THE AMERICAN JOURNAL OF SCIENCE AND ARTS. [THIRD SERIES.] ART. XLIX.-On the Geological History of the Gulf of Mexico; by E. W. HILGARD, of the University of Mississippi.* With a Map. THE colored outline map before you, without much pretension to accuracy of detail, shows the general geological features of the great embayment, once a portion of the Gulf of Mexico, whose axis is now marked by the course of the Mississippi river, from southern Illinois to its mouth. I have compiled this map from the best data now extant, accessible to me, with a view to the better elucidation of the succession and character of geological events; and especially with a hope of bringing to bear upon the later formations of the interior of the continent, the chronological record here left by the retiring waters of the sea. Marine deposits being better understood and more available for general comparison and conclusions than those of inland lakes, the series here shown would seem, by its original connection with the interior basins, to promise a key to the determination of equivalence in time, that, in view of the violent disturbances in the Rocky Mountain region, it might be difficult to find in that portion of the continent. The subject matter of the present communication is, for the greater part, embraced in publications made by myself during the past ten years; and to these publications I must refer for the corroborative detail, which in this general summary would be out of place. *Read before the American Association for the Advancement of Science, at Indianapolis, August, 1871. AM. JOUR. SCI.-THIRD SERIES, VOL. II, No. 12.-DEC., 1871. Cretaceous period. The most ancient shore-line of the Mississippi embayment is formed mainly by the various stages of the Carboniferous rocks. It is only in eastern Alabama and the adjoining portion of Georgia, that the Silurian and metamorphic rocks formed the shore of the Cretaceous seas. The deposits of the latter period have been traced by Safford up to the Kentucky line, along the western foot of the paleozoic ridge which compels the Tennessee river to make its long détour northward; and the Cretaceous outcrop doubtless extends, in a northwesterly direction, some distance into Kentucky. In the portion of this belt embraced within the States of Mississippi and Alabama, the dip is sensibly at right angles to the trend (i. e., between W. and S.) at the rate of twenty to twenty-five feet per mile. In its southerly portion especially, the Cretaceous beds are very distinctly divided into three prin cipal stages, viz: 1. A lower one 300 to 400 feet thick-Coffee group of Safford (Eutaw group mihi)-consisting of non-calcareous sands, and blue or reddish laminated clays, with occasional beds of lignite, and but very rarely (Finch's Ferry in Alabama) marine fossils, silicified; corresponds doubtless to Hayden's Dakota group, including, perhaps, in its upper part the equivalents of the Fort Benton group, to whose fossils those of Finch's Ferry, collected by Tuomey, bear a close analogy. 2. The middle or Rotten limestone group, not less then 1,200 feet in maximum thickness. Soft, mostly somewhat clayey, whitish, micro-crystalline limestones and calcareous clays; very uniform on the whole, if we except the locally important, but not generally extant, feature of the "Tombigby Sand," the special home of Inocerami, Selachians, and gigantic Am monites. 3. Ripley group: crystalline, sandy limestones, alternating with dark colored glauconitic marls containing finely preserved fossils. Thickness 300 to 350 feet. Equivalent of the highest bed of the Cretaceous of New Jersey, and doubtless of Hayden's Fox Hills beds. How far the Rotten limestone, as a whole, may be considered as embracing the whole of the series intervening between Hayden's Fox Hills beds and Dakota group, remains to be shown. The fauna of the Tombigby sand sub-group is distinguished, as already stated, by the great number, both of individuals and species of Inocerami, and of remains of (chiefly Selachian) fishes, wherein it corresponds to Hayden's Niobrara division; its fossils have been partly named and described (somewhat imperfectly) by Tuomey. According to Hayden's view of the New Jersey * Proceed. Philad. Acad. equivalents, the Rotten limestone would be represented by his Fort Pierre group. The distinctive features of these several groups become less marked the farther we advance northward, even in Mississippi. Non-fossiliferous or lignitic clays and sands mingle with the marine strata; and become altogether predominant, it would seem, near the northern termination of the outcrop. * West of the Mississippi, the continuous Cretaceous outcrop does not extend as far northward as on the east side, by some 150 miles. Nor have the more ancient lignitiferous beds (Coffee group) been observed there, with certainty, within the limits of this map. The Cretaceous area of Arkansas, according to Owen's description, falls almost altogether within the limits of the Rotten limestone group; and the same seems to be true of the greater part of the Cretaceous area of northern and middle Texas. Nevertheless, the series of isolated Cretaceous outliers, which traverses Louisiana from the head of Lake Bisteneau in a S.S.E. direction, terminating probably in the great rock-salt mass of Petite Anset, exhibit the main characteristics of the Ripley group; while deep borings have demonstrated the presence, for a thousand feet beneath, of the uniform Rotten limestone, such as it exists on the prairies of Mississippi and Alabama. I have elsewhere stated the stratigraphical as well as lithological reasons which induce me to consider both the rock-salt of Petite Anse, and the sulphur and gypsum deposits of Calcasieu, as lying within the limits of the Cretaceous formations. The data given by D. D. Owen seem to assign to the Cretaceous strata of Arkansas a dip S. or slightly W. of S. The outliers in Louisiana are too limited in extent for determinations of dip; but it can scarcely be doubted that they represent the summits of a (more or less interrupted) ancient ridge, a kind of "backbone" to the State of Louisiana, whose resistence to denudation has measurably influenced the nature and conformation of subsequent