represented by plant-beds.
A single Lower Pliensbachian marine fauna of the Jamesoni Zone has
been found in shaly clays with marcasite concretions, which have yielded
species of Uptonia and Polymorphites. These beds are grouped with the
Huayacocotla formation (below). Arieticeras occurs (Erben, 1954).
SINEMURIAN
The Huayacocotla formation (300-390 m. or more) in northern Veracruz,
northern Puebla and eastern Hidalgo, consisting of shales with subordinate
sandstone and conglomerate and some lenses of limestone, has yielded
a succession of 9 ammonite zones spanning the whole Sinemurian
(Burckhardt, 1930, pp. 9-23; Imlay & others, 1948). The ammonites
have not been figured and until this is done comment must be reserved.
The succession recorded by Burckhardt is:—
Beds with Microderoceras cf. bispinatum and Eoderoceras cf. armatum
Beds with Echioceras raricostatum, etc.
Beds with Arnioceras cf. james-danae Barcena
Beds with Arietites cf. deciduus Hyatt
Beds with Vermiceras cf. bavaricum
Beds with Oxynoticeras aff. oxynotum and aff. guibali
Beds with Euagassiceras cf. sauzeanum d'Orb. [= resupinatum Simpson],
etc.
Beds with Arnioceras cf. geometricum, etc.
Beds with Arietites aff. bisulcatus, etc.
The base of the formation has not been observed. It is presumed to rest
on metamorphic rocks, pebbles of which occur in its conglomerates.
CENTRAL AMERICA
In Chiapas, Guatemala and Honduras there is a widespread continental
formation, the Todos Santos Beds, consisting of yellow, red and brown
http://jurassic.ru/