"The Geology of Tarrant County" by W.M. Adkins and W.S. Adkins

[ pages: 1-23 ... 24-39 ... 40-59 ... 60-85 ... plates ... index]

University of Texas Bulletin
No. 1931: June 1, 1919

THE GEOLOGY OF TARRANT COUNTY
By
W. M. WINTON AND W. S. ADKINS

BUREAU OF ECONOMIC GEOLOGY AND TECHNOLOGY
J. A. Udden, Director

DIVISION OF ECONOMIC GEOLOGY
J. A. Udden, Head of the Division

Published by the University Six Times a Month, and Entered as Second-Class Matter at the Postoffice at Austin, Texas, Under the Act of August 24, 1912.

2
University of Texas Bulletin
The benefits of education and of useful knowledge, generally diffused through a community, are essential to the preservation of a free government.

Sam Houston

Cultivated mind is the guardian genius of democracy .... It is the only dictator that freemen acknowledge and the only security that freemen desire.

Mirabeau B. Lamar

TABLE OF CONTENTS LIST OF ILLUSTRATIONS...................................... GENERAL DESCRIPTION OF THE AREA............. PHYSIOGRAPHY AND TOPOGRAPHY................ Topographic Divisions of Tarrant County............... Black Prairie........................................................ Eastern Cross Timbers........................................ Grand Prairie....................................................... Western Cross Timbers....................................... Drainage................................................................... Geological Map...................................................... Type Localities in Tarrant County.........................	Page 5 7 9 9 9 10 11 12 12 14 15
FOSSILS AS HORIZON MARKERS...................... Mode of Preservation.............................................. Possibility of Recurrent Horizons........................... Utility of Fossils in Interpreting Well Logs............ Table of Identification Values of Comanchean and Cretaceous Fossils.................................................	16 18 18 19 21
THE GEOLOGICAL SECTION............................... Description of Formations...................................... Pennsylvanian..................................................... Comanchean......................................................... Trinity Division................................................... Trinity (Basement) Sands.................................... Glenrose Limestone.............................................. Paluxy Sands......................................................... Fredericksburg Division........................................ Washita Division................................................... Goodland .............................................................. Kiamitia Marl....................................................... Duck Creek Formation......................................... Fort Worth Formation.......................................... Denton Marl........................................................ Weno Formation.................................................. Pawpaw Formation.............................................. Mainstreet Formation........................................... Grayson Marl...................................................... Woodbine Formation........................................... Upper Cretaceous.................................................... Eagleford Shales.................................................. Cenozoic and Recent................................................	24 25 25 25 25 26 26 26 26 33 33 33 39 51 58 61 67 69 72 74 82 82 84
ECONOMIC GEOLOGY.......................................... Resources of Location.............................................	84 85

4   University of Texas Bulletin


    Resources of Soil................................................
    Resources of Plant and Animal Life..................
    Resources of Crude Material...........................
    Limestone Industries....................................
    Gravel and Sand...............................................
    Clay Industries..................................................
    Possibilities of Oil and Gas.............................
    Water Resources...........................................

DIP, ALTITUDES, WELL LOGS, ETC..................
    Dip..........................................................................
    Altitudes in Tarrant County..................................
    Precise Levels in Tarrant County.......................
Description of New Magnetic Station..................
Log of Well at Polytechnic, Texas. .....................
Log of well near Mansfield, Texas........................

INDEX..........................................................................

TOPOGRAPHIC MAP OF THE T. C. U. VICINITY......

GEOLOGICAL MAP OF TARRANT COUNTY..........

Page
87
88
89
89
91
92
95
97

100
100
101
102
107
107
115

119

122

122

The Geology of Tarrant County 5

LIST OF ILLUSTRATIONS

FIGURES

		Page
Fig. 1.	Columnar Section of the Frederickburg Limestone in the Valley of the West Fork of the Trinity River near the Lake Worth Dam...................................	31
Fig. 2.	Columnar Section of the Kiamitia Formation in Cut on the Azle Road, eight miles, northwest of Fort Worth......	36
Fig. 3.	Columnar Section of the Duck Creek Formation in Cuts near Forest Park, Fort Worth.........................	41
Fig. 4.	Columnar Sections of the Fort Worth Limestone. A. On Sycamore Creek, northwest corner of Glenwood Park. B. Eastward facing Exposure in a run one-half mile east of Texas Christian University and one-half mile south of Forest Park, Fort Worth.....................	53
Fig. 5.	Columnar Section of the Denton, Weno and Pawpaw formations on Sycamore Creek, 3 miles southwest of Fort Worth.....................................	63
Fig. 6.	Columnar Section of Portions of the Woodbine Formation in Cuts of the Rock Island Railway between Tarrant Station and the Tarrant-Dallas County Line.............	79

PLATES
(Inside back cover)

Plate 1. Goodland Limestone (top of Fredericksburg Division) at the Lake Worth Dam, 8 miles northwest of Fort Worth.

