After the fossils have been cleaned and tentatively identified, they should be cataloged. This is necessary to enable the collector to have a record of his collection and to furnish as much information as possible about each individual fossil.
The collecting data can be taken from the labels that were placed in each bag of fossils as they were collected, or from the field notebook. Actually, it is wise to check one against the other. This information should then be entered in some type of record book and also placed on a more permanent label which is put in the tray or box with the fossil. The catalog and label should contain such pertinent data as (1) the scientific name of the fossil, (2) the geologic formation from which the specimen was collected, (3) the exact geographic location of the collecting locality, (4) the name of the collector, (5) the date the fossil was collected, and (6) the catalog number of the specimen. The latter is usually placed in the upper right hand corner of the label (fig. 6) and corresponds with a like number in the record book.
Fig. 6. A brachiopod showing the catalog number on it, and the accompanying label that pertains to the specimen.
The entries in the catalog should be numbered consecutively, and all specimens from the same locality should bear the same number. This number should be written on the fossil with India ink, preferably on any remaining matrix or on some inconspicuous part of the specimen (fig. 6). If the surface of the fossil is too coarse or porous for ink, the catalog number can be written on a small patch of white enamel or clear nail polish painted on the specimen. After the ink has dried it should be coated with a dab of clear shellac or clear nail polish to help preserve the number. If each specimen is numbered, it can easily be identified even if it should become separated from its label.
Fossils are useful in a number of different ways, for each specimen provides some information about when it lived, where it lived, and how it lived.
Fossils are very important, for example, in tracing the development of the plants and animals of our earth. This is possible because the fossils in the older rocks are usually primitive and relatively simple; but a study of similar specimens that lived in later geologic time shows that the fossils become progressively more complex and more advanced in the younger rocks.
Some fossils, for example, the reef-building corals, appear to have always lived under much the same conditions as they live today. Hence, it is reasonably certain that the rocks containing fossil reef corals found in place (that is, where they were originally buried), were deposited in warm, fairly shallow, salt water. By studying the occurrence and distribution of such marine fossils, it is possible to outline the location and extent of prehistoric seas. Moreover, the type of fossils present will frequently give some indication as to the bottom conditions, depth, temperature, and salinity of these ancient bodies of water.
Probably the most important use of fossils is for purposes of correlation—the process of demonstrating that certain rock layers are closely related to each other. By correlating or “matching” the beds containing specific fossils, it is possible to determine the distribution of geologic units of similar age. Some fossils have a very limited vertical or geologic range and a wide horizontal or geographic range. In other words, they lived but a relatively short period in geologic time but were rather widely distributed during their relatively short life. Such fossils are known as index fossils or guide fossils and are especially useful in correlation because they are normally only associated with rocks of one certain age.
Fig. 7. Sketches of two types of micropaleontological slides. (a) Multiple space faunal slide. (b) Single-hole slide.
Microfossils are often very valuable as guide fossils for the petroleum geologist. The micropaleontologist washes the well cuttings from the drill hole and separates the tiny fossils from the surrounding rocks. The specimens are then mounted on special slides (fig. 7) and studied under the microscope. Information derived from these fossils often provides valuable data on the age of the subsurface formation and the possibilities of oil production. Microfossils are particularly valuable in the oil fields of the Gulf Coast region of Texas. In fact, some of the oil-producing zones in this area have even been named for certain key genera of microfossils. For example, the “het” zone of Oligocene age (geologic time scale, Pl. 1) is named for the genus Heterostegina, which is a tiny one-celled animal. Other microfossils, such as fusulinids, ostracodes, spores, and pollens, are also used to identify subsurface formations in many other parts of the State.
Plant fossils are very useful as climatic indicators but are not too reliable for purposes of age determination. They do, however, provide much information about the development of plants throughout geologic time.
