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). In connexion with the upper part of the following summary, use should be made of the correlation table for the Tithonian of South America and Mexico on p. 58r.

UPPER TITHONIAN At Mazapil this consists of ro m. of shaly and marly limestones with black cherts containing Substeueroceras cf. koeneni (Steuer) and Berriasella cf. calisto (d'Orb.). At Symon, Durango, from 21 m. of shaly limestone forming the top of the Jurassic were collected Substeueroceras sp., Paro­ dontoceras spp., Proniceras cf. idoceroides Burckhardt, Berriasella zacatecana Imlay, and Aulacosphinctes? (Imlay, 1939, p. 9). The fauna comprises many species of all these genera, plus Micracanthoceras spp. and Hildo­ glochiceras spp. (For complete list see Imlay, 1939, table 9; many figures in Burckhardt, 1906, 1921, and Imlay, 1939). Proniceras was placed by Burckhardt ( 1930, table 6) at the base of the Tithonian as here understood, below the Durangites and Kossmatia beds, but Imlay (1939, p. 23) states that this was an error, based on relative geographical position of separate outcrops, which might have been faulted. He has proved that it occurs at the top, with Substeueroceras. This fauna correlates with the zone of Substeueroceras koeneni in Argen­ tina, there also recognized by Leanza as the highest zone of the Jurassic (see p. 582). In southern Mexico the Tithonian Pimienta formation consists of tuffs and bituminous and bentonitic shales with lenses of limestone and a few thin bands of chert. From it have been obtained Substeueroceras, Paro­ dontoceras, Himalayites, Corongoceras, Hildoglochiceras, Pseudolissoceras and Durangites (Imlay, 1952, p. 971). In the type area in the southern Sierra Madre Oriental the thickness of the Pimienta is 100-200 m. (Heim, 1926).

MIDDLE ' TITHONIAN To this division belongs the next-lower horizon, characterized by numerous species of Durangites and Kossmatia, together with sub­ ordinate Hildoglochiceras and Micracanthoceras, and Grayiceras? mexicanum (Burckhardt) (which Burckhardt figured, 1912, pl. xxxiv, figs. 18-22, as a Simbirskites, but which Spath in 1923 likened to forms from the Spiti Shales). Many figures in Burckhardt, 1906, 1912, 1921; Imlay, 1939; list in Imlay, 1939, table 8. This horizon correlates with a Tithonian fauna in California (see p. 553).

LOWER TITHONIAN AND UPPER KIMERIDGIAN Between the Middle Tithonian Durangites and Kossmatia beds and the Middle Kimeridgian Hybonoticeras beckeri beds is a series of strata char­ acterized primarily by Perisphinctids of great variety and presenting great 2N