deposits. It is fair to presume that from this ridge the strata dip toward the axis of the Mississippi Valley, to meet those on the opposite side; and the depth at which these beds are found in the Calcasieu bores, seems to indicate, on the western slope, a south-southwesterly dip of three to four feet per mile. A glance at the map shows, nevertheless, that the general form of the northern Gulf shore was not materially influenced by the existence of this axis of elevation, which probably was marked merely by a series of disconnected islands in the early Tertiary sea that, after the emergence *Fide Safford. + Indicated on the map by the localities of Petite Anse, Chicot, Winfield and Bisteneau. This Journal, Nov., 1869, p. 345. of the immence Cretaceous area, already prefigured the present Gulf of Mexico. It would be bootless to speculate, at this early moment, on the precise origin of the great rock-salt, gypsum and sulphur deposits. That the prominent event of the epoch-the emerg ence of an extensive sea-bottom-afforded abundant opportunity for the accumulation of the two former substances, is obvious enough; it would seem to pre-suppose, however, a temporary or partial isolation at least from the general ocean, analogous to that which, apparently, occurred in later times. But as regards the sulphur, its ordinary co-occurrence with gypsum would hardly seem to afford a sufficient precedent for the present case: unless we assume the concurrent influence of volcanic or other "abysso-dynamic" agencies. Tertiary period. It will be perceived that during the Tertiary period, the northern Gulf shore receded from its extreme northern limits in southern Illinois and Missouri, to a shore-line which, though running near the latitude of Bâton Rouge, is not far from parallel to the present one, if we ignore the extreme projection of the Mississippi delta. This rapid filling-in of the embay ment, no less than the character of the deposits, prove that the depth of water was not great; especially in the remoter portions, where lignitic and lignito-gypseous deposits very sparingly interspersed with small marine beds (the remnants of estuaries) from the predominant material. Similar alternations of materials occur, in fact, throughout the older Tertiary deposits of the southwest; and hence, the divisions marked off by difference of color on the map, as "lignitic" and "marine" Tertiary, respectively, must be taken very much cum grano salis. Except only in southern Arkansas, few marine beds of any notable extent occur outside (i. e., north of) the limit of the area indicated as marine. But in northern Louisiana, where the dip is very slight, lignitiferous strata are altogether predominant on the surface; although the marine seem to underlie everywhere at no great depth, and in numerous localities crop out on the surface; forming, according to Hopkins, distinct beaches around some of the Cretaceous outliers mentioned above. So far, indeed, from considering the predominantly lignitiferous area as representing a period distinct from the older marine Tertiary, I have little doubt that the larger portion, if not all, of the beds I have heretofore designated as the Northern Lignitic (and Flatwoods clay) group (Lagrange and Porter's Creek groups of Safford) are the strict equivalents in time of the oldest marine beds observed in South Carolina and Alabama, and designated by Tuomey as the Buhrstone group ("Siliceous Claiborne" of my Miss. Report). The lithological continuity of the bedrocks of this group along the eastern border of the Tertiary, supported to some extent by paleontological evidence, strongly inclined me to this opinion ten years ago;* and it has received a strong confirmation from the latest observations of Dr. E. A. Smith, of the Miss. Geol. Survey, who has found the same rocks, substantially, continuous along the southern border of the lignitic area, nearly to the Mississippi bluff. At the same time, Safford mentions the occurrence of similar beds on the border of the Cretaceous, in Tennessee. The inference is inevitable that no beds outcropping in the fork of these two marine branches can be anterior in time; the interconnection, in fact, is such as to render the supposition that there can be any material difference of age almost stratigraphically impossible. The exclusively lignitic character of the central portion must, therefore, be ascribed rather to the inaccessibility of that region to the waters of the sea during their deposition; perhaps in consequence of a change of level, by which the upper portion of the embayment, from about the mouth of the Arkansas to Cairo, was converted, for the time being, into a littoral marsh. In Arkansas, nevertheless, small marine beds are more liberally interspersed among the lignitic clays, than is the case east of the Mississippi; and some of those mentioned by Owen as occurring on the territory here laid down as chiefly lignitic, are obviously more closely related to the celebrated Claiborne shellbed than to the Buhrstone group. The latter group does not, in fact, appear characteristically developed anywhere west of the river, so far as I know; and the occurrence of somewhat extensive marine Tertiary outliers on the Cretaceous territory of Arkansas, as well as of lignitic beds on that of Texas (e. g., the Cross Timbers, as approximately laid down on the map), proves that although the deeper water of the embayment followed substantially the lines of trend shown on the map, yet there still existed at that time a connection, in a northwesterly direction, of the Gulf waters with those of the great interior basin of the West. That this connection should not be uninterruptedly traceable at the present time, is not surprising when we consider the shallowness of the connecting trough, as demonstrated by the inconsiderable thickness of the deposits, that of course greatly favored their removal by the subsequent events of the Quaternary period. Nevertheless, enough seems to remain of these deposits to form a chain by which, with the aid of paleobotany, the equivalents in time of the Buhrstone and Claiborne marine groups, at least, can be determined among the fresh or brackish * Miss. Rep., 1860, §§ 162 and ff. 188, etc. |