Reynosa Formation in Lower Rio Grande Region, Texas1

BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA VOL. 37, PP. 455-462 SEPTEMBER 30. 1926

REYNOSA FORMATION IN LOWER RIO GRANDE REGION, TEXAS1

BY ARTHUR C. TROWBRIDGE

(Bead before the Society December 28, 192^)

CONTENTS Page Previous work...... 455 Distribution...... 456 Stratigraphic relations...... 456 Age ...... 458 Lithology ...... 458 Thickness...... 460 Origin and history recorded...... 460

P revious Work In 1890 Penrose,2 as a result of his trip down the Rio Grande with Dumble in 1889, described a deposit of limestone containing pebbles and cobbles, under the name “Reynosa limestone,” from the town of Rey- nosa, Tamaulipas, Mexico, across the Rio Grande from Hidalgo, Texas. At Reynosa the rocks unconformably overlie what was then called the Fayette sands, now known to include strata of upper Eocene, Oligocene, and Miocene age. Penrose found Recent shells embedded in the surface of the exposure and, thinking the formation was Recent, included it in his “post-Tertiary formations.” In 1891 Hill3 described patchy remnants of a formation that consisted of gravel cemented by a calcareous matrix, and that occupied terraces 400 to 1,000 feet above the Rio Grande, in Uvalde and adjacent counties in Texas, and called it the “Uvalde formation,” for the town of Uvalde,

1 Manuscript received by the Secretary of the Society September 2, 1925. Published by permission of the Director of the United States Geological Survey. 2 R. A. F. Penrose, Jr.: Report of Geology for eastern Texas. Texas Geol. Survey, First Ann. Rept., 1890, pp. 57, 58, 63. 3 R. T. H ill: Notes on the geology of the southwest. Am. Geologist, vol. 7, 1891, pp. 366-370. (455)

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/37/3/455/3430023/BUL37_3-0455.pdf by guest on 03 October 2021 '456 A. C. TROWBRIDGE--- REYNOSA FORMATION IN TEXAS

in Uvalde County, and in 1898 Hill and Vaughan4 interpreted the “Uvalde” formation as residual material washed out from the Edwards Plateau in late Tertiary time and deposited in fan-shaped areas on the downthrown side of the Balcones fault, which separates the Edwards Plateau from the Rio Grande plain, on which the gravels and conglom erates occur. In 1894 Dumble5 applied the name Reynosa “Division” to a thick and continuous formation of limestone and conglomerate between the Rio Grande and Nueces River and made the “Reynosa limestone” of Peruose the top member of his Reynosa “Division.” This correlation of Dumble’s has been questioned by Deussen.6 In 1923 the writer7 correlated the Reynosa of Penrose and Dumble with the “Uvalde” of Hill and Vaughan and adopted the term Reynosa formation.

D istribution Figure 1 shows the Reynosa cropping out as a continuous formation in a belt 20 to 35 miles wide in Webb, Jim Hogg, Starr, and Hidalgo counties, and Deussen9 has mapped a continuation of this, belt of out crop northward and eastward to Brazos River. Chiefly because of its porosity, and hence its superior resistance to erosion as compared with underlying clays, the formation gives rise to a cuesta, the eastward slope of which corresponds with the eastward dip of the formation. The steep west face of the cuesta is known as the Bordas scarp. In addition, there are numerous patches of gravel capping the higher hills west and north of the main belt of outcrop, as shown in Figure 1.