bttp:/ /jura~~ic.ru/ 562 MEXICO AND THE GULF REGION taxonomic difficulty, and (in the lower part at least) by the peculiar Oppeliid genus Mazapilites, not known outside Mexico (see Burckhardt, 1921, pis. i-iii, v-xiv, and list in Imlay, I939· table 7). Imlay (1939, I943) has figured a number of new forms from these beds and made valiant efforts to sort out the systematics of the Perisphinctids and the detailed succession. It is still impossible, however, to place all the forms satis­ factorily in existing genera, or to draw a definite boundary between Tithonian and Kimeridgian. The presence of Lower Tithonian in the upper part of these beds, probably above the range of Mazapilites, is proved by undoubted Virgato­ sphinctes, correlating with the Lower Tithonian of Argentina and farther afield (e.g. V. cf. denseplicatus Waagen sp., V. adkinsi Imlay, V. chihuahua­ ensis Imlay (I943), V. aguilari Burck. sp., 1906 pl. xxvii, 6-9). The predominant Perisphinctids, however, are a different group which seem to fall for the most part into the genus Aulacosphinctoides (Burckhardt, I 92I, pls. v-xiv, sub. Aulacosphinctes), characteristic of the Lower Tithonian and Upper Kimeridgian of Spiti, New Zealand, etc. Many of the species have a notable frequency of simple ribs, which strongly recalls the genus Torquatisphinctes, and if, as Imlay holds, both genera are represented, it is likely that these beds reach down into the Middle Kimeridgian. Another pointer in the same direction is the occurrence of forms of Perisphinctids (P. mexicanus Burckhardt, 1906, pl. xxxi, figs. 6-9) which appear to belong to the genus Virgataxioceras of the Beckeri Zone, and the overlap, according to Burckhardt, of Mazapilites and Hybonoticeras. Other forms occur which Imlay (1943, pp. 532-3, pls. 88, 89, 91) assigns to the late-Middle and early-Upper Kimeridgian genus Subplanites, the genus to which P. burckhardti Blanchet (1923) probably belongs (type Burckhardt, 1921, pl. xiv, figs. 1-3); and 'Perisphinctes nikitini' Burckhardt (1906, pl. xxxi, figs. 1-4) seems to be a Subdichotomoceras. Another fragment figured by Burckhardt (1906, pl. xxxii, fig. 2) may be identical with an early Virgato­ sphinctes of the Neuburg Lithographicum Zone, V. eystettensis Schneid (19I4, pl. iii, fig. 5). With these forms occur a considerable variety of Aspidoceras and Physodoceras, Taramelliceras, Pseudolissoceras, Haploceras, etc. It is difficult to believe that careful stratigraphical collecting will not one day yield a sequence and solve the problems. Some ammonites, mostly indeterminate fragments, that may represent a different Upper Kimeridgian fauna of the Pectinatus Zone have been figured from an isolated outcrop near Las Cuevas in eastern Durango (Imlay, 1939, p. 33-4). They have been assigned to Callovian genera and dated (p. 2I) to the 'Middle Oxfordian', but they bear a suggestive resemblance, in the photographs, to Pectinatites (pl. 7, fig. 7) and Wheat­ leyites (pl. 7, fig. I; pl. 5, fig. 8; pl. 6, fig. 1; pl. 8, figs. I, 2).

MIDDLE AND LOWER KIMERIDGIAN The Middle Kimeridgian is represented most recognizably by beds with Hybonoticeras cf. beckeri (Neum.), H. cf. harpephorum (Neum.), H. cf.

bttp:/ /jura~~ic.ru/ MEXICO knopi (Neum.), H. cf. hybonotum (Oppel) and H. cf. autharis (Oppel), a remarkably south European assemblage of the Beckeri Zone. With them occur the earliest Mazapilites, Physodoceras and a few Perisphinctids. The Lower Kimeridgian is the most richly fossiliferous and characteristic stage in Mexico, with an immense fauna of ldoceras and many other genera. Two zones are recognizable, an upper with ldoceras durangense and Glochiceras fialar, a lower with Idoceras of the group of I. balderum. Both must be mainly Lower Kimeridgian, for the higher zone contains many lnvoluticeras and Streblites, with Ochetoceras and Sutneria, also Pararasenia zacatecana (Burckhardt). Besides the genera mentioned there are numerous Aspidoceras, Nebrodites, Taramelliceras, Haploceras, etc., with subordinate Perisphinctids and some Phylloceras and Lytoceras. (For lists see Imlay, 1939, tables 4 and 5; numerous figures in Burckhardt's Mazapil (1906), San Pedro (1912), and Symon (1919-21) monographs and in Imlay, 1939). The Glochiceras fialar fauna also occurs in the Taman formation (altogether over 1000 m. thick) in the southern Sierra Madre Oriental (Heim, 1926). Many of the later Mexican Idoceras are clearly related closely to Ataxio­ ceras, from which the ventral smooth band alone provides a doubtful distinction (Burckhardt, 1912, pls. xxvi-xxxi). Peculiar local genera are Subneumayria, type S. ordonezi Burckhardt sp. (1906, pls. i, ii), and Epicephalites, type E. epigonus Burckhardt sp. (1906, pl. iii), both probably Rasenids related to Involuticeras.