The geologic history of our earth has been recorded primarily in marine sedimentary rocks, and this record indicates that our earth is very old and that life has been present for many millions of years. The earth is not only extremely old (more than 3½ billion years of age), but it has also undergone many changes which have taken place slowly but steadily and have greatly affected both the earth and its inhabitants. The earth’s physical features have not always been as they are seen today. Geologic research has shown that mountains now occupy the sites of ancient seas, and that coal is being mined where swamps existed millions of years ago. Furthermore, there is much evidence to indicate that plants and animals have also undergone great change. The trend of this organic change is, in general, toward more complex and advanced forms of life, but some forms have remained virtually unchanged and others have become extinct.
In order to interpret geologic history, the earth scientist must attempt to gather evidence of the great changes in climate, geography, and life that took place in the geologic past. The record of these changes can be found in the rocks, and here is found the story of the various events in earth history.
In order to discuss fossils and the age of the rocks containing them, it is necessary to become familiar with the geologic column and the geologic time scale (Pl. 1).
The geologic column refers to the total succession of rocks, from the oldest to most recent, that are found either locally or in the entire earth. Thus, the geologic column of Texas includes all rock divisions known to be present in this State. By referring to the geologic column previously worked out for any given area, the geologist can determine what type of rocks he might expect to find in that particular region.
The geologic time scale is composed of units which represent intervals of geologic time, during which were deposited the rocks represented in the geologic column. These time units are used by the geologist to date the events that have taken place in the geologic past.
The largest unit of geologic time is an era, and each era is divided into smaller time units called periods. A period of geologic time is divided into epochs, which, in turn, may be subdivided into still smaller units. The geologic time scale might be roughly compared to the calendar in which the year is divided into months, months into weeks, and weeks into days. Unlike years, however, geologic time units are arbitrary and of unequal duration, and the geologist cannot be positive about the exact length of time involved in each unit. The time scale does, however, provide a standard by which he can discuss the age of fossils and their surrounding rocks. By referring to the time scale it may be possible, for instance, to state that a certain event occurred during the Paleozoic era in the same sense that one might say that something happened during the American Revolution.
There are five eras of geologic time, and each has been given a name that is descriptive of the degree of life development that characterizes that era. Hence, Paleozoic means “ancient-life,” and the era was so named because of the relatively simple and ancient stage of life development.
The eras, a guide to their pronunciation, and the literal translation of each name is shown below.
Archeozoic and Proterozoic rocks are commonly grouped together and referred to as Precambrian in age. The Precambrian rocks have been greatly contorted and metamorphosed, and the record of this portion of earth history is most difficult to interpret. Precambrian time represents that portion of geologic time from the beginning of earth history until the deposition of the earliest fossiliferous Cambrian strata. If the earth is as old as is believed, Precambrian time may represent as much as 85 percent of all geologic time.
The oldest era is at the bottom of the list because this part of geologic time transpired first and was then followed by the successively younger eras which are placed above it. Therefore, the geologic time scale is always read from the bottom of the chart upward. This is, of course, the order in which the various portions of geologic time occurred and during which the corresponding rocks were formed.
As mentioned above, each of the eras has been divided into periods, and most of these periods derive their names from the regions in which the rocks of each were first studied. For example, the Pennsylvanian rocks of North America were first studied in the State of Pennsylvania.
The Paleozoic era has been divided into seven periods of geologic time. With the oldest at the bottom of the list, these periods and the source of their names are:
The Carboniferous period in Europe includes the Mississippian and Pennsylvanian periods of North America. Although this classification is no longer used in the United States, the term Carboniferous will be found in many of the earlier geological publications and on many of the earlier geologic maps.
The periods of the Mesozoic era and the source of their names are:
In Texas, the Cretaceous has two divisions, known as either Lower Cretaceous and Upper Cretaceous or as Comanche series and Gulf series, respectively. These designations are for rocks of nearly equivalent age, and both sets of terms have been used by geologists and in publications. In this handbook, both sets of terms are used interchangeably, that is, Lower Cretaceous and/or Comanche series and Upper Cretaceous and/or Gulf series.
