JURASSIC GEOLOGY OF THE WORLD BY W. J. ARKELL, D.Sc., F.R.S. FELLOW OF TRINITY COLLEGE, CAMBRIDGE, HONORARY MEMBER OF THE GEOLOGICAL SOCIETIES OF GERMANY, FRANCE AND EGYPT, THE PALEONTOLOGICAL SOCIETY OF AMERICA AND THE LINNEAN SOCIETY OF NORMANDY OLIVER AND BOYD LTD. EDINBURGH: TWEED DALE COURT LONDON: 39A WELBECK STREET, W.I bttp:/ /jura~~ic.ru/ CHAPTER 24 MEXICO AND THE GULF REGION MEXICO Most of Mexico is a continuation of the Cordilleras of North America. In it are still recognizable representatives of the Pacific geosyncline, the Cordilleran geanticline, and an analogue of the Rocky Mountain trough. The Pacific geosyncline continues southward through the north­ eastern state of Sonora and the long peninsula of Lower California and intervening Gulf of California, which may be a lineal descendant of the ancient geosyncline. In Lower California, however, no fossiliferous rocks older than Cretaceous have been found, and though from the lithology and the intrusive batholiths the peninsula would appear to be a continua­ tion of the Sierra Nevada and Coast Ranges, both the batholiths and the sediments are somewhat later. The only Jurassic known in NW. Mexico is some small outliers of Lias, mainly continental but partly marine, in the state of Sonora. The Cordilleran geanticline is continued in a general way, considerably narrowed, by the main NNW.-SSE. mountain range, the Sierra Madre Occidental, which though thrown up in its present form in the late Tertiary and even later, was already a land barrier in the Jurassic. To the east of the barrier lay a broad basin of Jurassic deposition, analogous with the Rocky Mountain trough and ancestral to the Gulf of Mexico, though differing greatly in outline. Like its northern analogue, this trough was deepest along its wester:n side, which developed as a geosyncline running obliquely SSE.-NNW. through what are now the Sierra Madre Oriental and the high central plateau (Mesa Central). It was in this geosyncline, open only at the south-east to the Gulf of Mexico, bounded on the west by the Cordilleran geanticline and on most of the east by a finger-like promontory from the northern land, that nearly all the Jurassic rocks of Mexico accumulated. The outstanding features of the Jurassic faunas of Mexico are their Mediterranean connexions and their evident severance from those that lived in the tectonically analogous Rocky Mountain trough of the United States and Canada. It is this discontinuity, and also the tectonic dis­ continuity in the southern part of the country, that make it logical to treat Mexico and the Gulf region in a separate chapter. The tectonic discontinuity consists in the sudden change of strike of the mountain ranges and fold axes to E.-.W. in the south of Mexico, in conformity with the general trend of the folding in Central America and the Greater Antilles. Palaeozoic, Cretaceous and Tertiary folds all follow this trend in the Central American-Antillean region. Lithology 557 IJttp :// jura$$ic.ru/ 558 MEXICO AND THE GULF REGION and faunas in southern Mexico indicate that in the Jurassic the same grain was followed by a seaway which at times linked the Gulf of Mexico with the Pacific and afforded a migration route across southern Mexico. It was not always open, and from its postulated closings and openings has been called the Balsas portal (Schuchert, 1935, p. 1 19). The times when it probably opened were the Upper Jurassic and Lower Cretaceous, at least intermittently (Imlay, 1940). A more permanent connexion between the Atlantic and Pacific Oceans lay farther south, through the Caribbean and across Costa Rica and Panama (fig. 88). The possibility of a more northerly connexion between the Mexican and Pacific troughs by way of a Sonoran portal (Imlay, 1940) cannot be ATLANTIC OCEAN Fw. 88.