Homer Montgomery

Paleogeography of Jurassic fragments in the Caribbean

Jurassic rocks of the Caribbean are a sampling of 100 million years of Farallon Plate history with fragments originating at diverse paleolatitudes and from varied tectonic settings. Fragments with clear paleogeographic signatures are components of the basement complexes of Duarte in Hispaniola, Bermeja in Puerto Rico and La Desirade off Guadeloupe. Paleolatitudinally sensitive radiolarian faunas document origination of Duarte as equatorial, La Desirade as higher latitude, and various Bermeja cherts as both equatorial and higher latitude. Red ribbon chert of Duarte and Bermeja of the same age, physical appearance, and lithological association are probably dismembered components of the same slab of Pacific crust. La Desirade red ribbon chert is slightly younger than the Duarte and Bermeja red ribbon chert and was deposited at higher latitude. Bermeja tuffaceous chert is also of higher latitude and probably had an arc-proximal origin. On the basis of modeled plate trajectories in the Pacific, the origin of various cherts from different paleolatitudes that end up in the same location requires different arrival times at the trench between North and South America. Based on radiolarian paleobiogeography plus indications of origin at a spreading ridge and ignoring the poorly constrained, modeled trajectories for the Late Jurassic, at least one of the higher latitude fragments may have originated in the southern hemisphere. The accumulation of multifarious chert, greenstone, and other ocean floor components was accomplished by offscraping strata transported to the subduction zone along the eastern Pacific margin and warehousing this material in an accretionary complex prior to entry of the Caribbean Plate into the gap between North and South America.

Jurassic (Tithonian) radiolaria from La Désirade (Lesser Antilles): Preliminary paleontological and tectonic implications

Jurassic (upper Tithonian) Radiolaria recovered from bedded, red ribbon cherts on La Desirade, Guadeloupe, are the oldest fossils yet discovered in the Lesser Antilles. This age not only corroborates contested isotopic ages for the igneous basement of La Desirade but also demonstrates that previously reported differences in basement ages for the central and eastern end of the island are invalid. In addition, La Desirade chert yielded a higher-latitude, Northern Tethyan to Southern Boreal Realm radiolarian assemblage (indicating deposition at a minimum of 22° north or south of the Jurassic paleoequator). Because Northern Tethyan or Boreal Jurassic oceanic crust did not exist in the spreading gap between North and South America, or east to Eurasia, we conclude that La Desirade oceanic crust formed in the Pacific. The fact that no Upper Jurassic red ribbon chert has been found in the Atlantic Ocean or in the Caribbean aside from in displaced oceanic fragments in Puerto Rico and the Dominican Republic lends additional support to a Pacific origin for the oldest crustal fragment in the Lesser Antilles.

Misconceptions concerning the Cretaceous/Tertiary boundary at the Brazos River, Falls County, Texas

Abstract Detailed biostratigraphic analysis (planktonic foraminifera and nannofossils) of sixteen K/T (Cretaceous/Tertiary) boundary sections in and near the Brazos River, Falls County, Texas indicates that a lithologically distinctive, coarse clastic event bed previously attributed to a meteorite impact-generated tsunami at the end of the Cretaceous was actually deposited during the early Tertiary (Danian: K/T + at least 230,000 years). A tsunami origin for this event bed is doubtful, but if a bolide-splashdown tsunami did generate the event bed, this putative meteorite impact must have occurred well into the Danian during the post-extinction, faunal recovery phase having little effect on extant foraminiferal and coccolith populations.

Rethinking the origins of the red chert at La Désirade, French West Indies

Abstract La Désirade in the Lesser Antilles contains one of the rare fragments of Jurassic oceanic crust known on Caribbean islands. Others in the northeastern Caribbean occur on Puerto Rico and Hispaniola. These fragments each include radiolarian-bearing chert that has been linked to an origin in the Pacific Ocean. Of these, a fragment in Sierra Bermeja, Puerto Rico is clearly of Pacific origin as it contains Lower Jurassic radiolarians that predate the opening between North and South America. Red ribbon chert at El Aguacate, Dominican Republic is essentially identical to widespread radiolarites found in accreted material of the Pacific basin and from Pacific Ocean ODP Site 801. Extensive sampling in the Atlantic basin has produced no Jurassic radiolarites. Thus, based on age (the older of the Sierra Bermeja outcrops) and lithology (El Aguacate), two of these fragments are definitely of Pacific origin. Re-evaluation of the chert/pillow lava sequence on La Désirade in light of recent discoveries at spreading ridges has resulted in a revised interpretation of their probable origin. A wide range of features of these cherts indicate pelagic and hydrothermal sedimentation at an Upper Jurassic spreading ridge, one that almost assuredly existed in the eastern Pacific realm. These features include: the types of chert found on the island, lack of argillaçeous partings, small outcrop size, discontinuous chert bodies, presence of limestone squeeze-ups into pillow lavas and indications of hydrothermal activity, including epidotization of basalt migrating outward from pillow margins with chert rinds that record pelagic and hydrothermal sedimentation at an Upper Jurassic spreading ridge, one that almost assuredly existed in the extreme eastern Pacific realm.

