Arkell.1956.Jurassic..

560 MEXICO AND THE GULF REGION
way contemporary with the Agassiz orogeny of Canada and the formation
of the Chisik conglomerate of Alaska; but both those events can be tied
down to a much narrower time-interval on the Callovian-Oxfordian
boundary.
The Nevadan orogeny is not discernible as such in Mexico but may
be represented by non-sequence between Kimeridgian and Tithonian.
As will be seen from the summary that follows, however, more stratigraphical
collecting is required before the existence and precise extent
of such a gap can be satisfactorily established.
FIG. 89.—Jurassic outcrops in Mexico. The broken line indicates approximately the
northern boundary of buried Jurassic rocks under the Gulf Coastal Plain of the
southern United States.
Some further volcanic activity at the end of the Jurassic is indicated by
tuffs and bentonitic shales in the Upper Tithonian in southern Mexico.
A complete picture of the ammonite faunas of the Middle and Upper
Jurassic of Mexico is presented in four sumptuously-illustrated monographs
by one of the most able and distinguished students of the Jurassic,
the Swiss geologist Carl Burckhardt, who died in 1935. The monographs
describe the Upper Jurassic faunas of Mazapil (1906), San Pedro del
Gallo (1912) and Symon (1919-21), and the Middle Jurassic and Callovian
faunas of the states of Oaxaca and Guerrero (1927), and they were followed
by a brilliant synthesis and discussion of the whole Mexican Mesozoic
(1930). Further figures and new information have since been published
in important papers by Imlay (1939, 1939a, 1943, 1943a; and Imlay &
others, 1948) and Mulleried (1942; and Burckhardt (posthumous) &
Mulleried, 1936). The Jurassic floras were monographed by Wieland
(1914). Comprehensive general accounts of Mexico and the adjacent
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