Absaroka Sequence

Absaroka Sequence
Name	Absaroka Sequence
Type	cratonic sequence
Period	Late Paleozoic to early Mesozoic
Namedfor	Absaroka Range
Region	North America
Namedby	geologists of North American stratigraphy

Absaroka Sequence The Absaroka Sequence is a major North American cratonic sequence recognized in regional stratigraphy as a large-scale cycle of transgression and regression that spans the Late Paleozoic into the Early Mesozoic. It is central to interpretations of Permian and Triassic basin development linked to orogenic events and global eustatic changes documented in studies by geologists working on the Absaroka Range, Rocky Mountains, Appalachian Mountains, Sierra Nevada, and the Cordillera. The sequence framework informs correlations among stratigraphic units such as the Permian Basin, Williston Basin, Anadarko Basin, and the Arctic Alaska Basin.

Overview and Definition

The Absaroka Sequence is defined as the cratonic-scale succession overlying the Kaskaskia Sequence and underlying the Zuni Sequence across much of North America, representing one of the major Phanerozoic depositional megacycles recognized in classical studies by stratigraphers associated with the U.S. Geological Survey, American Association of Petroleum Geologists, and university programs at University of Chicago, Stanford University, and University of Michigan. It encompasses stratigraphic packages traditionally assigned to the Permian and Triassic, and in some provinces includes latest Pennsylvanian and earliest Jurassic strata. Key regional names that fall within this framework include the Permian Marker, Triassic Chinle Formation, and various red-bed successions studied at the Grand Canyon, Petrified Forest National Park, and Zion National Park.

Stratigraphy and Time Range

Stratigraphically, the Absaroka Sequence spans roughly from the late Pennsylvanian through the Triassic and into portions of the Jurassic in adjacent provinces, though temporal limits vary by cratonal position and tectonic setting. Its chronostratigraphic bounding surfaces correspond to major unconformities correlated with the late Paleozoic icehouse-to-greenhouse transition documented by researchers at Smithsonian Institution, British Geological Survey, and Geological Survey of Canada. Lithostratigraphic units commonly correlated to the sequence include the Cutler Formation, Bell Springs Formation, and Dockum Group, and biostratigraphic tie points often utilize ammonoid zonations and fusulinid assemblages from sections studied by paleontologists at Yale University, Harvard University, and the Natural History Museum, London.

Depositional Environments and Lithology

Sedimentary facies within the Absaroka Sequence range from continental red beds and fluvial conglomerates to shallow-marine carbonates and evaporites. Typical lithologies include arkosic sandstones, conglomerates derived from uplifted sources such as the Ancestral Rocky Mountains, limestones hosting reefal assemblages comparable to those in the Permian Reef Complex of the Guadalupian, and evaporitic deposits analogous to the Zechstein of Europe. Depositional settings documented in the sequence include braidplain systems studied near Denver Basin, tidal flats examined at Monument Valley, and restricted basins with hypersaline conditions comparable to the Gogland Basin.

Tectonic and Sea-Level Controls

The evolution of the Absaroka Sequence is closely tied to tectonic episodes including the Alleghenian Orogeny, the waning pulses of the Alleghenian and Ouachita Orogeny, and the onset of rifting that later produced the Atlantic Ocean and influenced the Gondwana breakup. Sea-level changes recorded in sequence boundaries reflect global eustatic signals correlated with studies by teams at the Scripps Institution of Oceanography and the Woods Hole Oceanographic Institution. Foreland-basin development adjacent to orogens such as the Appalachians and sediment provenance studies tracing sources to the Ancestral Rockies and Sierra Madre Oriental illustrate tectono-sedimentary coupling.

Paleontology and Biotic Assemblages

Fossil assemblages in Absaroka Sequence strata document major biotic transitions including Permian faunas dominated by brachiopods, fusulinids, and diverse echinoderms, through Triassic faunas with ammonoids, bivalves, and early dinosaurs recorded at localities investigated by researchers from University of California, Berkeley, University of Utah, and Field Museum of Natural History. Plant fossils, including glossopterids and later cycadophytes, link floras to studies at Kew Gardens and the Royal Botanic Garden Edinburgh. Vertebrate sites yielding archosaurs and temnospondyl amphibians provide biostratigraphic and paleoecological context comparable to faunas described from the Karoo Basin and Ischigualasto Formation.

Economic Resources and Uses

The Absaroka Sequence hosts significant hydrocarbon reservoirs in provinces such as the Permian Basin, Williston Basin, and Anadarko Basin, where sandstone and carbonate reservoirs, sealed by evaporites, are targets for exploration by companies like Chevron Corporation, ExxonMobil, and ConocoPhillips. Mineral resources include potash and halite exploited in settings analogous to the Saskatchewan deposits, while widespread sandstone aquifers are important to municipal and agricultural supplies in regions governed by bodies such as the U.S. Bureau of Reclamation and Environment and Climate Change Canada.

Regional Distribution and Correlations

Regionally, the Absaroka Sequence is recognized across interior North America from the Canadian Shield margin through the Midcontinent Rift area to the western Cordillera, and correlates internationally with contemporaneous sequences in Europe, South America, and Africa that reflect similar Permian–Triassic histories. Correlation efforts draw on work by international panels including the International Commission on Stratigraphy, the Society for Sedimentary Geology, and collaborative mapping by the U.S. Geological Survey and Geological Survey of Canada.

Category:Geologic sequences