-Palaeogeographic sketch-map of middle America in the Upper Jurassic. Based on Imlay, and corrected to 1954. (From Journ. Paleont., 1941 modified.) excluded. The occurrence of outliers of marine Sinemurian in western and north-western Sonora (Burckhardt, 1930, pp. 23-4, 41-2 ; Imlay, 1952, p. 973) suggests the possibility of a seaway by this portal in early Jurassic times. Arnioceras and other marine fossils are interbedded in some of these outliers with a continental series (Barranca formation) which is in places as much as 750 m. thick. Further discoveries may show whether this fauna has closer affinity with those of southern Mexico or those of western Nevada and Oregon. The remarkable correspondence between the faunas of Mexico and the Mediterranean region, especially the south of France, at numerous succes­ sive levels of the Upper Jurassic, was made clear by Burckhardt as early as 1903 and again fully reviewed in 1930 (pp. 106-III). Burckhardt, however, pointed out a number of differences, which are enough to make detailed correlation difficult. The most striking is the abundance of the peculiar Mexican Oppeliid genus Mazapilites in the critical Upper bttp:/ /jurassic.ru/ MEXICO 559 Kimeridgian (to Lower Portlandian?) part of the succession, above the Beckeri Zone and below the definite Tithonian. This part of the sequence is still badly in need of further investigation. A general difference is the comparative rarity of Phylloceratidae and Lytoceratidae in the Mexican Kimeridgian and Tithonian, whereas in the Mediterranean area they are the dominant ammonites in numbers of individuals, though not in numbers of species. It is noteworthy that these families should be rare in Mexico in beds carrying an otherwise typical Tethyan fauna, whereas they are abundant in the Caucasus in beds carrying an otherwise typical NW. European fauna (seep. 362). Burckhardt (1910, 19II) believed that the Kimeridgian and Tithonian of Mexico contained a number of 'boreal' genera such as Simbirskites, Craspedites, Kachpurites, Virgatites, but this was contested by Uhlig (1911), and although Burckhardt in his last work (1930, p. 110) maintained his views, Uhlig is now generally held to have been right. The question will be dealt with further in connexion with the Andes of Argentin!lj (p. s8o). ' No orogeny involved Mexico during the Jurassic or Lower Cretaceous,: but earth-movements perhaps synchronous with the Dunlap (U pper1 Pliensbachian) and Nevadan (Upper Kimeridgian) orogenies can be: detected by local overlaps and disconformities or by interruptions of the. marine sequence. For instance the Sinemurian and Lower Pliensbachian' are marine, with a full ammonite sequence, although plants are present at several horizons; but above the Jamesoni Zone the marine record is cut' short and a purely continental regime of plant-beds persists until the Middle or Upper Bajocian. A great intercalation of red beds and gypsum (Huizachal formationr in the Upper Jurassic, up to 420 m. thick, is believed to be mainly of Oxfordian age (Imlay & others, 1948). It has a basal conglomerate up to nearly 50 m. thick, and other conglomeratic lenses may occur at any level. In eastern Durango this formation contains andesitic lavas 150 to 300 m. thick, interbedded with red shale and sandstone derived largely from disintegration of the lava. The age of these beds is a difficult problem, in the absence of fossils. They are, however, overlain disconformably by the Upper Oxfordian (La Gloria formation), which in its basal con­ glomerate contains pebbles derived from the red beds. In other places the junction is marked by an angular unconformity. The red beds usually rest on Palaeozoic or oldeJ; basement rocks and in these places may be of any date between Oxfordian and Permian; but in the Huesteca area of Veracruz, Puebla and Hidalgo they overlie marine Lower Jurassic, and they are believed .to correlate with similar but thinner red beds and gypsum in southern Mexico which are mainly of Lower Oxfordian age. (For summaries and discussion of the evidence see Imlay, 1943, pp. 1475-9; 1952, p. 972; Imlay & others, 1948, pp. 1753-61). It is possible that the earth-movements that caused this widespread continental episode in Mexico, with its conglomerates and volcanic activity, were in a general IJttp :// jura$$ic.m/ 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 strati­ graphical collecting is required before the existence and precise extent of such a gap can be satisfactorily established. FIG. 89.-}urassic 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 mono­ graphs 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 Miilleried ( 1942; and Burckhardt (posthumous) & Miilleried, 1936). The Jurassic floras were monographed by Wieland (1914). Comprehensive general accounts of Mexico and the adjacent IJttp :// jura$$ic.m/ MEXICO 56 I areas of Central America, the Gulf region and Antilles, have been published by Schuchert (1935) and Sapper (1937), and a synopsis of the Jurassic of the whole region by Imlay (1943).