OXFORDIAN Ochetoceras beds (1oo m.±) at San Pedro del Gallo: shales and sandy marls with nodules and a black limestone band containing 0. canaliculatum (d'Orb.), 0. pedroanum Burck., 0. mexicanum Burck., Taramelliceras neohispanicum Burck., Discosphinctes virgulatus (Quenst.) var. carribbeanum (Jaworski), Euaspidoceras (Burckhardt, 1912, pl. v, 5, 8, 9; pl. vii, 4-22). The date of this assemblage is probably Bimammatum Zone. Perisphinctes beds (I so m.) at San Pedro del Gallo: marls and shales with beds of sandstone and limestone and many pelecypods, including Trigonia hudlestoni Lycett. All the Perisphinctes figured from these beds are typical Dichotomosphinctes, strongly resembling common forms of the Plicatilis Zone, with which the beds certainly correlate (Burckhardt, 1912, pls. ii-vii). There are also Taramelliceras spp. and a Creniceras (pl. vii, 15-17). A characteristic Cuban form occurs: P. plicatiloides O'Connell (Burckhardt, 1912, pl. iii, 3-6). Below these beds are 6oo m. or more of shales and sandstones, with Nerinean and coralline limestone, resting on red shales, marls and sandstones, all without ammonites. In Zacatecas the Nerinean and coral­ line limestones are soo-1ooo m. thick (Burckhardt, 1930, table 6). In eastern Mexico are up to 420 m. of red beds of uncertain date but approxi­ mately Lower Oxfordian, the Huizachal formation (Imlay & others, 1948). This formation unconformably overlaps all previous Jurassic beds on to

bttp :/ / jurassic.ru/ MEXICO AND THE GULF REGION Palaeozoic or older rocks, but is said to be itself unconformably overlain by the Upper Oxfordian. Perhaps the explanation is that the red beds are of different ages in different places, since Burckhardt described them as underlying the Upper Oxfordian without a break in Durango.

CALLOVIAN Upper Callovian (Athleta Zone) is represented at El Consuelo, Oaxaca, and Cualac, Guerrero, by an assemblage of Peltoceratids, with which are associated Reineckeids and Perisphinctids. Characteristic forms are Peltoceras aff. athleta (Phil.) and P. cricotum Burck., Subgrossouvria suborion (Burck.), S. neogaeum (Burck.), etc. (Burckhardt, 1927, pls. xxx-xxxiv.) Ammonites identified with the Callovian genera Pseudopelto­ ceras ?, Subgrossouvria? and Indosphinctes? have also been figured from an isolated outcrop in Durango (Imlay, 1939, pp. 33-4) but, as remarked above (p. 562), from the figures they could be Upper Kimeridgian Pectinatites and Wheatleyites. In the Cualac district, Guerrero, there lies beneath the Athleta Zone a series of Reineckeia-bearing marls and shales, 66o m. thick, which contain Macrocephalitids in the lower part and bridge the Middle and Lower Callovian (Burckhardt & Miilleried, 1936, pp. 310-12; many figures in Burckhardt, 1927, pis. xv-xxix). Erymnoceras mixtecorum (pl. xiii) was found at another locality, El Consuelo, in the Reineckeia beds (see Burckhardt & Miilleried, 1936, p. 312). Among the Macrocephalitids are Eurycephalites boesei and Xenocephalites nikitini; and the spindle­ shaped Tulitid, Kheraiceras v-costatum, occurs (pl. xv).

BATHONIAN Under the Callovian of Cualac, according to Burckhardt & Miilleried, follow 250-300 m. of sandstones with pelecypod shell-beds. In the upper part of these beds are marls with some poorly preserved ammonites, of which the only one determined is Epistrenoceras paracontrarium Burckhardt sp. (1927, pp. 8o, 94-5, pl. xvi, figs. 14, 15). This genus is known nowhere in the world from any stage but the Bathonian. The Lower Bathonian yields a characteristic ammonite fauna of the Zigzag Zone at several localities in Oaxaca (summarized, Burckhardt, 1930, p. 25, as 'Bajocian Moyen'). From nodules embedded in clay­ shales, Burckhardt figured (1927, pl. xii) Zigzagiceras (Procerozigzag) floresi Burckhardt sp., aff. crassizigzag Buckman and allied forms (pis. xii, figs. 10-16, r8-2o, misidentified as Middle Bajocian Stephanoceras). With them are associated small Perisphinctids, some of which appear to be Siemiradzkia (pl. xi, fig. 8) or Planisphinctes (pl. xi, figs. 5-:7), or both, misidentified as Dactylioceras.

