Contributions to Geology 17.1

Geometry and origin of deformational structures in the Precambrian metamorphic rocks of the Hill City area, Black Hills, South Dakota

BERTRAM G. WOODLAND Department of Geology, Field Museum of Natural History, Roosevelt Road at Lake Shore Drive, Chicago, Illinois 60605

Pages
1-24

Keywords
Dakota, Hill City, Precambrian, metamorphic, phase, fold, structural

Abstract
Eugeosynclinal Precambrian sediments of the Hill City area, South Dakota, were probably deposited, deformed, and metamorphosed between 2,500 and 1,600 million years ago. The deformation was polyphase, but the phases were not equally developed throughout the study area. The earliest phase (D1) commenced while the sediments held a high content of pore water, as soft-sediment structures associated with minor folds occur. Continued compression during this phase drove out most of the pore water and produced major folds (trending approximately N-S and plunging gently to the S) and concomitant secondary planar and linear features. The resultant metamorphism was low grade, possibly no higher than the muscovite-chlorite subfacies of the greenschist facies. Penetrative cleavage formed by parallel-oriented phyllosilicates, mostly muscovite, is present in pelitic rocks, but it is not ubiquitous. In the more quartzose rocks cleavage is generally poorly developed, and muscovite commonly has little or no obvious preferred orientation.

The latest phase (D3) is represented by reclined and flattened folds which have a gentle plunge to the SW or W-SW. Axial plane cleavage is associated with these folds but, again, is not well-developed. These fold elements are recognized only in the central part of the area. Elsewhere, minor steeply-plunging folds are commonly visible in the field; some are rotated early folds, but others are folds superimposed on already-deformed rocks. These latter folds have been designated as an intermediate deformation phase (D2). These D2 folds are of a different deformation style and were developed to the N of the area of large-scale D3 structures. It seems possible, because of their distribution relative to the area affected by D3 folding, that they may have formed during an early phase of D3 rather than in a separate deformation.

The study area's structural interpretation based on the accepted stratigraphic order requires a very complex syncline (D1) refolded by tight reclined D3 folds. Inversion of the stratigraphic sequence and changes in correlation would result in a more simplified structure by removal of the D1 syncline. Such changes, however, introduce difficulties of interpretation in adjacent areas, so the resolution of this problem must await further detailed studies on correlation and sequence, as well as structural analysis of a wider area.

Silurian continental margin in northern Nevada and northwestern Utah

PETER M. SHEEHAN Department of Geology, Milwaukee Public Museum, Milwaukee, Wisconsin 53233

Pages
25-36

Keywords
Silurian, Great Basin, Nevada, Utah, continental, shelf, tectonic

Abstract
Evaluation of Silurian lithofacies distributions in the northern Great Basin permits the position of the shelf margin in northern Nevada to be determined. The north-south trend of the Silurian continental margin in central Nevada is abruptly deflected eastward more than 200 km from northern Nevada into northwestern Utah. The pre-Silurian shelf margin may have migrated 60 km or more toward the continent in the Early Silurian. Similar continent-ward migration of the continental margin in central Nevada and central Idaho has been reported previously, and apparently the margin subsided along a continuous strip at least a 750 km in length. The new Silurian shelf margin became the effective continental margin, and acted as a buttress during the Late Devonian-Mississippian Antler Orogeny, when the subsided strip was under-thrust beneath early and middle Paleozoic oceanic deposits. The east-west margin in northern Nevada may have been a significant structural feature in other post-Silurian tectonic events.

Vertebrate fauna of the Laramie Formation (Maestrichtian), Weld County, Colorado

KENNETH CARPENTER University of Colorado Museum, Boulder, Colorado 80309

Pages
37-50

Keywords
microvertebrate, Colorado, Laramie, dinosaur, turtle, mammals, fish

Abstract
A new microvertebrate locality in central Weld County, Colorado, has greatly expanded the previously known dinosaur and turtle fauna from the Laramie Formation. This is the first reported occurrence in the Upper Cretaceous of Colorado for many of the species. There are six taxa of fish, four amphibians, nine non-dinosaurian reptiles, six dinosaurs, and six mammals. All of the lower vertebrates indicate that this fauna is the southernmost known extension of the Lance-Hell Creek faunas.