Plate 2. Fig. 1. Top of Goodland (Fredericksburg) Limestone, looking east from near Benbrook. Figure 2. Goodland Limestone, Stove Foundry Road [Vickery], 4 miles west of Fort Worth.

Plate 3. Fig. 1. Top of the Goodland Limestone, looking east from near Benbrook. Fig. 2. Terraces in the Fredericksburg Division. Fig. 3. Terraces in the Basal Washita Division.

Plate 4. Fig. 1. Base of the Fort Worth Limestone, which underlies the business section of the city of Fort Worth. Fig. 2. Gate posts and flower beds in Forest Park, built of

6 University of Texas Bulletin

the large Ammonite, Desmoceras brazoense [Eopachydiscus], of the Duck Creek Limestone.

Plate 5. Fig. 1. The large Ammonite, Schloenbachia species J. [Mortoniceras], which characterizes the Fort Worth Limestone. Fig. 2. Illustration of the terms "cast" (left) and "mold" (right).

Plate 6. Fort Worth Limestone in Excavation at the northwest corner of Eighth and Houston streets.

7
THE GEOLOGY OF TARRANT COUNTY
BY W. M. WINTON AND W. S. ADKINS¹
GENERAL DESCRIPTION OF THE AREA

Tarrant County is in the third tier of counties of the east central province of Texas, and is included within the belt of densest population in the state. The estimated population of the county, January, 1919, is about 148,000, of which 130,000 is concentrated in the city of Fort Worth. This city, like the cities of Dallas, Waco, Austin, and San Antonio, lies on the outcrop of the calcareous formations of the Cretaceous system, Limestone areas have always been the areas of the greatest industrial development and its accompaniment of concentrated population: and of limestone areas those of the Cretaceous have in the past been the most favorable. Many writers have commented on the carious relationship between centers of civilization and the outcrops of Cretaceous rocks. In fact, even today, a map of the world showing the areas of greatest human development would bear a striking resemblance to the map of the world's outcrops of Cretaceous rocks. Many explanations have been attempted of this suggested connection. The simple facts seem ample. Cretaceous limestones furnish an abundant water supply, a firm substratum, break down into rich soil, furnish material for the construction of roads and buildings, and by their mode of weathering furnish a land surface which is a perfect compromise between the level surface most favorable to the development of elaborate transportation, and the rolling surface most favorable for perfect drainage.

Tarrant County epitomizes in a way the agricultural industries of the state. The western part of the county is in the rolling short-grass prairie region of West Texas, which is typical grazing land; while the eastern part of the county is made up of timbered country and bottom lands, including a typical cotton farming country. In a biological sense the county lies at the junction of the humid and semiarid divisions of the Lower Austral life zone. Its location implies a great mixture of wild life, both plant and animal; and such a condition exists.

¹ The order of names does not in any way indicate seniority. Ms accepted June 1, 1919, published March 1920.

8 University of Texas Bulletin

Besides this, the county lies at what might be considered the junction of life zone in a north-south distribution; that is, it contains life forms which belong both to the sub-tropical area and to the warmer temperate zones. There is probably no other region, in the state which shows such a variety of wild, life, especially of plant life, exhibiting a blending of semi-arid with humid forms and temperate with sub-tropical forms.

Tarrant County lies roughly between longitude W. 97° 1.8' and longitude W. 97° 32.6', and latitude N. 32° 33.3' and latitude N. 32° 59.3', giving it an area of about 903 square miles. The magnetic declination at the station of the United States Coast and Geodetic Survey on the Texas Christian University campus at Fort Worth is 9° 27.4' east. On the ordinary pocket compass used by hunters, boy scouts, and field geologists, reading to a minimum of two-degrees, the declination is ten degrees to the east. That is, when the pocket compass is held so that the needle points to 10? east, the zero or north mark of the compass is pointing to the true north.

The entire county is covered by rocks belonging to a single geological age of the Cretaceous, about equally divided between lower Cretaceous, or more properly Comanchean, and Upper Cretaceous. The underlying rocks dip gently toward the south east. The rocks are hard chalky limestones, soft limestones, marls, and red sandstones. The dip of the strata gives, in general, long gentle slopes on the southeast side of the uplands, where the surface of the ground approximates the dip of the underlying rocks, and it gives rather abrupt slopes on the northwest side of such uplands, where the successive ledges of the different formations emerge from below the surface. This is the so-called "cuesta" type of topography. Despite its recognizable nature, with the gentle dip plains on one side and steep bluffs on the other side, the fact must be kept in mind that in many cases the "dip plains" are not true dip plains, and the abrupt bluffs are not always the "ends'' of formations. This statement is made here because many writers in discussing this feature of North Texas topography have taken rather too literally the proposition that this cuesta type of topography indicates exactly the position, thickness, and attitude of the formations.

