Outline of "Minnesota's Geology"

Pages 87-94

Mesozoic and Cenozoic Geologc History of Minnesota

Introduction

Minnesota was part of the broadly emergent craton of North America during Triassic and Jurassic time (225 Ma-135 Ma). Terrestrial river deposits of silt and sand were accumulating on the craton to the west of Minnesota, but no deposits of Triassic or early and middle Jurassic age are preserved in Minnesota. Apparently, no subsiding sedimentary basin was present.

Dinosaurs had evolved by Triassic time and were no doubt roaming the surface, and flying reptiles were gliding through the skies. The first mammals, small and inconspicuous forms, began to appear in Triassic time but did not become dominant until the Cenozoic. After all, the Mesozoic was the age of the reptiles and dinosaurs.

To the east, the Appalachians were being rifted by the beginnings of the breakup of Pangaea which had formed by continental collisions at the end of the Paleozoic. The Atlantic Ocean was beginning to form.

The seas returned to the craton in Late Jurassic time, and may have reached extreme northwestern Minnesota. The so-called Sundance Sea extended from the Arctic regions southward to the western interior of the U.S., reaching as far east as the Williston Basin in eastern Montana and the Dakotas. In northwestern Minnesota, non-marine sediments of Late Jurassic age were preserved at the edge of the Williston Basin. These rocks are not exposed but have been drilled, are about 100' thick, and consist of red mudstones and subordinate dolomite. Evaporites are present a short distance to the north in Manitoba, but none have been discovered in Minnesota. Evaporites indicate a change toward an arid climate. By this time, Minnesota had drifted north of the equator into desert latitudes.

The seas retreated toward the end of the Jurassic and a broad river plain developed from the present-day position of the Rocky Mountains eastward into the Dakotas. The well-known Morrison Formation represents deposits of this alluvial plain west of Minnesota, and contains some of the best and most abundant dinosaur remains of this age in the world - [a true Jurassic Park].

Cretacous Period - 135-65 Ma

• Rocky Mountains began to rise at the western edge of the North American continent
  • result of development of a convergent margin and subduction zone at the western edge of the westward-drifting continent
  • area to the east of the rising mountains began to subside in response to the weight of the additional crust created during mountain building to the west.
• Western Interior Basin [the Rocky Mountain Foreland Basin]
  • western edge was against the rising mountains
    • early deposits of this basin on the west were non-marine and included thick accumulations of braided river sediments and lake sediments
    • river sediments are coarse-grained and diminsh in particle size toward the east
  • eastern edge of the basin merged with the low-lying landmass on the craton far to the east of the emerging Rocky Mountains, approximately in central Minnesota
    • sediments of the eastern edge of the basin are generally lower-energy and finer-grained
    • low-lying land provided a source for siltstone and black organic-rich shale
  • foreland basin was flooded by a world-wide Cretaceous rise in sea level
    • this inland or epicontinental sea was very extensive, and in it accumulated a great thickness of Cretaceous marine sediments
    • the marine sediments are largely sandstones nearer to sources in the Rocky Mountains on the western side of the basin, with interbedded tongues of shale representing episodic transgressions of the sea
    • toward the east the marine sediments are dominated by black shales and interbedded limestones including deposits of chalk (a fine-grained limestone made of microscopic shells of marine protozoans and algae)
• Cretaceous sediments in Minnesota
  • Cretacous rocks in Minnesota mainly known from water wells, but there are also a few exposures
  • dominated by siltstones in western Minnesota, grading to black shales in Dakotas
  • islands of Proterozoic Sioux Quartzite were probably present in the Cretacous sea in southwestern Minnesota
  • embayment of the sea projected east-northeast into the Mesabi District
    • shoreline rugged, with conglomerates containing hematite pebbles deposited at the edge of the sea
    • important timing information from the conglomerate - creation of the hematite iron ore by deep weathering must have happened prior to Late Cretacous time (100 Ma)
  • conglomerate, well-sorted sandstone, siltsone and lignite (very low-grade coal) are represent the range of rocks found throughout the state
  • environments of deposition included lakes, swamps and floodplains, as well as high-energy coastal environments where well-sorted sandstones accumulated
    • continental deposits
      • Windrow Formation of southeastern Minnesota
    • marine deposits
      • Coleraine Formation of northern Minnesota
      • Dakota and Pierre Formations of southwestern Minnesota
• Cretaceous climate and weathering
  • world-wide climates were warm during this time and mild conditions prevalied even into high latitudes, as is evidenced by fossil plants in these regions. Most of North America was subtropical.
  • Cretacous coal bed up to 35' thick found in Minnesota near Virginia. Made of the wood of conifers which probably grew in a small swamp at the edge of the Cretaceous sea
  • SW Minnesota - unconformity between Cretaceous and the Precambrian-Paleozoic rocks has a deeply weathered zone along its surface.
    • Underlying granites have thick clay zone up to 200' thick, formed by weathering in a humid environment. The dominant clay is kaolinite, a clay important in the ceramic industry. This weathering interval also concentrated the soft iron ores in the Mesabi and Cuyuna Ranges
    • Preservation of this weathered zone was possible because the surface was probably stabilized by plants, and erosion was not significant. The weathered zone developed on an early Cretaceous land surface of very low relief and extended from southwest Minnesota to Winnipeg, Manitoba
    • Studies of the relative resistances of the minerals in the Precambrian granite to chemical weathering during the interval which formed the unconformity at the base of the Cretaceous are classics in geologic literature.
      • Goldich established the well-known mineral stabiility series from these studies. He established the fact that quartz is the only stable silicate under conditions of intense chemical weathering
      • rocks rich in ferromagnesian silicates will weather faster than rocks rich in non-ferromagnesian silicates. Therefore, mafic igneous rocks will undergo more rapid and complete chemical weathering than sialic igneous rocks in the same climate
• Cretaceous life
  • dinosaurs (more than 70 genera), swimming and flying reptiles, sharks, and small mammals
  • animal fossils from Minnesota include remains of crocodiles and mosasaurs (a large swimming reptile) and sharks teeth. Coiled cephalopods called ammonites are also abundant, as are clam and snail shells. Many of the Cretacous fossil finds are from rocks deposited in an elongate bay of the Cretaceous sea in the vicinity of the Mesabi Iron Range.
  • plant fossils are also abundant, especially in rocks related to the coal beds. After all, coal is nothing more than lithified plant remains. The fossils consist of leaf impressions, wood fragments and pollen grains. Ferns and conifers were dominant, as in the modern temperate rain forests of the Pacific Northwest

Cenozoic Geologic History - Tertiary Period - 65Ma-2 Ma

Cretaceous seas retreated from the foreland basin and the Rocky Mountains continued to rise as subduction continued at the western margin of the continent. Vast amounts of sediment were shed from the rising mountains and spread eastward as great fans of gravel, sand and silt. These sediments underlie portions of today's Great Plains. But no sediment of Tertiary age is preserved in Minnesota.

Excellent exposures of Tertiary sediment are present in the badlands of North and South Dakota, and in these sediments are preserved a variety of mammalian fossils. Dinosaurs were all but extinct by this time, and the Cenozoic is the age of mammals.

While mountain uplift and erosion and sedimentation were going on to the west, Minnesota was above depositional base level and was becoming cooler and more temperate as a result of the climatic effects of the rising mountain barrier to the west. No evidence of tertiary events is preserved in Minnesota's geologic record, although some geologists have suggested that the iron ores of western Fillmore, southern Olmstead and eastern Mower counties were created when Paleozoic iron-bearing carbonate rocks of the Cedar Valley Formation were undergoing weathering in the temperate climate.