Public domain
[p. 727]
Introductory[1]
The history of the Cretaceous period, as formerly defined, was complex. At its beginning, the larger part of the North American continent was above the sea. During its progress, the sequence of events in our continent was somewhat as follows: (1) A somewhat wide-spread warping of the continental surface, resulting in extensive submergence in Mexico and Texas, and a lesser submergence along the Pacific coast. At about the same time the Atlantic and Gulf coasts and some parts of the western interior were brought into such an attitude as to become sites of deposition, though not submerged. A prolonged period of sedimentation followed. (2) The period of sedimentation was followed by other geographic changes which inaugurated a prolonged interval of erosion which affected the recent deposits as well as older formations. (3) Later, the sea encroached upon the Atlantic and Gulf borders, extending somewhat beyond the non-marine formations of the earlier stage. It again covered Texas, and presently extended northward over the Great Plains, probably to the Arctic Ocean. On the Pacific coast, too, the sea gained on the land. Few greater transgressions of the land by the ocean are recorded in the long history of the North American continent. A long period of deposition was initiated by the submergence. It was succeeded by (4) a wide-spread withdrawal of the waters from the continent, leaving the land area nearly or quite as large as now.
The formations of the Cretaceous period, as outlined under (1) and (3) above, have been divided, commonly, into two main series, a Lower and an Upper. To the former were referred the deposits [p. 728] of the earlier and lesser submergence, and to the latter, those of the later and more extensive submergence. The distinctness of the Lower and Upper Cretaceous is however so great, that it seems more in keeping with the spirit of modern classification to regard them as separate systems, and the corresponding divisions of time as periods. What was formerly called the Lower Cretaceous series is here called the Comanchean system. The propriety of this classification is the more striking, since it is applicable to other continents as well as to our own.
The Atlantic and Gulf Border Regions
That part of the Comanchean system along the Atlantic coast is called the Potomac[2] series; the part along the eastern Gulf coast, where conditions of sedimentation appear to have been similar to those along the Atlantic, is the Tuscaloosa[3] series. The approximate surface distribution of these series is represented in Fig. 498, which shows that the system outcrops near the inland margin of the Coastal Plain. It is indeed the lowest member of the Coastal Plain group of formations. Neither the Potomac nor the Tuscaloosa series is believed to represent the whole of the period, and the two are not strictly contemporaneous.
Conditions of origin, and constitution. By the beginning of the Comanchean period, both the Appalachian Mountains and the area of the present Piedmont Plateau had been degraded well toward base-level, so that little warping of the surface appears have been needed to convert portions of the coastal lands into sites of deposition, though more may have been necessary to provide lands high enough to furnish abundant sediments. The peneplanation of the eastern mountain and plateau region during the Jurassic period was no doubt attended by deep decay of the underlying crystalline rocks, and the consequent accumulation of a heavy mantle of residuary, insoluble earth. The warping which inaugurated the Comanchean period seems to have involved a rise of the Appalachian tract, and a consequent rejuvenation of the drainage from it, while the coastward tract was left relatively flat, or so warped as to become a zone of lodgment for the sediments brought down by the quickened drainage from the west. Lakes, marshes, etc., were probably features of the lodgment area. These conditions are in harmony with the constitution of the deposits, which consist of gravel (or conglomerate), sand (or sandstone), and clay, largely uncemented.
[p. 729]
[p. 730]
The gravel at any point is made up principally of materials derived from formations adjacent on the west, and subordinately from subjacent formations. It is often arkose in the immediate vicinity of the feldspar-bearing crystalline rocks, but elsewhere it is composed chiefly of the resistant products of mature weathering.
The sands are sometimes fine and the grains well rounded, 88 if transported far by moving water, and sometimes coarse and angular, as if they had been subjected to but little wear. Locally they contain feldspar grains, or bits of kaolin which have resulted from their decay. The presence of the feldspar in the sand, like the presence of pieces of schist in the gravel, shows that erosion sometimes exceeded rock decay. This betokens high land to the west whence the sediments were derived, and is one of the reasons for the belief that the region west of the site of deposition was tilted upward at this time.