The Cenozoic periods derived their names from an old outdated system of classification which divided all of the earth’s rocks into four groups. The two divisions listed below are the only names of this system which are still in use:
While the units discussed above are the major divisions of geologic time, the geologist usually works with smaller units of rocks called formations. A geologic formation is identified and established on the basis of definite physical and chemical characteristics of the rocks. Formations are usually given geographic names which are combined with the type of rock that makes up the bulk of the formation. For example, the Beaumont clay was named from clay deposits that are found in and around Beaumont, Texas.
The geologic history of Texas, like the geologic history of the rest of the earth, is recorded primarily in marine sedimentary rocks. These rocks provide some knowledge of the early geography and the first inhabitants of what is now the State of Texas. Most of these rocks were formed from sediments deposited in shallow seas which covered parts of the State at various times in earth history.
By studying these rocks and their relations to each other, geologists have established a geologic column for Texas.
In order to discuss the distribution and exposures of the rocks of Texas, it is helpful to be familiar with the physiography of the State. Physiography deals with the study of the origin and description of land forms, such as mountains, valleys, and plains. Plate 9 is a map of Texas which shows the major physiographic provinces within the State.
The majority of the land forms in Texas have been produced by the processes of erosion attacking the structural features of an area. Certain other land forms may be related to the effects of igneous activity which resulted in the accumulation of large masses of igneous rocks. The Davis Mountains are an example of surface features produced in this manner.
In discussing the physiography of Texas, three major physiographic provinces will be recognized. These are (1) the Trans-Pecos region, (2) the Texas Plains, and (3) the Gulf Coastal Plain (Pl. 9).
The Trans-Pecos region, located in the westernmost part of the State, is an area of mountains and plateaus with broad basins between the major mountain ranges. Many different types of rocks are exposed in Trans-Pecos Texas and these include marine, fresh-water, and terrestrial deposits. In many areas igneous rocks flowed out on the surface and now overlie sedimentary rocks. There are also many places where igneous rocks have been injected into the surrounding rocks, and these igneous rocks have been exposed by later erosion.
Included within this area is the Van Horn uplift of southern Hudspeth and Culberson counties, the Solitario uplift of southern Presidio and Brewster counties, and the Marathon uplift of northeast Brewster County. This region also includes the Big Bend area of Texas, a part of which has been set aside as a National Park where many interesting and important geological features may be seen.
The Trans-Pecos region is one of rugged topography with elevations as high as 8,700 feet, at Guadalupe Peak in the Guadalupe Mountains of northern Culberson County, and as low as 1,500 feet, in the Rio Grande valley.
Numerous invertebrate fossils occur in the Cretaceous limestones and shales of the Trans-Pecos region and in the Paleozoic rocks of the Marathon uplift. The Gaptank formation of Pennsylvanian age and the Permian reef limestones of the Glass Mountains are especially fossiliferous. In addition, many vertebrate fossils have been collected in Trans-Pecos Texas, particularly in and around Big Bend National Park.
The plains of Texas are broad expanses of country with very little surface relief. Most of the plains support grasses and some have wooded areas, particularly along stream valleys.
The plains of the northwestern part of the State have been subdivided as follows.
This area (Pl. 9), often called “the caprock,” is an elevated plateau which rises above the rolling plains which surround it. The High Plains are bounded by the Pecos River valley on the south, southeast, and west and by the North-Central Plains on the east.
The surface of the High Plains is very flat and characterized by a sparse cover of grasses and few trees. The surface strata consist largely of unconsolidated deposits of sands and gravels of Quaternary and Tertiary age, with remnants of Lower Cretaceous limestones along the southern margin. The rocks of the High Plains are mostly unfossiliferous, but mammalian remains have been found at several localities.
Plate 9
Physiographic map of Texas.
Surface strata of the North-Central Plains (Pl. 9) are westward-dipping Pennsylvanian, Permian, and Triassic rocks. Present also are extensive exposures of Quaternary sands and gravels which trend north-south across the central portion of the region. The area is bounded on the west by the High Plains, on the east by the Grand Prairie, and on the south by the Edwards Plateau and Llano uplift. Many vertebrate fossils have been collected from the Permian and Triassic rocks of this area. There are also many excellent outcrops of fossiliferous Pennsylvanian formations in the North-Central Plains region.