Paleolimnology of Uppermost Cretaceous Lacustrine Deposits in Western Texas

Abstract The association of vertebrate, invertebrate, plant, and ichnofossils from two Upper Cretaceous lacustrine environments in West Texas permits interpretation of the paleolimnology of part of the Javelina Formation. The relatively rich fossil assemblages permit studies of paleoenvironments and food web complexities, as well as provide paleohydrological information. In Big Bend National Park, at least three juvenile Alamosaurus sauropods were preserved in situ in anoxic mud among shallow-water lacustrine charophytes. Alamosaur remains in the Moon Valley section are both in situ and transported. Mussels, gastropods, gar, and crayfish trace fossils largely define the paleoecology, as well as reveal details of specific biological relationships, especially at several intervals in Moon Valley. The modern larval-host reproduction system of mussels may have existed in the possible case of an ectoparasitic relationship in the co-occurrence of fossil Unio and Lepisosteus. Relatively fine time-frame resolution is present based on investigations of the mussel, Unio, and the presence of crayfish burrows. Unio sclerochronology indicates continuous inundation for at least seven years at two different stratigraphic intervals. Horizons of fossil crayfish burrows record both periods of nondeposition and the level of the water table at those times. Viviparus gastropods suggest subtleties of water movement and oxygen concentration. Studies of the dinosaur bones and the presence of probable footprint impressions suggest dinosaurs visited these types of water bodies.

Paleontology of the Toa Baja #1 Well, Puerto Rico

The Toa Baja well penetrated 585.2 m of fossiliferous Oligocene and Miocene limestone, sandstone, and shale above 2119 m of Eocene, dominantly volcaniclastic strata. Oligocene and Miocene rocks were deposited on a broad, shallow shelf. The unconformity between Eocene and Oligocene strata spans at least the Late Eocene, and probably part of the Early Oligocene at Toa Baja. Below the unconformity, unfossiliferous volcaniclastic and igneous rocks are interstratified with fossiliferous limestone beds containing Middle and possibly late Early Eocene planktic foraminifera probably deposited in deep water. No Paleocene or Cretaceous fossils were recovered.

Using Problem-Based Learning to Deliver a More Authentic Experience in Paleontology

ABSTRACT This study is a research project focused on the implementation of problem-based learning in an honors, paleontology-oriented, Earth Science course. The course, the Age of Dinosaurs, is taught at the University of Texas at Dallas to undergraduates from a range of majors who seek core-curriculum science credit. All class work is centered on fossils and rock samples selected from a research collection accumulated from dinosaur-bearing beds in the Chihuahuan Desert of West Texas. A list of goals for the course was prepared during the design phase for the class. This list was used as a Likert scale questionnaire following completion of the course and dissemination of grades. Results from the goals questionnaire and from the official course evaluations present highly favorable responses to the course and to the problem-based learning methodology. Students provided free-response assessments and advice that influenced the next offering of this course.

PALEOECOLOGY OF THE GADDIS SITE IN THE UPPER CRETACEOUS AGUJA FORMATION, TERLINGUA, TEXAS

Abstract Diverse faunas at the Gaddis site near Terlingua, Texas are present at four stratigraphic levels along a hill in the Upper Cretaceous Aguja Formation providing a rich fossil record of the transition from an aquatic environment to dry land while recording exceptional events such as predation and wildfires. Given evidence for minimal transport the microvertebrates, macrovertebrates, coprolites, and plants may be reliably utilized for paleoecological analysis. Paleoenvironments shift upsection from nearshore marine, through tidal channel, to swamp, and, finally, to well-vegetated dry land. The faunas change from a basal layer of oysters with shark teeth to a microvertebrate assemblage just above logs with Teredolites borings and pristine leaves. Above the microvertebrates are diverse macrovertebrates including numerous dinosaurs associated with leaves, logs, and scrambling vines. The macrovertebrate layer is a rich assemblage of herbivorous and carnivorous dinosaurs, crocodiles, turtles, and rare pterosaurs. Several skeletal elements exhibit signs of predation that include punctures and gouges that precisely match crocodile and theropod teeth also recovered at the site. Abundant plant material that includes charred vines and logs is likely evidence of wildfires having ranged across the area. The preponderance of young dinosaur remains may support a wildfire scenario. An exposed surface with 24 coprolites, two hadrosaur vertebrae, logs, and in situ stumps caps the section. The larger coprolites are likely dinosaurian. Most contain plant materials while one contains a bone fragments suggesting the presence of herbivorous and carnivorous dinosaurs on the same surface, an occurrence that echoes herbivore/carnivore interactions in the underlying bonebed.