Beds of clay of such purity and magnitude are found in the Potomac series, that they have been extensively utilized, especially in New Jersey,[4] for the manufacture of clay wares. The clay often shows little stratification, and is notable for its bright and variegated colors, black, white, yellow, purple, and red being not uncommon. White is to be looked upon as the normal color; the others are the result of various impurities, the black being due to organic matter.
The clay, sand, and gravel are irregularly disposed, doubtless [p. 731] the result of the physical conditions where the sedimentation took place, conditions which might have existed along the lower courses of rivers or at their debouchures, where shore-waters had little effect upon them.
In addition to the clastic sediment, there is a little lignite, and some iron ore, and though both are widely distributed, neither is of much commercial value.
Stratigraphy and stratigraphic relations. The Potomac and Tuscaloosa series are nearly horizontal, with a gentle dip seaward. The Potomac series rests unconformably on Triassic and other formations (Fig. 499), and the Tuscaloosa series on Paleozoic or older strata. Both series are overlain unconformably by the Upper Cretaceous.
Thickness. The Potomac series rarely reaches a thickness of 700 feet. The thickness of the Tuscaloosa series is about twice as great.
The Texas Region[5]
The Comanchean system is more fully represented in Texas than farther east, but its stratigraphic relations are the same. The beds appear at the surface over an area distant from the coast, dip seaward at a low angle, and are concealed near the coast by younger formations. The system includes three distinct series, (1) the Trinity, (2) the Fredericksburg, and (3) the Washita. The first was perhaps contemporaneous with the Potomac series; and the last is probably younger than any part of the system on the Atlantic coast. The system here is much thicker than farther east, ranging from 1,000 feet to about 4,000. Some parts of it are marine, and [p. 732] some terrestrial in origin. The marine part of the system includes much limestone.
From Texas, the Comanchean formations, or some of them, originally spread northward into Kansas,[6] northwestward to Colorado, and westward to Arizona. Though they appear at the surface in small areas only, their extent may be considerable beneath younger formations.
i** The Comanchean of Mexico is mainly limestone, and, though but imperfectly known, has been estimated to have the extraordinary thickness of 10,000 to 20,000 feet. Its distribution is such as to show that a large part of that country was beneath the sea. It has been conjectured that the waters of the Atlantic and Pacific mingled over the site of some part of Mexico at this t i me. but this is uncertain. If the oceans were connected, it was probal >1 v across southern Mexico, or perhaps Central America. At any rate, there does not seem to have been free faunal intermigration between the Gulf coast and the coast of California. In its abundance of limestone, the series of Texas and Mexico resemble the Lower Cretaceous of the northern part of South America, and southern Europe.
The Northern Interior
Though the sea is not known to have had access to the westers interior of North America north of Kansas during the Comanchean period, clastic beds of terrestrial origin, which are perhaps Comanchean, are known at various points farther north. The beds in question, the Morrison,[7] Como, and Atlantosaurus beds, are best known along the Front Range through Colorado and Wyoming, and in the Black Hills,[8] though they probably reach northward to Montana. If all the beds thought to be the equivalent of the Morrison, are really so, the formation is widely distributed. These beds are often regarded as late Jurassic.[9] and this may be their proper classification.
In Montana, Alberta, and Assiniboia, there are beds (the Kootenay [p. 733] and Cascade formations,[10] etc.) similar in character to rhose just mentioned. They are mainly clastic, and contain some coal. Their fossils are mostly of plants of early Cretaceous types. In Montana, the Kootenay formation overlies the Morrison. The exact relations of these formations to the Comanchean of the Atlantic and Gulf coasts is not known.
To the Morrison and Kootenay formations a lacustrine origin has usually been assigned, and there is perhaps no adequate ground for questioning this conclusion for some parts of the formations; but the character of some of the beds and the nature and distribution of their fossils suggest a fluviatile origin for parts, and perhaps for large parts, of the series. The position of these formations with reference to the Rocky Mountain axis, is much the same as that of the Potomac series to the Appalachian axis, and a similar conception as to the mode of origin may be entertained.