The Geology of Tarrant County 9

The underlying geological formations, aside from climatic factors, are the most important natural features in determining the configuration of a region, and therefore, also, its possibilities for cultivation and its suitability for a dense population location of large cities, and other cultural developments.

The area included in Tarrant County illustrates this intimate relation between the geology of a country and its surface features. A complexity of surface features is possible with a combination of alternate hard and soft strata, an even dip, and a well developed drainage. These factors result in a striking diversity of land forms and a pleasing variety of scenery.

The highest point in the county is in the western part, where an elevation of 1050 feet above sea level is reached. The lowest point is where the Trinity River passes out of the county to the east. This latter point has an elevation of 450 feet, giving the entire county a maximum relief of 600 feet. In general this is represented by a gradual slope to the east, broken by the westward facing escarpments. The escarpments are steep and conspicuous, but the much longer and gentler eastern slopes are not so readily noticed. The trip from Fort Worth to Dallas "by the automobile pike is decidedly downhill in its entirety, the fall being about 160 feet in a distance of a little more than thirty miles; but because of the several conspicuous escarpments: which must be ascended in traveling east, the average individual fancies that he is traveling mostly up hill.

TOPOGRAPHIC DIVISIONS OF TARRANT COUNTY

The area included within Tarrant County consists of four broad belts, two prairie strips alternating with two sandy strips or "cross timber" zones. These are as follows, from east to west: the Black Prairie, the Eastern Cross Timbers, the Grand Prairie, and the Western Cross Timbers.

The Black Prairie has only a small part of the lower and western portion of its great area in Tarrant County. This black land prairie is underlain by the whole Cretaceous series, above

10 University of Texas Bulletin

the Woodbine sand¹. Due to the dip of the strata toward the Gulf Coast, this is the lowest of the belts in altitude, but the highest in the geological series. It is a gently rolling slope with a treeless surface. The soil is black, waxy, and carbonaceous. It is deep and of a superior grade. Because of the deep soil, the cuesta topography is not prominent, but even in the soft marly shales, as in the eastern part of Tarrant County, the escarpment and dip plains are recognizable. The Eagleford subdivision of the Black Prairie is separated from the Austin chalk subdivision by the prominent Austin chalk escarpment which extends for hundreds of miles across Texas parallel to the general direction of outcrop of the strata, and is finely exposed in White Cliff on the Fort Worth-Dallas pike, a few miles west of Dallas. This westward-facing escarpment overlooks the successive strips of Eagleford prairie, which arise from under it, and ascending westward divide the prairie by small crests which are themselves miniature escarpments. These correspond to the subdivisions of the Eagleford formation. Approaching Port Worth from the east, these rise in altitude until just east of Arlington the top of the Woodbine sand emerges from underneath the basal member of the Eagleford shales.

The Eastern Cross Timbers are underlain by a homogeneous strip of red sandstone composing the Woodbine formation; and are heavily timbered with black jack oaks and post oaks. The exposure of the Woodbine formation is an important catchment area for water, as noted by Dr. Hill, furnishing artesian water in areas far to the east of Tarrant County where this sand is deep underground. A conspicuous feature of the rock under the Cross Timbers is its tendency to form large lens-like masses and to show cross bedding. These features should be considered very cautiously by prospective oil drillers and are explained and discussed under the section on the Woodbine formation At the western border of the Cross Timbers where the limestone emerges, an interesting feature is the string of small "islands"' or outliers of the red sandstone. These form the numerous "Brushy Knobs", seen in this strip from the Red River to the Brazos. Some of

¹ The formations referred to in this discussion are subsequently more fully described under the section on Geology.

The Geology of Tarrant County 11

these ''knobs" are distinctly peak-like, and are possibly due to lenticular masses in the sandstone.

The Grand Prairie has a surface which is varied corresponding to the diverse groups of rock strata underlying it. The columnar section (fig. 1-6) indicates roughly a series of limestone layers alternating with marl members. The limestones weather out as upland surfaces which are more or less flat, except for the effects of erosion, while the softer marl groups break down more readily under weathering and are exposed as slopes connecting the levels of the limestones above and below. This produces a terrace effect which is one of the features of the region. The limestones do not always or even usually weather out into dip plains whose surfaces are entirely upon a single stratum. This sort of plain is rare, despite frequent reference to such occurrences in the geological literature of this region The most common upland slope is an erosion slope dipping in the direction of the greatest dip of the underlying strata but also slanting off towards the drainage and consequently not lying entirely on one stratum, but including many successive strata. The statement that the cuesta type of topography prevails must be accepted with reserve so far as Tarrant County is concerned.

