Paleogeography of the Late Paleozoic World. 186-233A Earth & Life History (Fall 2001). Recommended reading: STANLEY “Earth System History” Chapter 15, pp. 414-431.
186-233A Earth & Life History (Fall 2001)
STANLEY “Earth System History”
Chapter 15, pp. 414-431.
Keywords: Carboniferous (Mississipian, Pennsylvanian) and Permian periods, coal cyclothems, concretions, Ancestral Rocky Mountains, seasonal climate,
In Europe, the Carboniferous system is split in Early, Middle and Late intervals. (The right hand side names as “stages” defined on the basis of biozones.
In North America, the Carboniferous system is split in the Mississipian and Pennsylvanian periods.
The Mississipian strata are separated from the younger Pennsylvanian strata by a 4-million year gap.
This “discomformity” is an interval of non-deposition. Bedding is parallel in strata below and above it. Many genera of crinoids and ammonoids disappear at that level.
A disconformity separates two sequences that are flat-lying (unlike an angular unconformity) but shows evidence of erosion at the top of the older formation.
In Pennsylvanian (or Carboniferous) strata, coal beds occur between other rock types. The different rock types form remarkably repetitive cycles at any one locality.
Predominantly marine section covers the coal.
Coal covers a predominantly non-marine section
Coal forms from plant remains accumulating in swamps found between channels of river deltas.
Part of the non-marine sequence within cyclothems is due to the movement of delta lobes, flooding the swamps with coarser sediment (sands) carried by the meandering river.
Ice caps grew and shrank repeatedly... Sea level changes caused the cyclothems.
During the Carboniferous,Gondwanaland joins Euramerica.
When glaciers shrank, during the Upper Carboniferous, lycopods and seed ferns were replaced by spore ferns, which were better adapted to moister conditions.
Cliffs are washed by the tides of the Bay of Fundy. New material is constantly eroded and exposed.
- plant fossils
Dendrerpeton skull, Joggins (NS). Its limbs and skull structure suggest a more terrestrial than most amphibians.
Calamites trunk and its modern relative, a horsetail.
This tree trunk was buried in sediment on the floodplain. Some tree trunks contain complete skeletons of small animals, like this Hylonomous.
A 2-meter long myriapod (as drawn by Dawson, above), left trackways (right) at Joggins. The same arthropod is known from other localities.
Spectacular fossils of Carboniferous age are also found south of Chicago, Illinois, in an area named from a local river, Mazon Creek. By 1979, 320 species had been described from the deposit.
Most Mazon creek fossils can be assigned to either a marine fauna or a freshwater-terrestrial fauna.
The best fossils are found by cracking open siderite (FeCO3) concretions that stick out from the weathered siltstone.
These concretions can form in coal beds during early burial.
A carbonate mineral (CaCO3) is dissolved and redistributed within the sedimentary layers.
The carbonate mineral reprecipitates as tiny crystals around an intact, dead animal. This produces a hard envelope which preserves delicate fossils from being crushed during deeper burial.
The reassembly of continents into the supercontinent “Pangea” continued throughout the late Paleozoic.
Uplift and downfaulting in southwestern U.S. along deep, nearly vertical faults. This suggests that the crust was being stretched rather than compressed. The exact cause is unclear...
The Permian Reef complex
encrusting algae under microscope
fusulinid foraminifer under microscope (below)
reef rock with sponge
Low sea level: clastics eroded from land are deposited.
Limestone layers become interbedded with red sandstones during periods of lower sealevel.
Sandstones fill channels dug in the basinal shale.
Dark layers are CaCO3 and the white layers are gypsum (CaSO4. 2H2O). Thin varves (layers) correspond to yearly cycles of wet and dry seasons.
Bottom: Gypsum crystal in the basinal shale.
The magnitude and suddenness of the Permian extinction are unique in the fossil record.
Tectonic movements are slow... The drier climates of Pangea and the glaciations did not develop particularly abruptly.
What other mechanisms could have dealt a fatal blow to marine communities that were already under significant stress?
Similar episodes occurred several times through Earth’s history.
On this map of igneous provinces of the world, interpreted as mantle plume events, the late Permian “Siberian Traps” stand out by their extent.
Meishan, China, contains ash beds, critical for dating the two pulses of extinction that mark the late Permian.