Stratigraphic Relations Where the formation is continuous between Rio Grande and Nueces rivers it lies upon the Eocene Frio clay and on the Miocene Oakville sandstone, with a notable erosional and structural unconformity between, the Pliocene Lapara and Lagarto formations being missing or over lapped; and near the Gulf coast the Reynosa formation is overlain un-

4R. T. Hill and T. W. Vaughan: Geology of the Edwards Plateau and Rio Grande Plain adjacent to Austin and San Antonio, Texas. XJ. S. Geol. Survey, Eighteenth Ann. Rept., pt. 2, 1808, pp. 244-247, 254-255. 5 E. T. Dumble: The Cenozoic deposits of Texas. Journal of Geology, vol. 2, 1894, p. 560. 6 Alexander Deussen : Geology of the coastal plain of Texas west of Brazos River. U. S. Geol. Survey Prof. Paper 126, 1924, p. 102. 7 A. C. Trowbridge: A geologic reconnaissance in the Gulf coastal plain of Texas near the Rio Grande. U. S. Geol. Survey Prof. Paper 131, 1923, p. 99. 9 Alexander Deussen : Ibid., plate viii.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/37/3/455/3430023/BUL37_3-0455.pdf by guest on 03 October 2021 STRATIGRAPHIC RELATIONS 457

SE R IE S GROUP FORMATION SYSTEM

RECENT Dune sand

Deposits of Rio Grande delta rüb~i Beaumont cl a y PLEISTOCENE'S o n Lissie gravel UNCONFORMITY

PLIOCENE « Rey noaa formation UNCONFORMITYAND OVERLAP

U valde Frio clay

F ayette sandstone

Yegua formation Jaibornes | Tcm [ Cook Mtn.formation »arrizo 5| >M I T Mt.Selmanformation

Biéford formation ^»Encinal vvilcox< Í tTI Carrizo sandstone

Palafox\ Indioformation UNCONFORMITYAND OVERLAP

Laredol M id way form ation

|v^ebbronville ■ m Cretaceous undivided

¡naciol HOGG¡BROOKS k I Qds KENEDYj

RoT*r*uS I WILLACY Rio Grande Cit) Qrd CAMERON O 10 20 30 40 50 MILES

Br.qwnsvilli

F i g u r e 1 .—Surface Distribution of the Reynosa Formation in the Lower Rio Grande Region Adapted from Trowbridge.8

8 A. C. Trowbridge : Ibid., plate xxviii.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/37/3/455/3430023/BUL37_3-0455.pdf by guest on 03 October 2021 458 A. C. TROWBRIDGE REYNOSA FORMATION IN TEXAS

conformably by the Lissie gravel, from which the remains of Pleistocene horses have been taken. On the landward side of the belt of continuous outcrop the formation, as isolated patches, lies unconformably by overlap on all Eocene forma tions and in places overlaps the Eocene and lies on the Cretaceous (fig ure 1). A ge Except for the remains of Eecent land snails, crayfish, jackrabbits, and a few other animals which have become embedded in the surface as the limestone has been dissolved and reprecipitated, and except for fossils originally deposited in the formations from which the gravel was de rived, no fossils have been found in the Eeynosa formation, so that its precise age cannot be determined; but its stratigraphic position, uncon formably above the Miocene and above the early Pliocene and uncon formably below the Pleistocene, indicates late Tertiary or early Pleisto cene age, and it is tentatively assigned to the Pliocene. These sedi ments were included in the “Lafayette” of McGee and are of about the same age as the Pliocene Citronelle formation of east Texas, Louisiana, Mississippi, and Alabama.10

Lithology The Eeynosa formation is an intricate mixture of gravel cemented by lime carbonate, uncemented gravel, limestone in which are embedded peb bles and cobbles, almost gravelless limestone, sand, sandstone, gravelly sand, and a relatively small amount of clay. The pebbles and cobbles consist of chert, limestone, vein quartz, and igneous rocks of various sorts. Where the gravel or conglomerate overlies the Payette, Frio and Oakville formations it includes petrified wood. Most of the pebbles and cobbles are well shaped by abrasion, many of them are highly polished, and some show markings made by hard impact during transportation. The gravel deposits are roughly sorted into lenses and pockets of different textural grades (fig. 2). The limestone is gray and sandy and weathers into rough, irregular surfaces, due to irregular concretions and impurities. In places it has surficially a tuffaceous appearance. Its basins, rims, terraces, channels, and concentric banding are suggestive of deposition by springs. Patches south of Espejo Eanch, in north-central Webb County, and at Carrizo