BAJOCIAN Although most of the supposed Middle Bajocian Stephanoceratids are Lower Bathonian Zigzagiceras, there remains Stephanoceras undulatum

bttp :// jurassic.ru/ MEXICO

Burckhardt (pl. xii, figs. 1-4) which is a Normannites and can only be Bajocian. Some doubtful forms on pl. xi (figs. 5, 11, 12) could also be Bajocian (Leptosphinctinae ?). A Garantiana ? fragment found at another locality (pl. xvi, figs. 10, 11, 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 Barc;ena Beds with Arietites cf. deciduus Hyatt Beds with V ermiceras 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

bttp:/ /jnra~~ic.ru/ 566 MEXICO AND THE GULF REGION sandstones, marls, shales and conglomerates. The thickness varies, but 250 m. is normal for Chiapas. The beds rest unconformably on folded Palaeozoics and on gneiss. The upper part of the formation is proved by marine intercalations to be Cretaceous (Sapper, 1937, pp. 26-8) but plant-beds in the lower part ('several hundred metres') contain species reported to be identical with some of those in the Lower and Middle Jurassic of Oaxaca (Miilleried, 1942, p. 129). That marine Jurassic exists in some parts of Central America is indicated by reports of Amaltheus in river gravels in Honduras (Sapper, 1937, p. 28) and 'a few ammonites similar to Macrocephalites' at Tegucigalpa, also in Honduras (Haas, in Imlay, 1952, p. 970).

MALONE MouNTAINS, WEST TEXAS The Jurassic sea of the Mexican geosyncline extended all through the northern state of Chihuahua and came to an end just beyond the frontier in New Mexico and the extreme west corner of Texas. In trans-Pecos Texas the plains are broken by a small, isolated mountain group, the Malone Mountains, near Torcer station, in Hudspeth County. These provide the most northerly outcrops belonging to the southern sea. The length of the range is only 6i miles and the width 2f miles, with a relief of about 700 ft. above the surrounding plain. There are also some outlying parallel hills a mile to the north-east. These small arid mountains and hills are built largely of folded Jurassic rocks, the Malone formation, about 300 m. thick, which rests uncon­ formably on fossiliferous Permian and is overlain conformably by more than 900 m. of Lower Cretaceous. The Malone is a near-shore marine formation with a rich pelecypod and somewhat sparse ammonite fauna, which has given rise to prolonged controversy, having been regarded by some geologists as Jurassic and by others (notably Kitchin, 1926) as in part Jurassic (the ammonites) and in part Cretaceous (many of the pelecy­ pods, especially the Trigoniae). The ammonites have been figured by Cragin (1905) and revised by Albritton (1937), who has also revised the stratigraphy and found that the Trigoniae and other pelecypods asserted by Kitchin to be Cretaceous in reality are found in the lower part of the formation, in the same beds as Lower Kimeridgian ammonites, and below beds (Upper Malone) with Tithonian ammonites (Albritton, 1937; 1937a; 1938, with detailed geo­ logical map of the area). The succession is as follows:-

(VALANGINIAN Torcer formation (about 120m.). Black impure limestone, sandstone. sandy shale, and occasional beds of limestone conglomerate. Age given by the genus Neocomites but fauna sparse. At the base a quartzitic sandstone and chert-pebble conglomerate member, with average thickness of 12m.]

bttp :/ / jurassic.ru/ MALONE MOUNTAINS, WEST TEXAS

(BERRIASIAN AND UPPER TITHONIAN missing)

MIDDLE TITHONIAN Upper Malone formation (45-99 m.). Predominantly limestones, but the highest 7 to 8 m. consists of sandy limestone and sandstone and is the only part in which fossils have been found. At the top were collected Kossmatia aguilerai (Cragin) and K. zacatecana Burckhardt. (The lower part could represent the Lower Tithonian, which is not necessarily missing.)