The Edwards Plateau (Pl. 9) is located in south-central Texas and is bounded on the south by the Balcones fault zone and on the north by the North-Central Plains. The surface of the area is typically flat with a gentle slope to the south. The rocks of the Edwards Plateau consist primarily of Lower Cretaceous limestones and shales, many of which are very fossiliferous.
This area (Pl. 9) has a relatively flat surface but there are areas of gently rolling hills. The eastern boundary of the Grand Prairie is marked partly by the Balcones fault zone. North of McLennan County, however, the Balcones fault zone is not expressed at the surface and in this area the eastern boundary is defined by the western edge of the Woodbine exposures. Upper and Lower Cretaceous rocks occur at the surface and dip to the southeast; many of these rocks contain a large number of invertebrate fossils.
The Llano uplift (Pl. 9) is located in the central part of the State where Precambrian igneous and metamorphic rocks and sedimentary rocks of early Paleozoic age occur on the surface. The area, which now appears as a basin-shaped depression, was at one time covered by Lower Cretaceous rocks and perhaps also by Devonian, Mississippian, and Pennsylvanian strata. These have since been removed by erosion. The east, south, and west sides of the uplift are surrounded by Lower Cretaceous rocks, and the northern margin is marked by the Mississippian and Pennsylvanian formations of the North-Central Plains. The area is, in general, composed of unfossiliferous rocks, but some invertebrate fossils (primarily trilobites and brachiopods) have been collected.
The Gulf Coastal Plain (Pl. 9) is composed of Cretaceous, Tertiary, and Quaternary rocks and includes the eastern, southeastern, and southern portions of the State. The rocks of the area consist of sands, clays, shales, and limestones. The Texas Gulf Coastal Plain is bounded on the north and west by the Balcones fault zone, on the south and southwest by the Gulf of Mexico, and extends eastward into Arkansas and Louisiana.
The region has broad river valleys and uplands of low relief, but there is an increase in relief toward the interior of the State. The surface of the area slopes gradually toward the Gulf and successively younger formations are encountered gulfward.
The rocks of the Texas Gulf Coastal Plain are relatively unfossiliferous, but many of the Upper Cretaceous rocks contain fossils. In the central portion of the region some marine formations of Tertiary age locally contain well-preserved invertebrate fossils.
Geologic studies of the State of Texas have indicated the presence of rocks formed during every era and period of geologic time. These range from the Precambrian granites of the Llano uplift to the Quaternary gravels of the High Plains.
Plate 10
GENERALIZED GEOLOGIC MAP OF TEXAS
Modified from Geologic Map of Texas, 1933
One of the best ways to become acquainted with the geology of Texas is to study the geologic map of the State (Pl. 10). A geologic map shows the distribution and age of surface rocks and may also indicate what kind of geologic structures are present. The types of rocks that crop out at the surface may be shown by means of symbols, colors, or patterns, and these are explained by a legend which accompanies the map. On Plate 10, colors are used to show the distribution and geologic age of the surface rocks of Texas. Reference to this map will give the collector some idea of the age of the fossils that might be found in a given area. Some special geologic maps may have the location of geologic structures and formation contacts indicated by means of symbols, such as dashed lines, arrows, and similar special markings. However, the map included in this publication does not show any of these special markings.
The Precambrian rocks of Texas are composed of igneous and metamorphic rocks and some sedimentary rocks. Most of the Precambrian outcrops are in the Llano uplift and El Paso and Van Horn regions.
Alterations produced by vast amounts of time, heat, and pressure have obliterated any trace of fossils that may have been present in these rocks. With the exception of some questionable primitive plants collected in the Van Horn region, no Precambrian fossils have been reported from Texas.
Rocks of Paleozoic age are widespread in Texas, and rocks of each period are well exposed. Outcrops are found in the Llano uplift, North-Central Plains, and Trans-Pecos region. The most extensive exposures are of Pennsylvanian and Permian age, and the former are highly fossiliferous in parts of the North-Central Plains.
