The Late Paleozoic: End of an Era

• Main Highlights
• Assembly of supercontinent Pangea.
• Gradual inundation in early Devonian begins Kaskaskia sequence
• Global glaciation and sea-level drop marks end of Kaskaskia
• Absaroka characterized by cyclical sedimentation (Icehouse)
• Transition to arid climate in Permian (Greenhouse)
• Kaskaskia Sequence
• Basal sediments include clean quartz sands containing heavy mineral deposits.
• Terrestrial clastics blanket western edge of Acadian orogeny from two source areas:
• Older sedimentary units of east and north (tourmaline, zircon & rutile);
• Igneous and metamorphic origin from Taconic orogeny (unstable pyroxene, amphibole, biotite & garnet)
• Kaskaskia in the Devonian
• Transgression resulted in lime-muds and reef complexes
• Restricted sites accumulated salt & gypsum.
• Late Devonian Black Shales
• Near "eastern" craton (across transcontinental arch) are black shales (O₂-deficient waters; Chattanooga shale ±10 m thick).
• Result of orogenic activity (Acadian Orogeny).
• Shales contain uranium radiometrically dated at 350 MY BP.
• Kaskaskia in the Mississippian
• Mountains erode and non-marine shales and sandstones deposited (e.g., Pocono Group)
• Reduction in suspensed mud results in extensive limestone platforms
• Mississippian Carbonate Platforms
• Limestone deposition blanketed middle craton west of highlands
• Cherty LS, shelly LS, bioclastic LS, ooid LS
• Late Mississippian regression associated with initiation of Gondwana glaciation
• Kaskaskia in western Craton
• Limestone, shale, and reef deposition
• Restricted circulation (e.g., Williston Basin) resulted in salt & gypsum (evaporite) deposition.
• Absaroka Sequence
• Widespread regional unconformities mark the boundary between the Kaskaskia & Absaroka sequences
• Karst topography at boundary
• Absaroka begins in Pennsylvanian and continues into the Triassic
• Pennsylvanian Icehouse Deposits
• Pennsylvanian reflects extensive coastal lowlands with thick peat swamps
• Continental sandstones, shales, mudstones, and coal predominate
• Terrestrial clastic rocks alternate cyclically with shallow marine deposits.
• In mid-continent marine limestones alternate with sandstone and shale.
• Rhythmic Cyclical Deposition
• Peat-accumulating swamps
• Paleosol
• Coastal Plain Deposits
• Nearshore Marine Sands
• Offshore Marine Shales
• Deep Offshore Marine Deposits
• Peat to Coal
• Forest biomass accumulates in swamps
• Burial of coastal swamps
• Biochemical alteration
• Increased temperature & pressure
• Chemical alteration of remaining biomolecules
• Coalification
• Reasons for cyclothemic deposition
• Tectonic activity causes loading of the craton resulting in subsidence (marine deposition) with periods of tectonic stability represented by terrestrial deposits
• Glacio-eustatic seal level fluctuations causing repetitive transgressions during interglacial periods and regression during glacial periods
• Absaroka in the West
• Influenced by orogenic activity
• Deposits reflect isolation
• Paradox Basin - shales overly karst limestones
• Restricted marine in response to mountain building results in evaporite deposition (salt, gypsum & anhydrite)
• Out of the Icehouse - Into the Greenhouse
• Evaporite areas surrounded by limestones and reef complexes
• Arkosic sandstones from erosion of highlands
• Permian Greenhouse Deposits
• West Texas consisted of isolated yoked basins (up to 500 m in depth) in which extensive reef communities and evaporite deposits formed
• Texas' Oil Reserves
• Marfa, Delaware, & Midland basins
• Barrier reefs (El Capitan, Guadelupe Mountains on margin of Delaware Basin)
• Regression to the southwest, followed by continental red siltstones and sandstones.

GE142