KIMERIDGIAN Lower Malone formation (o-205 m.). Thin-bedded sandstone, sandy shale, impure limestone, and limestone conglomerates. Ammonites are practically confined to the highest 30 m. They include Haploceras cragini Albritton, ldoceras schucherti (Cragin), I. clarki (Cragin), Physodo­ ceras smithi Albr., P. booni Albr., P. bakeri Albr., Aspidoceras laevigatum Burckhardt, Nebrodites nodocostatus Burckhardt, and two Perisphinctids, 'Lithacoceras' (?) malonianum (Cragin) and 'L' (?) shuleri Albr. (The latter bears no resemblance to any Lithacoceras and is not identifiable from the figures, but might be a crushed Katrolt"ceras or Subdichotomoceras; the former is perhaps a Progeronia). Idoceras ranges from the top of the Lower Malone down to within 30 m. of the base, and below it no ammonites are known. At least six­ sevenths of the Lower Malone is therefore Lower Kimeridgian, of the age of the Idoceras beds of Mexico. The basal3o m. is undated. The Malone formation thins rapidly towards the east and north, and since it contains fossil driftwood (one log 15 ft. long and 2ft. thick) and abundant conglomerate, the shore probably lay not far in that direction. Mapping shows that the Malone rests with angular unconformity on the Permian. The basal part contains some gypsum, probably reworked from the Permian below.

BuRIED JuRASSIC OF THE SouTHERN UNITED STATES As remarked above (see fig. 88) the Mexican geosyncline in Jurassic times was partly separated from the Gulf of Mexico by a long, narrow peninsula, possibly tapering to a chain of islands, which projected south across eastern Mexico from the North American continent, somewhat as Florida does to-day. East of this the Gulf of Mexico extended north of its present shores to fill a large bay over eastern Texas and the states of Louisiana, southern Arkansas, Mississippi and western Alabama, embracing much of the Gulf coastal plain and lower Mississippi valley. The Jurassic rocks which formed at the bottom of this bay are buried beneath thick cover of Cretaceous and Tertiary sediments and were discovered only by deep drillings for oil. Examination of the drill cores has yielded much information on the thickness, facies and age of the

lJttp:/ /jura~~ic.rtt / 568 MEXICO AND THE GULF REGION Jurassic formations present and led to many interesting deductions (Imlay, 1941, 1943, 1945; Swain, 1944, 1949). The Jurassic (which is all Upper Jurassic) thins northward, from over 2000 m. in NE. Louisiana to 6oo m. and less under Arkansas. The thinning is accompanied by overlaps within the system and by change of facies from shallow off-shore sediments in the south to shallower near­ shore in the north. Besides many well-preserved ostracods (Swain, 1949), pelecypods, gastropods and ammonites have been figured (Imlay, 1945). The Tithonian and Kimeridgian succession is comparable lithologically and palaeontologically with the La Casita formation in northern Mexico. It has yielded Glochiceras fialar, Metahaploceras and three forms of Idoceras comparable with Mexican species. Beneath these were found Upper Oxfordian Perisphinctids. This is the earliest fossil assemblage proved. The age of the lowest, unfossiliferous formation (Eagle Mills) is problematic. It consists largely of red beds and contains masses of salt. These beds may be equivalent to Lower Oxfordian red beds and salt in Mexico (as believed by Imlay), or to the saline series with intercalations of fossiliferous marine Permian on which the Upper Jurassic is trans­ gressive in the Malone Mountains (see e.g. Eardley, 1951, p. 544). (For the latest summary of the evidence and arguments, see Imlay, 1952, pp. 973-4). The Eagle Mills formation rests with a basal conglomerate on Palaeozoic or presumed-Palaeozoic rocks, including siltstone, phyllite, schist, etc. (Imlay, 1943, p. 1434). The formations may be tabulated as follows:-

Max. thickness

Tithonian? Cotton Valley group : marine in the south, 1200 m. with Idoceras fauna in lower part, and Lower Kimeridgian Exogyra virgula ; passing north into red beds and clastics

unconformity ? 144m. Buckner formation: red shales, anhydrite and some dolomite ; few fossils, none diagnostic

Upper Oxfordian soo m. Smackover formation: oolitic limestones in upper part with Perisphinctes spp.; lower part darker with argillaceous layers