Rocks of late Cambrian age are exposed in the Llano, Marathon, and Solitario uplifts, and the Franklin Mountains near El Paso. These are sedimentary rocks consisting of conglomerates, sandstones, shales, limestones, and some dolomites.
Some of these formations are relatively fossiliferous, but the specimens are commonly fragmental and very poorly preserved. Fossils that are apt to be found in the Cambrian rocks of the Llano uplift include brachiopods, gastropods, trilobites, and small rounded objects believed to have been formed by algae (primitive one-celled plants). In other parts of the State, Cambrian rocks are sparsely fossiliferous and the fossils consist primarily of fragmental brachiopods, trilobites, and algae.
Ordovician outcrops are present in the Llano uplift of central Texas and in the Marathon, Solitario, El Paso, and Van Horn regions of Trans-Pecos Texas. These are sedimentary rocks and consist largely of sandstones, cherts, limestones, and dolomites.
Although some of the Ordovician formations are fossiliferous, they are seldom collected by amateur paleontologists because they are exposed in relatively inaccessible places and the fossils are usually poorly preserved. Ordovician fossils reported from Texas include sponges, corals, brachiopods, gastropods, cephalopods, and trilobites. In addition, the Marathon formation of the Marathon uplift contains large numbers of well-preserved graptolites (fig. 24, p. 86).
The Silurian of Texas is poorly represented in surface exposures, and only one formation, the Fusselman, has been described. The Fusselman crops out in the El Paso and Van Horn regions where it is a white dolomitic limestone. Fossils are not abundant in this formation, but brachiopods and corals have been collected at a few localities.
Devonian rocks are best developed in Trans-Pecos Texas, especially in the Marathon, El Paso, and Van Horn regions. In addition to the Trans-Pecos exposures, there are minor outcrops of Devonian rocks in the Llano uplift of central Texas.
Fossils are rare and fragmental in the Trans-Pecos exposures and consist primarily of radiolarians and brachiopods. The Devonian rocks of central Texas are predominantly calcareous and, although the material is usually poorly preserved, many fossils have been collected from them. These include bryozoans, corals, brachiopods, gastropods, and trilobites. Conodonts and fragments of primitive armored fishes (Pl. 37) have also been reported.
Mississippian rocks are exposed in the Llano region and in the Hueco Mountains of the Trans-Pecos area. The Trans-Pecos rocks primarily contain brachiopods with some bryozoans and gastropods.
The central Texas Mississippian rocks are much more fossiliferous and some of the material is well preserved. Fossils reported from this area include brachiopods (Pl. 17), crinoids, gastropods, cephalopods, trilobites, and ostracodes.
Pennsylvanian rocks are well represented in Texas and are exposed in the Llano uplift, north-central Texas, and Trans-Pecos Texas.
In Trans-Pecos Texas fossiliferous rocks crop out in the Hueco and Diablo Mountains. Fossils found in this area are algae, fusulinids, corals, brachiopods, pelecypods, gastropods, cephalopods, and crinoids. There is also a thick section of Pennsylvanian rocks in the Marathon uplift, but only one formation, the Gaptank, is very fossiliferous. It contains many fossils including fusulinids, sponges, corals, bryozoans, brachiopods, gastropods, pelecypods, cephalopods, and crinoids.
Certain Pennsylvanian strata in the Llano region are very fossiliferous, and the material is well preserved. The more abundant forms are fusulinids, corals, brachiopods, gastropods, pelecypods, cephalopods, and crinoids.
Probably the best Pennsylvanian collecting areas are to be found in north-central Texas. Here the thick marine limestones and shales contain large numbers of well-preserved invertebrate fossils, and the terrestrial or shallow marine strata have yielded an abundance of plant fossils. Invertebrate fossils are apt to be found along the banks of streams and gullies and in railroad and highway cuts. Many of the limestones bear large numbers of fusulinids or crinoid stems, and the shales may contain many corals, brachiopods, and mollusks. The best collecting will, of course, be found where the rocks have been sufficiently weathered.