30 G. C. Matson and E. W. Berry: The Pliocene Citronelle formation of the Gulf coastal plain and its flora. U. S. Geol. Survey Prof. Paper 98, pp. 167-204.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/37/3/455/3430023/BUL37_3-0455.pdf by guest on 03 October 2021 LITHOLOGY 459

Spring's are clearly spring deposits, but these are not typical, for they contain little or no gravel. The sands and sandstones are gray or brown or red and almost uni versally weather into a dark-red sand. The grains are quartz coated with red iron oxide, but in places where this red coating has been worn off by the wind the sand is gray or even white. The few clays are generally sandy, but some are almost fat. The surficial material derived from the Reynosa is typically deep-red, pink, or gray sand, through which white or gray limestone projects at many places. Laboratory analysis of a number of samples of limestone from the Reynosa formation shows that 70 per cent to 90 per cent is CaC03, with

F ig u r e 2 .—Reynosa Gravel Location is on the International and Great Northern Railway at Greens, Webb County.

an average of 78 per cent; that 6.4 per cent to 20.4 per cent is quartz, orthocla.se, plagioclase, and minute quantities of heavy minerals, with an average of 13.3 per cent, and that 2.0 per cent to 17.6 per cent is fluffy or flaky white or buff clay, with an average of 12.1 per cent. Some of the percentages given for CaC03 include small amounts of manganite. The colors include gray, white, cream, and tan. A typical mechanical analy sis of the insoluble residue is given in figure 3, which shows a textural range from medium sand to clay, with a double maximum in the fine sand and clay grades. In all samples a large proportion of the grains in grades

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/37/3/455/3430023/BUL37_3-0455.pdf by guest on 03 October 2021 460 A. C. TROWBRIDGE--- REYNOSA FORMATION IN TEXAS

above 1/16 millimeters are rounded and some of the grains are well rounded and frosted, indicating eolian action. Some of the samples have a roughly radial and concentric structure, in which the laminae curve and govern the distribution of the grains, and others have horizontal laminae with the sand grains oriented and the CaC03 evenly distributed.

T hickness The thickness of the formation near the Rio Grande is not definitely known. The patchy deposits west of the Bordas do not exceed 30 feet in thickness, but at the east border of the main belt of outcrop the formation

Per DUm. Cent, in mm. g s s s i — 1048 1 1 1 1 1 1 1 1 1 ------______Vo* 02061 ----- 32------16______8______-2___ -1____t

— 1/512 —1/1084 —J/2048

F ig u r e 3.—Mechanical Analysis of Residues from Sample 'Number 02051 This sample is from the Bordas scarp, 10 miles southeast of Aquilares. The analysis was made after dissolving 70 per cent of its contents in dilute acid.

may be 500 feet thick or more, and farther north it is reported by Deus- sen11 to vary in thickness from 200 to 1,505 feet.

Origin and H istory Recorded Such an unusual mixture of limestone with clastic sediment, including coarse' gravel and clay, the whole making up a formation of such thick ness and continuity, constitutes a problem in geological genetics of more than usual interest and importance—a problem upon which it is hoped the field and laboratory work on which this paper is based casts light. After the Oakville sandstone was deposited, and probably after the