? 530 m . Eagle Mills formation: red beds and salt, with basal conglomerate

The Buckner formation indicates regional uplift in latest Oxfordian or earliest Lower Kimeridgian times, and this was followed by some gentle folding and a transgression, which produced regional unconformity at base of the Lower Kimeridgian Idoceras beds. This corresponds to the beginning of the fossiliferous Jurassic sequence in the Malone Mountains. The Cotton Valley group in places has a basal conglomerate,

bttp :// jurassic.ru/ BURIED JURASSIC OF THE SOUTHERN UNITED STATES 569 and in SW. Alabama (that is, towards the north-east) this rises diachroni­ cally and at the same time becomes thicker and coarser and contains pebbles derived from the Appalachian Mountains, which are consequently inferred to have experienced a revival of uplift at this time. 'It seems logical to correlate the uplift of these mountains with the Palisade dis­ turbance, which formed block mountains from Nova Scotia to the Carolinas sometime after the Upper Triassic and before the Upper Cretaceous' (Imlay, 1943, p. 1474). According to Swain (1949, p. 1250) the movements between Upper Oxfordian and Lower Kimeridgian in the southern States were insignificant

NORTHERN PART Of M!:SOZOIC GULf COAST GEOSYNCLINE

AT CLOSE. OF" LOWER CR!:TACEOUS

..... ,... ~W.t.II. L ' 090 .... ~$M Al. [ ~~~~!AI.T os:::,;( .A .... 'IO.If[ ~~~~~w · ECIJLIIII[$T0Nt

FIG. go.-Generalized horizontal section through the Jurassic and Cretaceous of southem Arkansas and northem Louisiana, after Imlay. (From Journ. Paleont., 1941 .) compared with later ones which occurred at or near the end of the Kimeridgian. In that case 'the most intense interval of uplift of Jurassic time' (Swain) coincided with the Nevadan orogeny.

CUBA Excepting only Kiogar in the Himalayas with its 'erratic blocks of Malla J ohar', no small Jurassic area in the world has proved so baffling to geologists or produced so many conflicting interpretations as the Organ Mountains in Pinar del Rio province in western Cuba. In the geological literature on this region published during the last 35 years it is standard practice for every account to contradict its predecessors on almost all important points. The same formations have been declared to be Palaeozoic, Jurassic, and Upper Cretaceous, the Jurassic has been stated to be 120 m. thick in one account and 10,500 m. thick in another; the junction between the Upper Oxfordian and Tithonian has been interpreted as a major unconformity demonstrably due to an important Jurassic

IJttp :// jura$$ic.ru/ 570 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 meta­ morphosed 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. 8r) 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 ro,2oo m. thick), though very sparsely fossiliferous, is probably Upper Cretaceous. Above the Oxfordian J agua 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 thick­ ness 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

bttp :/ / jura~~ic . ru / CUBA 571 Kimeridgian and Lower Tithonian, however, a large non-sequence must be inferred, and from the fact that the Oxfordian is confined to the west end of the island while the Tithonian occurs over a much larger area almost to the east end, a regional unconformity is probable. Palmer, indeed, seems to suggest this, when he ascribes the restricted distribution of the Oxfordian to overlap by the Tithonian ( 1945, p. 26).

(IU.OC.I'It.., .., ... nUNI

tAITAtiOU• " .. " • TITNIUIIAII MILlS I ,.,,•• 6 FIG. 91.-Sketch-map of western Cuba. After Palmer, 1945.

According to Palmer's interpretation (figs. 91, 92) the Oxfordian has been thrust up southwards from beneath the Gulf of Mexico. Palmer's finding that the aptychus beds are later than most of the Middle Tithonian ammonites confirms a remark by O'Connell in 1921 (quoted by Schuchert, 1935, p. 516), that 'a definite horizon has been traced for several miles, at which nothing but aptychi and an occasional

FIG. 92.-Section through the Organos Mountains, western Cuba. After Palmer, 1945. The 'scar' on the south marks the south edge of the overthrust.

