Alycia L. Stigall

The Ordovician rocks of the conterminous United States (US) have a complex history, spanning multiple ancient basins, shifting palaeoclimate and evolving tectonic regimes. The US portion of the palaeocontinent of Laurentia occupied a relatively stable and isolated position around the southern tropics during the Ordovician. In general, Lower Ordovician rocks form a vast autochthonous blanket of fine-grained (tropical) carbonates that covered much of Laurentia, named the ‘Great American Carbonate Bank’. Outboard, ribbon carbonates and graptolitic shales are found in allochthonous fragments of the ancient continental margin. Middle Ordovician strata are more lithologically diverse, including the addition of several regionally distributed sandstones of the inner detrital belt, mostly overlying the Sauk–Tippecanoe unconformity. Upper Ordovician strata show the greatest lithologic and faunal diversity, reflecting steepening topography resulting from regional compression along the south Laurentian (Appalachian) margin. Recent advances in the interpretation of the US Ordovician come primarily from studies of carbon and oxygen stable isotopes, sequence stratigraphy, palaeoecology, tephrochronology, redox geochemistry, strontium isotopes and geochronology.