The Grand Prairie has two areally prominent members, the Mainstreet limestone and the Fort Worth limestone. These together make up 80 per cent of the total area; and the other members of the series form essentially narrow strips connecting these two with the adjoining areas and with each other. For instance, the Grayson marl connects the eastern prairie strip, the Mainstreet limestone, with the Woodbine sand. Although fifty feet thick, the Grayson is an inconspicuous strip areally (exaggerated in mapping) and the formation is very rarely exposed or entirely free from overwash. The Mainstreet with the same thickness forms a broad upland many miles wide. At many places the Denison beds, the Denton, Weno, and Pawpaw formations, form a sharp, even slope connecting the Mainstreet limestone upland of the eastern side with the Fort Worth limestone upland of the western side. Yet the thickness of the Denison beds far exceeds the combined thickness of the Mainstreet and Fort Worth limestones:

12 University of Texas Bulletin

The Western Cross Timbers and the adjoining rocks to their east are complicated by the greatly dissected headwaters of the Trinity River. This dissected lowland is likewise composite, and is underlain by three groups of strata in addition to the river alluvium. The three groups are timbered but are very dissimilar in the rock composing them.

The upper and eastern strip is the Walnut formation, which is conspicuous by the great amount of slabs composed entirely of Grypheas, a fossil oyster-like shell. The region is considerably dissected into steep hills, deep laterals of the Trinity River, with frequent waterfalls, cliffs and small rapids. It is well timbered with post-oak, black-jack oak, and water oak. Next underlying it geologically, and farther west, is a strip of Paluxy sand. This weathers into flat bottom lands of dull reddish sand. The Glenrose limestone, which is westward and immediately under the Paluxy sand, is not exposed at the surface in Tarrant County.

Throughout the four great belts of country above described, there are two types of gravel, an upland and a lowland type, and probably several stages of alluvium. These two gravels are recognized commercially as "pit" (upland) and "stream" (lowland). In these gravels are found the few fossils of the Pleistocene age which are known for this region, such as mammoths, and a few others.

DRAINAGE

Tarrant County is drained entirely by the Trinity River. The Clear Fork of the Trinity, arising in Johnson County near the Noland's River divide (Brazos drainage) passes northeast to join at Fort Worth the main branch of the Trinity, the West Fork, which rises in Wise County. These two forks drain the entire western half of the county, and the east half is drained by smaller laterals of the Trinity, such as Sycamore, and Big and Little Fossil creeks. The Trinity descends from an elevation of 1050 feet at the southwest corner of the comity to 450 feet at the Tarrant-Dallas County line, giving a drop of 600 feet in 40 miles, or 15 feet per mile. Much of this drop is almost in the strike, for where the river runs directly across the outcrops of the formations near Fort Worth, the rock strata are

The Geology of Tarrant County 13

dipping eastward shout ten times as fast as the river. The result is that the river in passing eastward crosses successively younger formations until northeast of Arlington it has passed into the Eagleford shales.

The drop of the Trinity River is rather even across Tarrant County where throughout its course it has a timbered lowland alluvial floodplain, increasing in width to five miles at the eastern border. The floodplain is bordered by Goodland and Duck Creek escarpments capped by Goodland and Fort Worth uplands west of Fort Worth, and east of Fort Worth by Weno and Pawpaw escarpments capped by Mainstreet uplands. In the southwest part of the county the Clear Fork of the Trinity passes through a Goodland floodplain as described, but in the northwest the West Fork cuts down a much dissected head-waters region. To the east the alluvial floodplain is bordered by Cross Timbered Woodbine sands.

The strata of Tarrant County have suffered erosion from the Trinity drainage according to their hardness and other characteristics so that the topography of the areas bordering the river valley shows three general types: (a) Dissected Head-waters; (b) Dissected Uplands; (c) Cross Timbered Bottom Lands. These types have distinct individualities, divide the county into natural regions by producing each a different topography and scenery, and have differing possibilities of development.

Dissected Headquarters of the Trinity: —This is an area of about one-sixth that of the county, on the upper course of the West Fork. The inequalities in hardness and composition of the underlying Walnut clays and shell marl produce a very rough and precipitous headwaters region whose aspect is totally different from that of the uplands. The laterals of the Trinity run mostly in deeply cut narrow valleys which are tortuous and carry swift streams during the rainy season. The hills make indented and irregular ridges converging towards the river valley and to the east capped by Goodland limestone. From the top of the Goodland escarpment are broad vistas of this dissected timbered landscape ascending toward the west and uncovering in its turn the underlying sandy laud of the Western Cross Timbers. (See page 26.)

14 University of Texas Bulletin

Dissected Uplands: — These strips of upland prairie, mainly underlain by the Fort Worth and Mainstreet limestones (page 51) are topographic units over the whole Cretaceous area in North Central Texas, where their direction of outcrop is a little east of north. It is notable that several railroads approaching Fort Worth from the north and south have found at the same time an even gradient and a firm substratum by running in the strike of these formations.