The Pacific Border
In the United States. The Lower Cretaceous beds have great development in California, where they attain their maximum known thickness. They are known here as the Shastan group,[11] made up of the Knoxville series below and the Horsetown above. The deposits are thickest in the Sacramento valley, the sediments having been furnished by the newly uplifted mountains (p. 710). Most of the thick system, including its basal beds, bears the marks of a shallow-water origin. The Shastan group is represented in Oregon also.[12]
Where the base of the Shastan series has been observed, it is unconformable on Jurassic rocks, or on metamorphic rocks of unknown age. It is overlain unconformably in some places, and without apparent unconformity in others, by the (Upper) Cretaceous (Chico [13] series) . In the Coast Range of California, the system contains some igneous rock.
[p. 734]
The fauna of the Shastan group is markedly unlike that of the Comanchean of Texas, and since the differences do not seem referable to climate, it seems most rational to suppose a barrier to marine life between the two regions. In the United States, this barrier was perhaps wide; but in Mexico it was probably narrow, for the Comanchean fauna, or some part of it, extends west to the western part of Mexico, while farther south the Pacific fauna reached eastern Mexico. The exact position of the barrier which separated the oceans is not known. It is possible, on the other hand, to think of ocean currents of different temperatures, as determining the differences of faunas. Though the exact time relations of the Comanchean and Shastan series have not been determined, they are believed to be approximately equivalent.
North of the United States. Farther north, Lower Cretaceous beds (Queen Charlotte series) occur in the Queen Charlotte Islands,[14] where they have an estimated thickness of between 9,000 and 10,000 feet. In British Columbia, the coast line was east of the Coast ranges, and extended farther and farther east with increasing latitude, until the ocean swept clean across the site of the Cordilleras in the early part of the period, and extended south along the area which is now the east base of the mountains.[15] The Kootenay formation is perhaps partly contemporaneous with these marine beds. The Comanchean system of British Columbia generally rests unconformably on the Triassic system, and contains some volcanic material and, locally, coal.
Farther north, the Lower Cretaceous has not always been separated from the Upper, but the former has extensive development in some parts of northern Alaska,[16] where it contains coal. It is also believed to occur on the west coast of Greenland, where the beds are believed to represent some such horizon as that of the Kootenay, or Potomac.[17]
[p. 735]
The Close of the Comanchean (Lower Cretaceous) period in North America
Considerable changes in the geography of North America brought the Comanchean period to a close. Along the Atlantic and Gulf borders considerable tracts were converted from areas of deposition into areas of erosion. The sea was withdrawn from Texas, and the Comanchean system somewhat deformed and faulted; in Mexico the deformation of the system was notable. Along some parts of the western coast, there was folding of the Lower Cretaceous beds,[18] accompanied by volcanic activity, as in the southern Coast Range of California, while in other places the sea spread itself over areas which had been land. Still other areas in the west appear to have emerged at this time, and never to have been submerged since.[19]
On the whole, the deformative movements at the close of the Comanchean period were more extensive than those which occurred in the midst of any one of the Paleozoic periods as now denned, if the Mississippian and Pennsylvanian are regarded as separate periods. The force of this point in its bearing on the distinctness of the Comanchean period from the Cretaceous, is increased by the fact that the latter was inaugurated by a notable submergence, affecting great areas. During this submergence, Cretaceous beds of marine origin were deposited on the eroded surface of the Comanchean system, somewhat generally, and the younger system overlapped the older in most regions where both are present. The reverse was the case, however, in British Columbia. On stratigraphic grounds, therefore, the distinctness of the two systems is clear. The case is hardly less clear on the paleontological side. In Texas, for example, no species of marine life is known to have lived over the time-interval recorded by the unconformity between the two systems.
[p. 736]
Europe. The deposits in some of the lakes, marshes, estuaries, and other lodgment basins which resulted from the geographic changes at the close of the Jurassic period in Europe, record the transition from that period to the early Cretaceous. The interruption of marine sedimentation in Southern Europe was not so general, and over considerable areas the Lower Cretaceous succeeds the Jurassic conformably, both being marine. In Russia, it is difficult in many places, to define the upper limit of the Jurassic, so complete is the gradation into the (Lower) Cretaceous.
In Europe, as in North America, the Cretaceous, as that term is there used, is divisible into two major parts, a lower and an upper, as distinct as successive systems usually are. As in North America, the lower division is much more restricted in its distribution than the upper, and is, to a large extent, of non-marine origin.