small Haploceras are found'. Imlay (1952, p. 969) states that in several of the collections studied by him 'the ammonites and aptychi are associated and in some cases are even on the same rock slabs. Also, some of the collections consisting mostly of aptychi have Portlandian ammonites such as Pseudolissoceras and Virgatosphinctes'. However, it is possible that not all the mixed collections of ammonites and aptychi came from the main aptychus beds, which O'Connell and Palmer both observed to be separate. In a thickness of 1700 m. it would not be surprising to find that there are

bttp:/ /jura~~ic.ru/ 572 MEXICO AND THE GULF REGION representatives of both Tithonian and Neocomian, even though Upper Tithonian ammonites have not been found. This might account for the inclusion in the collections of so many anomalous uncoiled ammonoids of strongly Cretaceous affinities. The Middle Tithonian having received expert treatment in the readily­ accessible monograph by Imlay (1942) there is no need to list the species. The genera include Phylloceras, Metahaploceras, Pseudolissoceras, Hildoglochiceras, Simoceras, Virgatosimoceras, Aspidoceras, Physodoceras, Virgatosphinctes, Corongoceras, Dickersonia, Micracanthoceras, Durangites, Lytohoplites, Parodontoceras, Berriasella, Spiticeras and various 'hetero­ morphs'. Dickersonia has not been found outside Cuba. Three species of 'Berriasella' are added by Sanchez Roig (1951, pis. 21, 22), but one (pl. 22, figs. 3, 4) seems to be an Oxfordian Dichotomosphinctes (? aff. ouatius Buck.). It can perhaps be assumed that a similar mistake in collecting was respon­ sible for the record by Brown & O'Connell (1922) of Bajocian in Cuba on the strength of a Strenoceras sp. nov., considering the strong resem­ blance to this genus of some Cuban Corongoceras and Dickersonia (cf. Imlay's pis. 5 and 6). No ammonites earlier than Upper Oxfordian have ever been figured from Cuba, although Bajocian and Bathonian have been quoted without evidence (Schuchert, 1935, p. 520; Palmer, 1945· p. 6). The rich Upper Oxfordian fauna, on the other hand, still lacks a reviser. Figures of variable quality (some unrecognizable) have been published by Sanchez Roig (1920, 1951), O'Connell (1920) and Jaworski (1940). Dr Sanchez Roig in his latest paper (1951) still assigns some of the forms to Kimeridgian genera (Ataxioceras and even Virgatosphinctes) but these are misidentifications. In my opinion his Virgatosphinctes (pl. 20, figs. 1, 2) is the inner whorls of an Upper Oxfordian Perisphinctes sensu stricto, and his Ataxioceras lictor cubanensis (pl. 23) is a typical Arisphinctes, close toP. (A.) ringsteadensis Arkell, and its date is early-Bimammatum Zone; it has no resemblance to Progeronia lictor (Fontannes) of the Kimeridgian. Jaworski (1940, p. 134) and Imlay (1952, p. 969) were right in concluding that all the ammonites from the Jagua formation figured hitherto either are Upper Oxfordian or belong to peculiar local subgenera or genera (such as Vinalesphinctes), some not yet named, but not incon­ sistent with a Bimammatum Zone dating. Having been concerned with European Perisphinctids of Oxfordian age for more than thirty years, I think it worth while to attempt a revision on the basis of the published figures. Professor J aworksi sent me a few specimens from Cuba before the war and my comments are incorporated in his valuable paper (1940, pp. 124, etc.). The extraordinary suture of Perisphinctes plicatiloides as figured by O'Connell, which I pointed out (1939, Mon. Am. Engl. Corallian Beds, p. 149) seemed to set it apart from all European subgenera, was found by Jaworski (1940, p. 120) to have been misinterpreted.