It has been mentioned that while the outcrops of these upland limestones and their interbedded marl formations conform in general to the cuesta type of topography, it is nearly always with modification due to erosion slope toward the drainage bottoms. The uplands are covered with 'young' stream valleys cutting their way into the original dip plains or their remnants and producing a rolling topography. This upland dissection is active and rapid in Tarrant County, while in the adjoining counties of the Cretaceous strip the exposures seem rarer. As the Trinity cuts across these limestones it produces the conspicuous escarpments which border its valley for over half its course in the county. The city of Fort Worth owes its favorable location to the Fort Worth limestone escarpment as exposed along the south bank of the Trinity and its resulting good drainage and broad outlook over the adjacent valley. Although the Trinity is a small stream at low water there has been considerable lateral swing as evidenced by the comparatively broad valley, the bordering escarpments, the fluviatile shell deposits, and other features.

GEOLOGICAL MAP

Because of the lack of an aerial map which is reliable enough to be used as a base for geological mapping, our base map is composite and contains certain errors which affect the exactness of the formation contacts. In general it may be pointed out that the formations most nearly follow the roads as mapped, although even sight compass intersections show the roads to be slightly mislocated in places. In addition an even degree of refinement has been attempted over the whole area, and this has necessitated a simplification of the geology at places where more detail could have been shown. On the map the finest pen

The Geology of Tarrant County    15

line drawn represents a width of 106 feet, and at many places the geology can he mapped more closely than this. It is hoped that there may be produced an aerial map on which the known detail of Tarrant County geology can be represented.

Tarrant County has been more worked than any other county in the North Texas Comanchean, except Grayson County. It is the scene of investigations by Bill, Taff, Leverett, and others. Part of the county or all of it is included in the following maps.

1892 *Taff and Leverett: Cretaceous Area North of the Colorado
River. 3rd Annual Report, Texas Geological Survey.

1898 *Hill: Black and Grand Prairies of Texas.   21st Annual Report, Part VII, U. S. G. S.

1898 Reconnaissance Map, U. S. G. S., Fort Worth Sheet.

1912 J. B. Hawley, Topographical map of the Lake Worth region.

1913 J. C. Travilla: Road map of Tarrant County.

1916 *Geological Map of Texas; Udden, Baker and Bose. Bureau of Economic Geology and Technology, University of Texas, Bulletin 44. (Third edition, 1919).

----   Missouri, Kansas and Texas Railway of Texas. Contour map of 4 square miles southeast of Fort Worth, Texas.

1918-19 Corps of Engineers, U. S. Army. Progressive Military Map, Advance Sheet 487 N, II & IV.

TYPE LOCALITIES IN TARRANT COUNTY

Within this county are found the type localities of many of the species described by Cragin and Clark. For convenience of reference there is here given a list of these localities, the original terminology being quoted:

Epiaster elegans var. praenuntius Cragin¹ Comanche Peak, Benbrook.

Epiaster hemiasterinus Cragin¹ Grayson marl, 6 miles east of Fort Worth.

Heterodiadema ornatum Clark² Washita group, Fort Worth.

Leptarbacia argutus Clark² Washita group, Fort Worth.

*Geological map.
¹ Cragin, 4th Ann. Rept. Tex. Geol. Sure., 1893.
² Clark and Twitchell, U. S. G. S., Mon. LIV [54], 1915.

16 University of Texas Bulletin

Ophioglypha texana Clark³ Denton marl, 6 miles north of Fort Worth, on the banks of Little Fossil Creek.

Astarte acuminate Cragin¹ Weno marl, 3 1/2 miles east of Fort Worth.

Vola bellula Cragin¹ Half mile above T. P. bridge on Sycamore Creek, near Fort Worth.

Exogyra plexa Cragin¹ T. P. Ry., 3 miles east of Benbrook, 20 feet below the top of a bed of Exogyra texana

Stearnsia robbinsi White³ Little Fossil Creek, 6 miles north of Fort Worth.

Dalliconcha invaginata White³ Same.

Cinulia tarrantensis Cragin¹ Top of Exogyra Texana bed, T. & P. Ry., 2½ miles east of Benbrook, with Tylostoma mutabilis and Cylindrites formosus.

Cylindrites formosus Cragin¹ Same.

Trichotropis shumardi Cragin¹ E. texana beds. 1½ miles east of Benbrook.

Neritopsis tramitensis Cragin¹ Woodbine sands, top, Bear Creek, Tarrant County, near Dallas County line.

These localities are all still recognizable. The type localities of the Fort Worth limestone (Hill⁴) are practically obliterated by overwash and by street grading.