During the initial stages of the Lower Cretaceous, the areas of sedimentation were more or less isolated; but later, advances of the sea enlarged some of these areas, and finally united many of them by bringing them beneath a common sea. The Lower Cretaceous formations embrace all sorts of clastic rocks, together with limestone, glauconitic beds, beds of coal (northwestern Germany), and iron ore. They embrace, indeed, about all varieties of sedimentary rock except chalk, the rock from which the name “Cretaceous” was derived. In southern Europe, much of the system is limestone.
Other continents. In other continents, the Lower and Upper Cretaceous have been less clearly differentiated; yet enough is known to show that the Lower and Upper Cretaceous systems are, in general, markedly different, both in origin and distribution. Lower Cretaceous formations of marine origin are wide-spread in Siberia, Japan, and the southern part of Asia, but in limited areas only in most other parts. The system is believed to have slight development in the mountain regions of northwestern Africa, where it is really an extension of the Lower Cretaceous of southern Europe [p. 737] and is unconformable beneath the Upper Cretaceous, and in South Africa.[21] Marine Lower Cretaceous is wide-spread in the northern part of South America, but not elsewhere east of the Andes. It is generally absent about the borders of the South Atlantic. On the other hand, marine Lower Cretaceous beds occur in many places about the southern Pacific and Indian Oceans. The areas where the system is exposed are, however, mostly small.
Close of the period. Geographic changes of importance occurred in various parts of the earth at the close of the early Cretaceous period, and are recorded (1) in the unconformities between the Lower and Upper Cretaceous systems, as at some points in Europe, north Africa, Australia, and South America, and (2) in the differences in their distribution, as will appear in the account of the following system.
Climate. In the aggregate, the known fossils of the Lower Cretaceous of America are not such as to indicate great diversity of climate. Even in Greenland, the climate seems to have been as warm as that of warm temperate regions to-day.
The fresh-water fossils of those deposits of central Europe which represent the transition from the Jurassic to the Lower Cretaceous, are, on the whole, of such a character as to indicate a climate far from tropical. So far as they afford a warrant for inference, the climate of central Europe would seem to have been comparable with that of the temperate portions of America to-day. 'he fossils of lower latitudes denote a warmer climate. On the rhole, European fossils seem to afford better evidence of the existjnce of climatic zones than those of America. From them paleontogists have thought to find warrant for the hypothesis that the dimate underwent more or less fluctuation during the course of the eriod.
The terrestrial vegetation. Fossil plants constitute the chief record of the life of the early stages of the Comanchean in America. The earliest flora was akin to that of the Jurassic, in that the cycadeans [p. 738] (Fig. 500), conifers, ferns, and horsetails were the dominant forms. In most of Europe, this group held possession of the land throughout the period, though angiosperms appeared in Portugal before its close. Descendants of Jurassic types of plants also continued throughout the period in northwestern America.
The introduction of angiosperms. The eastern and central America angiosperms, including both monocotyledons and dicotyledons, appeared early in the period, and developed so rapidly that by the beginning of the next period they seem to have overrun the continent. This is one of the most radical evolutions in the [p. 739] history of the plant kingdom. The precise time and place of origin of the angiosperms is not known, but present data point to the borders of the north Atlantic as the place of origin, and the late Jurassic or earliest Comanchean as the time.
About 400 species of Comanchean angiosperms are known from the Atlantic coast. They were in a minority in the lowest Potomac, but increase to an overwhelming majority in the upper beds. The earliest forms are ancestral, but not really primitive, and throw little light on the derivation of the group. The majority bear definite resemblances to modern genera and some (as Sassafras, Ficus, Myrica, and Aralia) are referred to living genera. Before the end of the period, figs, magnolias, tulip trees, laurels, cinnamon and other forms referred to modern genera, but not to modern species, had appeared. The cycadeans had dropped to an insig ificant place, and the conifers and ferns, while not equally reduced, were subordinate to the angiosperms.
The land animals. The aspect of the vertebrate life was inter tediate between that of the Jurassic and Upper Cretaceous, and, so far as it is known, has been sketched already (p. 720). Little is known of other forms of terrestrial animal life.
