Donald F. Reaser

Style of Faults and Associated Fractures in Austin Chalk, Northern Extension of the Balcones Fault Zone, Central Texas

ABSTRACT Distributions, geometries, and densities of faults and associated fractures in the Cretaceous Austin Chalk were studied in outcrop within the northernmost extension of the Balcones Fault Zone in Ellis and northern Hill Counties, Texas. Description of the fracture systems may be applicable to hydrocarbon exploration and production from this unit and to locating the proposed Dallas-Fort Worth Area Superconducting Super Collider site in Ellis County. Inactive normal faults with throws of less than 30 m compose this northernmost extension of the fault zone. Most of the major faults strike northeastward to east-northeastward, oblique to the regional strike of bedrock, and dip 45° to 80° northwest and southeast. Smaller faults strike northwestward. Calcite mineralization occurs along most of the fault planes, and slickensides indicate normal to slightly oblique slip. Developed within the fault zone are a series of narrow grabens that are 250 to 600 m wide and as much as 5.5 km long. In plan view these small grabens occur either as (1) depressed strips of bedrock bounded by subparallel hinge faults that increase in displacement to the northeast or as (2) irregularly subsided blocks bounded by double hinge faults that increase in displacement toward the central part of the trough and pinch out toward the northeast and southwest. Areas of high fracture density within the Austin Chalk occur adjacent to the major faults and within gently warped beds located where faults terminate along strike. Narrow zones characterized by different fracture styles and densities occur subparallel to the major structures. These zones vary in width and are composed of (1) complex minor faults (2 m wide), (2) minor faults and joints (15 to 90 m wide), or (3) abundant joints (60 to 140 m wide). Most minor faults have throws of less than 3 m and strike subparallel to an associated major fault or flexure, although some minor faults strike subperpendicular to major structural trends. Minor faults dip between 40° and 70° and are usually partly filled with calcite. Most systematic joints are nearly vertical and also strike subparallel to an associated major fault or flexure. Joints are only rarely filled with calcite, and abutting relationships suggest that minor faults predate joints. Average fracture spacing near faults or within flexures for 0.5- to 1-m-thick chalk beds generally ranges from 0.5 to 1.3 m. Locally joint densities may be as high as 10 joints/2 m. Minor faults and many joints cut through adjacent beds of different thicknesses.

Balcones Fault System: Its Northeast Extent: GEOLOGICAL NOTES

The Balcones fault zone is a system of en echelon normal faults that extends from about 20 mi. N. of Del Rio to a short distance N. of Waco, Texas. Recent field work has shown large normal faults, with displacements up to 100 ft., in NE. Ellis County (between Waco and Dallas). These faults are probably part of the Balcones fault system because 1) they lie along a northeastward projection of the Balcones zone, 2)they have the trend (NE. strike) and the order of magnitude of the northern Balcones faults, and 3) they are about 30 mi. W. of faults in the Luling-Mexia fault system.

Austin Chalk (uppermost Santonian) Discontinuity Surface, North Central Texas: ABSTRACT