FOSSILS AS HORIZON MARKERS

It is desirable to place on record here certain observations regarding the range of the fossils found in the formations ex-

¹ Cragin, 4th Ann. Rept. Tex. Geol. Surv., 1893.
³ Hill, Bull. Geol. Soc. Amer., 5, 1893, p. 328.
⁴ Hill, Amer. Jour. Sci., 1888: and 21st Ann. Rept., U. S. G. S..
Pt. VII, 1901, pp. 259-61.

The Geology of Tarrant County 17

posed in this county, and to indicate so far as practicable the relative utility of different species horizon markers, including the mode of preservation of the fossils, and the possibility of recurrent horizons.
In Tarrant County the limestones and interbedded marls follow each other in alternating beds, so that the whole geological column presents a remarkable uniformity. In the first place, the formations alternate between harder and softer groups of rocks, so that each limestone formation is overlain by a soft marly or clay formation. For example, the Goodland limestone is overlain by the Kiamitia marl; then comes the Duck Creek limestone, overlain by the Duck Creek marl; then the Fort Worth limestone, overlain by the Denton shell marl; then the Weno limestone and marl, overlain by the Pawpaw clay; then the Mainstreet limestone, overlain by the Grayson marl. In the second place, within a formation are alternating lime and mail strata.

With such a confusing similarity of strata some means of locating the exact geological level is necessary other than that of a mere inspection of the rock. This is furnished by certain of the fossils contained in the beds, of course some fossils run throughout the series or through considerable portions of it, and therefore are worthless as horizon markets. Of these practically nothing will be said in this paper. Certain others are strictly limited in their range and occur only at particular levels. These are key fossils, or horizon markers. If their range of occurrence in Tarrant County is roughly less than ten feet vertically, the range, in this paper, will he called the zone of occurrence. If' it is two vertical feet or less, it will he referred to as a restricted zone. If it is more than ten feet, but of limited range, it will he referred to as a horizon. Some fossils have zones in which they are particularly abundant, lying within horizons in which they occur, but less abundantly. Some fossils appear in recurrent zones, that is, zones between which the fossils in question have never been found, even after considerable search, although admittedly they might occur there. It seems likely that many fossils now known only at certain levels will be found more widely distributed; and that the main reliance is to be placed upon their zones of abundance as here outlined.

² Tarrant

18 University of Texas Bulletin

MODE OF PRESERVATION

Fossils in this region have been found in the following modes of preservation:

(a) In the condition of lime compounds, as aragonite, and calcite. This is the most common mode. There are two conditions: The original shell material may be replaced by lime, preserving the original appearance and the material may be either indurated or chalky; or the original shell may be present, sometimes with the nacre and lustre of the living form.
(b) In the condition of iron compounds, as pyrite, hematite, or limonite.
(c) In the condition of silica, as in some fossilized or opalized wood fragments and sandstone fossils.
(d) In the condition of mud casts, consisting of compacted marl, often yellowish, and differing from ordinary shells in appearance, usually casts of the interior of the original shell. These casts are especially prevalent in the Goodland limestone, but are common also in the Washita division. The impressions may be internal, as casts; or external, as moulds.

If the original shell is replaced by iron oxide it is then called an iron pseudmorph. Sometimes, but rarely, calcareous shells are colored pinkish.

POSSIBILITY OF RECURRENT HORIZONS

Certain species which are doubtfully recurrent will be mentioned. The matter of recurrent forms is at present debatable, and we confine ourselves to bare facts. To establish recurrence in a given region, a fossil must occur in certain zones and not between these, and it must be proven that the recurrent zones contain the same species. If these things are true, there was at intervals a presumable migration of the species in and out of the region to one or more "reservoirs" in other regions. In practice it is essential not to confuse fossils from different levels by supposing them to indicate the same level. The chief recurrent forms are:

(a) Kingena wacoensis and other species. In Tarrant County, these brachiopods have been found in only four zones: base of the Duck Creek limestone; Lower Kingena zone and upper Kingena zone of the Duck Creek marl; and base of the Mainstreet

The Geology of Tarrant County 19

limestone. Of these, the Kingena zone near the base of the Mainstreet limestone is the Kingena zone of the literature, and is stated to lie at the top of the Georgetown limestone at Georgetown, Austin, and other Central Texas localities. At Blum, in Hill County, near the Brazos River, brachiopods occur near the top of the Gryphea and Ostrea conglomerate of the Denton formation, and at the classic locality on Duck Creek north of Denison and other places on the Red River they occur at the base of the Hamites zone in association with fossils of this zone. The Mainstreet zone extends from the Red River to the Rio Grande, according to Hill. The Duck Creek marl zones extend for miles in every direction from Fort Worth, but have not yet been found at the Red River. We know of no occurrences between the zones above mentioned. For practical purposes, it can he said of the Kingenas, first, that they are not confined to one zone; and second, that there are probably several species of brachiopods in these zones.