The fresh-water fauna. The molluscan fauna of the inland waters had assumed a pronouncedly modern aspect, as illustrated in Fig. 501. It had probably attained considerable importance through the extension of the fresh waters, but the record is by no [p. 740] means so ample as would be expected if the deposits were made mainly in lakes and river channels. This is an additional reason for the growing opinion that the terrestrial deposits were in considerable part the products of land-wash of the more transient type, due to overflows, storm-wash, sheet-wash, and other forms of more strictly subaerial aggradation.
The marine faunas. Two very distinct marine faunas are found in North America, implying two distinct maritime provinces — that of the Mexican Gulf and that of the Pacific. The former had its connections eastward with Portugal and the Mediterranean region, the latter, northward and westward with Asia and Russia, though the boreal element is less conspicuous in the upper part (Horsetown). No species common to the two provinces is known. The decline of the boreal aspect of the western fauna may have been due to the closing of Bering Strait, thus shutting off cold currents from the Arctic.[22]
[p. 741]
The Comanchean faunas are said to represent three distinct facies, the reef facies, most conspicuous, and the littoral and deeper water facies.
For a full review of the American Cretaceous, up to 1891, see White (C. A.), Bull. 82, U. S. Geol. Surv. ↩︎
McGee, Am. Jour. Sci., Vol. XXXV, 1888, pp. 120-143; Clark and Bibbins, Jour. Geol., Vol. V, 1897, pp. 479-606, Bull. Geol. Soc. Am., Vol. XIII, pp. 187-214 j also reports of the Geological Surveys of New Jersey and Maryland, and folios of the U.S. Geol. Surv. covering parts of the Atlantic Coastal Plain. ↩︎
Smith, Geol. Surv. of Ala., 1894. ↩︎
Cook, Report on Clays, Geol. Surv. of Now Jersey, L868, and Kumrael, 1904. ↩︎
Hill, 21st Ann. Rept., U. S. Geol. Surv., Pt. VII, and Bull. Geol. Soc. Am., Vol. V, 1893, pp. 297-338. Also Hill and Vaughan, 18th Ann. Rept., U. S. Geol. Surv., Pt. II, pp. 217-237. ↩︎
Prosser, “Comanchean Series of Kansas,” the Univ. Geol. Surv. of Kans., Vol. II, 1897. ↩︎
Cross, Pike’s Peak folio, U. S. Geol. Surv., 1894. ↩︎
Ward, Jour. Geol., Vol. II, p. 250. ↩︎
Knight, Bull. 46, Wyo. Exp. Station, p. L34. ↩︎
Fort Benton, Mont, folio, U. S. Geol. Surv. ↩︎
Turner, Bull. Geol. Soc. Am., Vol. II; Diller, Am. Jour. Sci., Vol. XL, 1890, and Bull. Geol. Soc. Am, Vol. IV, 1892; Diller and Stanton, idem, Vol. V, a Summary for the Pacific Coast brought up to 1894. ↩︎
Diller, Am. Jour. Sci., Vol. XXIII, 1907. ↩︎
Fairbanks, Jour. Geol., Vol. Ill, pp. 415-430, and San Luis, CaL, folio, U. S. Geol. Surv. ↩︎
Dawson, Geo. M., Am. Jour. Sci., Vol. XXXVIII, 1889, pp. 120-127. ↩︎
Dawson, Science, March 15, 1901; and Bull. Geol. Soc. Am., Vol. XII, p. 87. ↩︎
Professional Papers 15 and 20, U. S. Geol. Surv. ↩︎
White and Schuchert, Cretaceous Series of the West Coast of Greenland. Bull. Geol. Soc. Am., Vol. IX, pp. 343-368., 1898. ↩︎
Fairbanks, Jour. Geol., Vol. Ill, pp. 415-430, and San Luis folio, U. S. Geol. Surv. ↩︎
Ransome, Bisbee, Ariz, folio, U. S. Geol. Surv. ↩︎
The term Comanchean is not applied to the Lower Cretaceous formations outside of North America. ↩︎
Ann. Rept.Geol. Com. Cape of Good Hope, 1901, p. 38; Corstorphine, History of Stratigrapbical Investigations in South Africa, Rept. S. Af. Assn. for Adv. of Sci., 1904; and Rogers, Geology of Cape Colony, 1905. ↩︎
Stanton, Jour. Geol., Vol. XVII. ↩︎