ABSTRACT The Austin Chalk-Taylor (Ozan) Marl contact marks the Santonian-Campanian boundary which is a regional unconformity in north-central Texas. This distinctive surface has been examined at three localities where it records evidence of complex sedimentologic and diagenetic histories. This surface is highly irregular and has been stained pervasively with limonite. The most conspicuous aspect of the uppermost Austin discontinuity surface is the abundance of phosphatized and pyritized nodules and bioclasts (gastropods, bivalves, corals, coprolites and Baculites sp.). Fish teeth and bone fragments are also present. These nodules and bioclasts have been penetrated by small-diameter chalk-filled borings. Close examination reveals the presence of abundant trace fossils, especially Rhizocorallium jenenese. These trace fossils have been infilled with reddish-brown clay piped downward from the overlying Ozan Marl. Phosphatic nodules and glauconite also occur in these clay-filled burrows. Well-preserved chalk-filled Rhizocorallium jenenese are present locally. The occurrence of Rhizocorallium sp. in the uppermost Austin Chalk is noteworthy because Rhizocorallium has not been recorded in other Cretaceous chalks of North America or Europe. The exquisite preservation of Rhizocorallium in the uppermost Austin Chalk is indicative of a firm ground paleosubstrate. Based on sedimentologic, mineralogic, and paleontologic data, the Austin Chalk-Taylor Marl contact is interpreted as a condensed horizon (omission surface). This phosphatized condensed horizon overlies the Rhizocorallium firm ground. This thin stratigraphic interval records a complex history of soft-sediment bioturbation, early marine lithification, submarine erosion, and mineralization. The amount of erosion along the Austin-Taylor contact in the study area appears to have been relatively minor. Elsewhere in north-central Texas the Austin-Taylor boundary has been subjected to significant erosion. The Austin-Taylor discontinuity surface formed in a relatively shallow marine (inner to mid-shelf) paleo-environment.

Biogenic Mounds and Associated Trace Fossils: Wolfe City Formation (Upper Cretaceous), North-Central Texas

ABSTRACT Small, conical sandstone mounds occur in some outcrops of the lower Wolfe City Formation (Upper Cretaceous) in Ellis and Navarro Counties, North-Central Texas. Biogenic mounds are common in modern shallow-marine environments (e.g., Callianassa mounds) but are rare in the geologic record. These biogenic mounds resemble the ichnogenus Chomatichnus sp. described originally from Carboniferous limestones of Lancashire, England (Donaldson and Simpson, 1962). This ichnogenus occurs as well-defined conical mounds having a central vertical burrow. Wolfe City Chomatichnus consists of calcite-cemented, glauconitic, bioclastic, argillaceous, fine-grained quartzarenite. The central vertical burrow is lined with shell debris; striae radiate from the central burrow. Thalassinoides, Gyrolithes, and Pseudobilobites occur in lower Wolfe City strata with Chomatichnus. The upper Wolfe City contains a profuse ichnoassemblage of Thalassinoides; Pseudobilobites; Gyrolithes; U- and star-shaped trace fossils; large-diameter, sinuous, horizontal burrows; and tripartite digitate burrows. The Wolfe City ichnoassemblage records the activities of infaunal crustaceans in a low-energy shallow-marine paleoenvironment. Chomatichnus has probably been preserved here because of early marine cementation and/or rapid sedimentation (storm) events.

Foraminifers of Lower Ojinaga Formation (Cretaceous), Southern Quitman Mountains, Hudspeth County, Texas: ABSTRACT

Three partial sections of the lower Ojinaga Formation were measured in the southernmost Quitman Mountains, near the Rio Grande. The lower Ojinaga is composed largely of medium-gray to black, variably calcitic shale, except for dark, flaggy, calcarenitic limestone and shale in the basal 20 m and for a few thin limestones and a dolomite above. Thirty-two residues and 75 thin sections were studied. The foraminiferal populations are overwhelmingly planktonic; plankton-benthos ratios exceed 99:1 without exception. The fauna consists primarily of Hedbergella amabilis Leoblich and Tappan, H. brittonensis Loeblich and Tappan, H. delrioensis (Carsey), H. planispira (Tappan), H. simplex (Morrow), Rotalipora cushmani (Morrow), R. montsalvensis Mornod, R. brotzeni (Sigal), and R. greenhornensis (Morrow), although Heterohelix occur sporadically and poor preservation obscures possible Praeglobotruncana and Whiteinella. In addition to foraminifers, calcispheres and radiolarians equal or exceed the foraminifers in many samples. In varying proportions, the three taxa form minute laminae, commonly microscopically cross-bedded and disconformable in the calcarenitic biogenic limestones. Juvenile ammonites and protoconchs are common at the top of the section. Inoceramid prisms, oyster fragments, and fish debris occur in the coarser and thicker laminae. The sequence sampled is generally correlative with the middle and upper Cenomanian and the lower Turonian. A more precise correlation suggests middle Cenomanian and lower Turonian with an intervening unconformity, although the taxonomic foundation is questionable. The microfauna is markedly pelagic and typical of midbathyal or deeper depositional environments. The dark color and general lack of bioturbation suggest the possibility of bottom anoxia and consequent absence of benthic forms, but tests for organic carbon show only a modest elevation of total organic carbon, with a maximum of 2.7%. The microscopic sedimentary structures of the biogenic flaggy limestones testify to gentle but definitely tractional current action at bathyal depths. End_of_Article - Last_Page 1428------------