(b) Ostrea carinata Lamarck. The taxonomy is again confused here and the Texas material requires critical study and comparison with European individuals. This is one of the two or three species still remaining from a large number of species formerly thought to be common to the Texas and European Cretaceous. The zones of recurrence of this species have been so greatly extended that they are only doubtfully recurrent, yet between these zones these forms have not yet been found in the Texas region. In Europe where the species ranges widely in the Cenomanian it is not considered to be recurrent.

(c) Gervilliopsis invaginata (White.) The situation is essentially the same as for Ostrea carinata.

(d) Exogyra americana Marcou. This large conspicuous oyster occurs near the top of the Duck Creek limestone near Denison (Dr. Boese) and persistently in the top of the Fort Worth limestone in Tarrant County and other places. It has so far not been found elsewhere.

UTILITY OF FOSSILS IN INTERPRETING WELL RECORDS

All of the following fossils are of much practical importance, since even from minute fragments such as are found in well drilling,

20 University of Texas Bulletin

the level may be determined always approximately, and sometimes with exactness. These levels have been carefully checked and their sequence verified in the field in numerous localities and over wide areas. Where there is variation within Tarrant County, this is stated under the discussion of the fossil horizon.

These key fossils will be seen to unequal advantage in well washings. If, as sometimes estimated, the largest compact fragment likely to be found in a standard rotary drill cutting is about 5/8 of an inch, it is evident that some fossils will survive the drilling process intact, while others will be ground to unrecognizable fragments. Fossils will survive because of their small size, as some of the small pyrite fossils mentioned in this paper; or because of their hardness, as certain Grypheas; or because of both, as Kingena; or because, of their abundance, as some of the Grypheas in the shell marls; or for other reasons. With the number of key fossils cited in this paper, it is believed that a sufficient sample of one or a combination of several fossils can be had from practically any level of the Washita division, to determine the level.

To assist in the certainty of these determinations, the value of the fossils for locating specific levels has been designated in the following table of identification values. The list as given in this table is incomplete and subject to revision. As in the European section the ammonites and echinoids are most reliable for determination of stratigraphic level and other fossils are variable in value. Certain associations and zones of abundance are valuable, as has been explained, if recurrences of the same fossil are carefully distinguished from each other. (See page 18). In the following list, the fossils are arranged within each formation, in the order of their occurrence,
from top to bottom.¹

¹ In this table fossils which are most likely to be found and which have not been found at other levels than those indicated are marked with two asterisks. (**)

Fossils which are less likely to be found, but which identify the level indicated within narrow limits, are marked with one asterisk. (*)

Fossils occurring in zones of abundance or other fossils occurring in zones important for locating stratigraphic levels, are left without asterisk.

The Geology of Tarrant County 21

TABLE OF IDENTIFICATION VALUES OF COMANCHEAN AND CRETACEOUS FOSSILS.¹

Eagleford:
**Ostrea belliplicata, gastropods, fish teeth and bones.
    Schloenbachia sp. P.
*Acanthoceras swallovi (Shumard).

Woodbine:
*Conglomerate of Ostrea soleniscus, Ostrea carica, Barbatia
    micronema (?), Exogyra sp., Aguilera cumminsi, Cerithium,
    fish vertebrae and teeth.
    Dexter sands: no fossils observed.

Grayson Marl:
*Exogyra sp. 1.
*Gryphea mucronata Gabb.
*Exogyra sp. 2 and Exogyra arietina Roemer.
    Pecten subalpina (abundant), Cyphosoma, Hemiaster calvini, etc.

Mainstreet Limestone:
    Leiocidaris (highest),
**Turrilites brazoensis Shumard.
**Kingena wacoensis Roemer (?).
     Schloenbachia sp. O., aff. inflata.
*Exogyra arietina Roemer. ,
    Ostrea quadriplicata Shumard (highest).
*Holectypus sp., aff. limitis Boese.
    Ostrea carinata Lamarck.
    Pachymya austinensis Shumard .
*Pecten cleburnensis Adkins and Winton (sp. 1).
    Pecten sp. 2.

Pawpaw Clay:.
    Hemiaster sp. 1; Nautilus sp. 1.
**Pyrite fauna: Baculites sp.; Scaphites hilli Adkins and
    Winton (sp. A) ; Hamites tenawa Adkins and Winton;
    Enallaster sp. 1; Salenia sp. 1; Trochosmilia sp. 1; Turrilites
    worthensis Adkins and Winton (sp. A) and spp.;
    Mortoniceras sp. A; Acanthoceras sp. A.
*Starfish zone: Metopaster hortensae Adkins and Winton;
    Comptonia sp.; other starfishes:
    Nodosaria texana Conrad.
    Fish teeth, vertebrae, plates.
    Enallaster sp. 3; aff. bravoensis Boese.

