paleogeogwus.html

Middle Jurassic Paleogeography

Paleogeography of the Southwestern US

Images from a talk presented to the Annual Meeting of the Geological Society of America, Salt Lake City, Utah, Oct. 1997

Introduction

The images presented here show the paleogeography of the SW US from 1.8 billion years ago to 10 million years ago. When played in order, the paleogeographic evolution of the region unfolds. Emphasis is on the Mesozoic where two or more images of each period are shown. Two different interpretations are presented for some of the periods. One shows a tectonic interpretation in which most elements of the western US were derived from North America; the second shows a tectonic interpretation in which some elements originated far from North America (exotic elements). Debate continues concerning these two different tectonic models.

Paleogeographic Maps and Globes
  • Text
  • Precambrian
  • Cambrian
  • Ordovician
  • Silurian
  • Devonian
  • Mississippian
  • Pennsylvanian
  • Permian
  • Triassic
  • Jurassic
  • Cretaceous
  • Tertiary
    The images were prepared by placing various stratigraphic, tectonic, and sedimentologic data on a base map, plotting land vs. sea over this data, and then adding more detailed landforms such as mountains, shelf edges, rivers, arc-trench systems, and lowlands. The data was then painted in Photoshop 5.5. Climatic data is also added as tones of browns and reds for arid and greens for humid.

    The database for the images consists of many papers published on the geology of SW NAM. My own work is incorporated onto the Colorado Plateau portion of the maps. Plate patterns are from PGIS MAC, a plate-tectonic computer program by the Paleomap Project. Major sources of data are listed below:

  • Geology of North America, vol G3; GSA DNAG Series, Burchfiel et al, 1992: Poole et al; E. L. Miller et al; Saleeby et al; Cowan and Bruhn; D. M. Miller et al
  • Ingersoll and Schweickert, 1986, Tectonics, vol 5, p. 901-912
  • Pacific Coast Paleogeography Syp 1 (Pacific Sect SEPM), Stewart et al, 1977: Ross; Poole et al; Poole and Sandberg; Rich; Stevens
  • Riggs and Blakey, 1993, in Dunne and McDougall, Mz Paleogeography Western US (Pacific Sect SEPM)
  • Blakey, 1994; 1996 (see my reference section elsewhere on this homepage)

    Abstract

    PALEOGEOGRAPHIC EVOLUTION OF THE PASSIVE-MARGIN TO ACTIVE-MARGIN TRANSITION, EARLY MESOZOIC, WESTERN NORTH AMERICA

    BLAKEY, Ronald C., Dept of Geology, Northern Arizona Univ, Box 4099, Flagstaff AZ 86011, ronald.blakey@nau.edu

    The early Mesozoic was a time of great change for southwestern North America (NAM) as the continental margin evolved from chiefly passive during most of the Paleozoic to active in the early Mesozoic. This fundamental tectonic evolution produced dramatic changes in the paleogeography of the region from passive-margin coastal plains to active-margin mountains, arcs, and tectonic basins. A series of shaded-relief paleogeographic maps have been prepared to visually emphasize these changes. These maps were constructed using tectonic, stratigraphic, and sedimentologic data. Although consensus of geologic interpretation is rare in this complex region, certain basic inferences can be made regarding gross paleogeography. This results in general trends in the distribution of major landforms, both structural and depositional. For the Colorado Plateau region, rather detailed paleogeomorphic maps can be prepared and the resolution of the maps is less than the detail of the known geology. For the Cordilleran region, the detail shown on the maps is more hypothetical and the detail shown exceeds the geologic resolution. This is particularly true of terranes that have had great lateral transposition since their initial formation and deformation.

    The change from passive to active margin was not sudden, but rather evolved as a series of oceanic arcs approached and collapsed against NAM. As the Cordilleran margin was built, it developed as continental (Andean) to the south and marine to the north. The marine arc collapsed against NAM by Late Jurassic and an Andean arc was established across the region. These broad trends were punctuated by accretion of exotic microplates, some of which produced local to regional mountain building. Additional complications to the paleogeography were created by transverse motions of small and large plates along the Cordilleran margin. Until the history of these blocks becomes better understood, the details of the paleogeography will remain tentative.

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