Structural and Stratigraphic Framework of Lower Mesozoic and Upper Paleozoic Strata, Northeast Texas: ABSTRACT

The lower Mesozoic and upper Paleozoic were investigated in a 19,430 sq km area centered about Ellis County, Texas, in an effort to define the pre-Cretaceous surface, determine the westward extent of Jurassic rocks, analyze thickness and lithic nature of the lowermost Cretaceous, delineate the extent of faulting, and evaluate the economic potential of the section studied. The pre-Cretaceous surface dips east-southeast and consists of Paleozoic rock in the updip third of the study area and Jurassic rock in the downdip two-thirds. Regional dip increases southeastward into the East Texas basin, but is interrupted by Balcones and Mexia-Talco faults. According to seismic data, many of these normal faults extend into the Paleozoic section. They are Jurassic and younger in age and formed along preexisting lines of weakness in response to the structural development of the East Texas basin. Jurassic rock extends updip beyond the Mexia-Talco system in an onlap fashion, each carbonate formation becoming more clastic as it nears its own pinch-out. The overlying Hosston Formation was deposited in a fluvial to nearshore environment, in the study area, on the basis o reported lithologies and isopach form. The interval studied has economic potential as geothermal, ground-water, and hydrocarbon sources. Possible hydrocarbon traps include fractured Arkansas Novaculite, updip pinch-out of and porous facies within the Upper Jurassic formations, and traps against the downdip sides of faults in the Hosston Formation and underlying Jurassic formations. End_of_Article - Last_Page 607------------

Sedimentary Facies and Depositional Environments of Cenomanian Buda Limestone, Northern Coahuila, Mexico: ABSTRACT

Buda Limestone in northern Coahuila ranges in thickness from 15 to 35 m and consists mostly of lime wackestone and mudstone. Two major facies developed in the area in response to contrasting environmental conditions: a northern mudstone-wackestone facies composed mostly of benthic fossils deposited in water depths less than 100 m and a southern wackestone facies with abundant pelagic fossils deposited in water as deep as 500 m. A fossiliferous, intraclast packstone facies developed on shoal areas in adjacent trans-Pecos Texas. These shoals may have been related to areas of tectonic adjustment. Subfacies developed in the north differ in intensity of bioturbation, fossil content, and amount of terrigenous clastic content. The lower and middle Buda contain a diverse benthic fauna, abundant burrows, and up to several percent quartz silt and clay in places. The upper Buda contains a benthic fauna low in diversity and number of specimens, rare burrows, and a lesser amount of terrigenous clastics. Distribution of the two principal facies of the Buda in northern Coahuila conforms to late Albian paleogeographic features--the broad flat carbonate platform north of the Stuart City reef, and the deeper basinal area on the south. In the north, a broad low-relief shelf over the preexisting platform was characterized by open marine conditions although there is some evidence of restricted water circulation in the northeast part. The water was deep enough so that the sea floor was rarely disturbed by wave action. Normally clear waters were periodically muddied by an influx of terrigenous clay and silt. South of the preexisting Stuart City platform margin, waters were clear and deep. End_of_Article - Last_Page 247------------