Weno Limestone and Marl
   "Quarry group": Ostrea quadriplicata, Ostrea carinata (abundant).
    Gryphea washitaensis (abundant), Homomya sp.
----
¹These and other key fossils are described and figured in: Adkins and Winton, Univ. of Texas Bull. 1945.

22   University of Texas Bulletin

*Pentagonaster texensis Adkins and Winton (sp. 1).
    Schloenbachia sp. L-M.
*Engonoceras sp. 1
*Nodosaria texana Conrad.
    Venericardia sp. 1, Corbula spp. 1-2, Turritella sp. 1.
*Gervilliopsis invaginata (White).
*Pecten georgetownensis Kniker (?), Turritella sp. 2,
    Remondia (?) acuminata Cragin, Hamites sp. B, Cottaldia sp. 1.

Denton Marl:
**Gryphea washitaensis (abundant).
    Ostrea carinata (abundant).
    Protocardia sp., Trigonia sp.

Fort Worth Limestone:
*Enallaster longisulcus Adkins and Winton (sp. 2).
*Nerinea sp. 1.
*Exogyra americana Marcou.
    Ostrea carinata Lamarck.
*Hemiaster elegans Shumard (abundant).
**Pecten bellula Cragin.
*Schloenbachia sp. K (size of austinensis Lasswitz).
**Holaster simplex Shumard (abundant).
*Pecten sp. 3.
    Schloenbachia sp. I.

Duck Creek Marl and Limy Marl:
*Upper Kingena zone.
    Upper Gastropod zone:  Cerithium, Turritella,
    Pleurotomaria, Gyrodes (?).
    Schloenbachia sp. I.
*Pecten wrightii Shumard.
**Lower (Main) Kingena zone.
*Goniophorus sp. I.
*Crania sp. 1.
**Hamites tanima Adkins and Winton; Hamites spp. D-G,
*Pinna sp. 1.
*Scaphites worthensis Adkins and Winton (sp. B).
    Lower Gastropod Horizon: Cerithium, Lunatia, Cinulia. Turtle.
    Nerinea sp. aff. pellucida Cragin, Schloenbachia sp.

Duck Creek Marly Lime and Limestone:
*Schloenbachia sp. H. aff. trinodosa noose.
*Schloenbachia sp. G. *Schloenbachia sp. F.
*Desmoceras sp. B ("brazoensis", "graysonense"?).
*Desmoceras sp. A.
*Inoceramus comancheanus, I. munsoni.
**Hamites comanchensis Adkins and Winton (sp. A);

The Geology of Tarrant County   23

    H. nokonis Adkins and Winton (sp. B);
    H. fremonti Marcou (sp. C); H. spp. H-J.

Kiamitia Marl:
**Gryphea navia Hall.
    Gryphea washitaensis (lowest).
    Pholadomya sp.
*Exogyra plexa Cragin (highest).
*Schloenbachia belknapi Marcou (highest).
*Pecten irregularis Boese (highest).
*Schloenbachia acutocarinata Shumard (highest).
*Exogyra texana Roemer (highest).
    Cyprimeria sp. 1.

Fredericksburg Division:
    Cyprimeria sp. 1.
*Schloenbachia sp. near belknapi.
    Schloenbachia acutocarinata Shumard
*Diplopodia taffi Cragin.
**Coral syndrome: Parasmilia sp., Trochosmilia sp.
*Holectypus sp.
*Enallaster texanus Roemer (abundant).
*Hemiaster sp. near whitei Clark.
**Salenia mexicana. Upper level.
    Exogyra plexa Cragin (small form).
**Syndrome of Chondrodonta sp. aff. munsoni (Hill),
    Pecten irregularis (Boese), Cinulia, Lima, Pinna sp. near
    comancheanus Cragin, etc.
*Engonoceras sp. aff. piedernale von Buch.
*Exogyra plexa Cragin (large form).
**Schloenbachia acutocarinata (Shumard) (abundant).
    Pholadomya sancti-sabae Roemer (abundant).
    Gryphea marcoui Hill and Vaughan (abundant).
*Schloenbachia sp. A.
*Gryphea marcoui (Upper Conglomerate).
    Exogyra texana (Upper Horizon, abundant).
    Salenia sp. (lower level).
    Protocardia filosa (Conrad).
    Cerithium bosquense Shumard.
    Natica sp. aff. pedernalis Roemer.

Walnut conglomerate and clay:
*Schloenbachia acutocarinata Shumard (lowest)
    Enallaster sp., Turritella sp.
*Gryphea marcoui conglomerate.

Paluxy sands: No fossils noted in Tarrant County.

[ pages: 1-23 ... 24-39 ... 40-59 ... 60-85 ... plates ... index ]