11 Alexander Deussen : Ibid., p. 103.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/37/3/455/3430023/BUL37_3-0455.pdf by guest on 03 October 2021 ORIGIN AND HISTORY RECORDED 461 formation of the Lapara sand and Lagarto clay, the coastal plain of south west Texas appears to have been slightly elevated, tilted, and eroded, so that a peneplain was formed, beveling all formations and doubtless ex tending northward and westward across the Edwards Plateau toward and perhaps to the Cordilleran Mountains of west. Texas and New Mexico. On this low-lying surface the products of weathering accumulated. The cherty Cretaceous limestones of the Edwards Plateau gave rise to cal careous, agrillaceous, and arenaceous residual soils and to fragments of chert which could not be dissolved and were left at the surface. The higher surfaces west of the Edwards Plateau were probably subdued and likewise covered by residual materials, such as clay, sand, and fragments of vein quartz. The Tertiary sediments of the coastal plain were similarly weathered to form surficial residual soils consisting of clay, sand, silicified wood, et cetera. On this whole surface such residual materials probably accumu lated to considerable thickness. At some time late in the Tertiary, probably in the Pliocene, this con dition was changed by a diastrophic disturbance in which the Balcones fault was formed, the Edwards Plateau and perhaps the plateaus and mountains farther west were upthrown and elevated, and the coastal plain was downthrown and left as a low and relatively reliefless plain, extend ing from the foot of the Balcones scarp to the Gulf coast. The elevation of the surfaces west of the escarpment, bv increasing the declivity and perhaps by interference with winds giving rise to increased precipitation, caused the rejuvenation of streams, which were thus enabled to remove large quantities of the accumulated residuum, even including the fragments of chert and other insoluble residues. Some of the streams in time penetrated the surficial materials and eroded their valleys into the unweathered rocks below. This elevation also increased the ground-water circulation and gave rise to gulfward-moving water beneath the surface, carrying large quantities of calcium bicarbonate and other soluble salts in solution. The materials thus removed from the elevated surface to the west were deposited on the relatively depressed plain south and east of the Balcones scarp. The shallow valleys were filled and the streams wandered over the general surface, spreading a more or less continuous sheet'of sediment. In general, the finer materials were carried on to the Gulf and the coarser ones were left on the surface to make up the Beynosa gravels. Thus a sort of piedmont plain was formed topographically and a sort of compound fan lithologically.

XXXI— B u l l . G e o l . S o c . A m ., V o l . 37, 192D

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/37/3/455/3430023/BUL37_3-0455.pdf by guest on 03 October 2021 462 A. C. TROWBRIDGE--- REYNOSA FORMATION IN TEXAS

The larger and more vigorous streams, such as the Rio Grande, carried and deposited the coarser gravels. Most of the pebbles and cobbles of the gravels and conglomerates came from the Edwards Plateau, but the Rio Grande brought vein quartz and igneous rocks from the western mountains across the plateau, to be deposited on the qoastal plain with the rest. The limestone of the Reynosa was contributed mainly by ground water and perhaps to some extent by the streams. The ground water, rich in dissolved calcium carbonate, coming from the elevated western area with invigorated circulation, stood high in the pores and close to the low sur face of the eastern plain. Although rainfall was at least fairly abundant in the high plateaus to the west, the coastal plain was arid or semiarid, then as now, and evaporation caused the precipitation of the ealcium car bonate to form the limestone. Some of the precipitation took place in the pores of the gravel near the surface, as the surface was built up by the deposition of the gravel. Some of the limestone, such as that near Heb- bronville and elsewhere in the main belt of outcrop, and that at Carrizo Springs and at Los dos Hermanos, in Webb County, was precipitated at the surface from springs, forming cones and terraces where the springs issued, and deposits of bolson and alkali-flat types in shallow basins into which the spring water flowed and evaporated. Perhaps, also, some of the stream water flowed into sloughs, bayous, resacas and lagoons, and into shallow flats, where it was evaporated and deposited its dissloved salts. The laboratory analyses, which show rounded and frosted sand grains embedded in the limestone, demonstrate that sand and dust were shifted about by the wind and blown into these surface waters to be in closed in the tufEaceous deposits. The faulting which caused and conditioned the deposition of the Rey nosa was doubtless renewed from time to time, as deposition on the coastal plain progressed, and that portion of the plain near the fault line or zone was probably dragged up somewhat periodically, resulting in partial or complete removal, or at least a thinning, of the surficial sedi ment there and a thickening of the formation toward the coast. Thus the Reynosa came to be thicker east of the Bordas scarp than west of it. After the^ deposition of the Reynosa its landward margin, where the sediments were thin, was eroded until only remnants remain on the higher portions of the surface, and the thicker deposit nearer the coast remains as a continuous formation dipping below the Pleistocene and Recent coastal clays.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/37/3/455/3430023/BUL37_3-0455.pdf by guest on 03 October 2021