K-Ar age for the Fowkes Formation (middle Eocene) of southwestern Wyoming

MICHAEL E. NELSON Department of Earth Sciences and Sternberg Memorial Museum, Fort Hays State University, Hays, Kansas 67601

Pages
51-52

Keywords
Fowkes, Wyoming, Eocene, biotite, radiometric

Abstract
A radiometric age of 47.9 + 1.9 million years was obtained on biotite from the Fowkes Formation in the Fossil Basin of southwestern Wyoming. This date, along with fossil mammals, suggests that the Fowkes is middle Eocene in age and correlative with the Bridger Formation of the Green River Basin.

Description of a large trionychid turtle shell from the Eocene Bridger Formation of Wyoming

EUGENE S. GAFFNEY Department of Vertebrate Paleontology, The American Museum of Natural History, Central Park West at 79th Street, New York, New York 10024

Pages
53-58

Keywords
turtle, Eocene, Bridger, Wyoming, trionychids, Trionyx

Abstract
A nearly complete carapace and plastron of a large trionychid turtle from the Eocene Bridger Formation near Opal, Wyoming, has features formerly considered diagnostic of Axestemys Hay (1908) and Paleotrionyx Schmidt (1945) and supports the synonymy of these genera with Trionyx as proposed by Romer (1956). However, an examination of the literature on generic level taxa of Recent and fossil trionychids leads me to the conclusion that the genus Trionyx as presently conceived is recognized by the absence of features or the presence of features primitive for Trionychidae, and that assigning a species to this "waste-basket" genus is equivalent to Trionychidae incertae sedis.

Angiosperms from the late Tertiary Keller local fauna of Ellis County, Kansas

JOSEPH R. THOMASSON Division of Science and Mathematics, Black Hills State College, Spearfish, South Dakota 57783

Pages
59-64

Keywords
angiosperm, Tertiary, Keller, Kansas, grasses, borages

Abstract
Angiosperm fossils recovered at and near the immediate vicinity of the late Tertiary Keller local fauna (Clarendonian or Hemphillian) include two grasses and four borages not previously recorded from this site. These are: Berriochloa intermedia (Elias) Thomasson, Berriochloa undescribed spp., Biorbia fossilia (Berry) Cockerell, Cryptantha coroniformis (Elias) Segal, Cryptantha cf. C. auriculata (Elias) Segal, and Eliasiana cf. E. aculeata Thomasson. Eliasiana cf. E. aculeata and Berriochloa intermedia previously have been recorded only once from the Ogallala Formation. The biostratigraphic implications of this association are as yet unclear.

Aspects of continental crustal structure and growth: targets for scientific deep drilling

SCOTT B. SMITHSON Department of Geology, The University of Wyoming, Laramie, Wyoming 82071

Pages
65-76

Keywords
crustal, reflections, tectonics, Precambrian, seismic

Abstract
The nature of continental growth in space and time is a central problem in earth science. Observations suggest that continental crust consists of a vertically and horizontally complex, variable framework of igneous and metamorphic rocks; metamorphic rocks predominate, and the crust cannot consist of a few simple, laterally homogeneous layers. Crustal growth represents a prime target for investigation through techniques of deep drilling. Seismic reflection profiling complemented by geological, geochemical, and other geophysical studies represent major interpretive tools to precede deep drilling, the ultimate test. Questions that arise in crustal reflection interpretation involving the common short reflection segments and transparent zones may be resolved by drilling; such activities will allow "calibration" of crustal reflections, and render them more powerful in refining geological interpretations. Major problems that could be attacked by deep drilling include: (1) the nature of the "Conrad discontinuity" and crustal layering; (2) estimation of volumes of water in the crust; (3) extent of underplating and subcrustal erosion; (4) the nature of igneous intrusions, high grade metamorphism, and suture zones; and (5) the role of plate tectonics in the structural history of the Precambrian.