bttp:/ /jura~~ic.ru/ TRINIDAD 573

UPPER OXFORDIAN AMMONITES OF THE JAGUA FORMATION (SR = Sanchez Roig) Phylloceras jaguaense SR Phylloceras lagunasense SR Glochiceras aff. microdomum (Oppel) (Jaworski, 1940, p. 97) Ochetoceras canaliculatum var. burckhardti O'Connell Ochetoceras mexicanum Burckhardt (O'Connell, 1920) Ochetoceras (Cubaochetoceras) imlayi SR (includes 'Neoprionoceras' girardoti Jaworski, 1940) Ochetoceras (Cubaochetoceras) constanciae SR Ochetoceras (Cubaochetoceras) vignalense SR Euaspidocems vignalense Spath (SR 1951, pl. 28) Euaspidoceras o' connelli SR Vinalesphinctes niger Spath (Jaworski 1940 and SR) Vinalesphinctes roigi Spath Vinalesphinctes brodermanni SR Vinalesphinctes grossicostatum SR sp. (sub Arisphinctes) Perisphinctes of the following subgenera:­ Perisphinctes sensu stricto Perisphinctes anconensis SR (sub Virgatosphinctes) Perisphinctes? diversicostatus SR (deformed; sub Dichotomosphinctes) Dichotomosphinctes spathi SR (note lappet) ' Dichotomosphinctes plicatiloides O'Connell, including cubanianus SR and probably also Dichotomosphinctes catalinensis SR sp. (sub Berriasella) Arisphinctes sanchez-roigi sp. nov. (castroi SR, sub Dichotomosphinctes, non Choffat) Arisphinctes cubanensis SR sp. (sub Ataxioceras lictor) Arisphinctes? aguayoi SR (sub Dichotomosphinctes) Arisphinctes? planatus SR (sub Dichotomosphinctes) Arisphinctes? gregarius SR (sub Dichotomosphinctes) Arisphinctes? guanensis SR (sub Discosphinctes) (The last four names might with more material be reduced; they seem to be forms of the group of P. berlieri de Lorio!) Pseudarisphinctes vignalensis SR (sub Dichotomosphinctes) Pseudarisphinctes imlayi SR (sub Decipia) Discosphinctes antillarum Jaworski Discosphinctes carribeanus Jaworski (sub 'Planites' virgulatus) Discosphinctes subguanensis sp. nov. (Per. (Planites) virgulatus var. guanensis SR, non Per. (Discosphinctes) guanensis SR) Orthosphinctes (or close derivatives thereof) cubanensis O'Connell, including P. delatorii O'Connell Orthosphinctes rutteni Jaworski (sub 'Planites') Subgen. nov. (possibly related to Vinalesphinctes) subconsociatus Spath (SR 1951, pl. 16, fig. 2, sub Prososphinctes) Second suhgen. nov. petrosus SR (sub Ataxioceras)

TRINIDAD It is possible that the Vinales Limestone is represented among partly metamorphosed limestones and schists of unknown age that form the axial mountain range of the island of Hispaniola (Haiti), but no fossils have been found. Otherwise the only other Jurassic in the West Indian islands is in Trinidad. Trinidad is a direct continuation of the northern coastal mountains of Venezuela, from which only a narrow channel separates it, and it is a long way from Cuba and the Gulf of Mexico. Nevertheless it is described in this chapter because it is one of the West Indian islands and is more likely to be looked for in a chapter on middle America than in one on South America. No Jurassic is known in eastern Venezuela, and Trinidad is nearly as remote from the Jurassic in the West Venezuelan and Colombian Andes as from that in Cuba. This extremely isolated occurrence of Jurassic was discovered in the

bttp:/ /jnra~~ic.ru/ 574 MEXICO AND THE GULF REGION Northern Range, a schist chain about 50 miles long by 10 miles wide, densely clad in jungle, which is a detached fragment of the Caribbean Range of Venezuela. In eastern Venezuela transgressive Cretaceous rests on Palaeozoics and Trias, but the age of the schists and associated metamorphics is unknown in either Venezuela or Trinidad. They are believed to be Palaeozoic by some (Liddle, 1946, pp. 691, 702) and Cretaceous by others (Bucher, 1952, p. 79). Caught up in the schists and obviously incorporated tectonically are isolated blocks and lenses of limestone. Some of these exposed on the north-east corner of Trinidad had been found to contain Upper Cretaceous fossils. In 1938 excavations for a dam at Hollis Reservoir, on the south flank of the North Range, exposed under schists a mass of dark fiaggy limestone from which some recognizable Upper Jurassic ammonites were obtained (Hutchison, 1938). They were identified as Virgatosphinctes transitorius (Zittel), zonal index fossil of the European Tithonian (Spath, 1939). Subsequently 'further ammonites of Tithonian­ Neocomian age' have been reported (Suter, 1951, p. 190; Barr, 1952). It thus appears that the Cretaceous transgression on the south side of the Caribbean may have begun with the Tithonian, as in Cuba; and the major orogeny to which the Tithonian and Cretaceous rocks have been subjected in Trinidad is consistent with Palmer's interpretation of the structures in western Cuba (seep. 571).

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