THE LAU PRIMO-SECUNDO OCEANIC EVENT (LUDLOW, SILURIAN) IN SOUTHERN LAURENTIA
The late Ludlow Lau Primo-Secundo Event is recognized in two regions in southern Laurentia, the Arbuckle Mountains in southern Oklahoma and the western edge of the Illinois Basin in Missouri. Major changes in conodont faunas, shifts in rock type, and a positive del13C excursion characterize the Lau Event in both regions, but the sections in Oklahoma and Missouri preserve somewhat different records of the Lau Event.
Pre-Lau strata in southern Oklahoma comprise brownish argillaceous, silty wackestones and shales with graptolites. A diverse Dapsilodus-dominated fauna includes species characteristic of the Havdlem Primo Episode in moderate abundance: Polygnathoides siluricus, Oulodus siluricus, Ozarkodina confluens, Walliserodus sp., Kockelella sp. and Panderodus recurvatus. Above a 0.1 m dark brown shale bed, a new Dapsilodus–dominated fauna, characterized by O. snajdri, abundant Wurmiella excavata and rare Panderodus, replaces this fauna. Clay and silt content declines sharply above the dark shale and resistant wackestones appear slightly higher that yield Pedavis latialata and Parazieglerodina plodowskii? Values of del13C dip from +1.0 o/oo in strata below the dark shale bed to –0.5 o/oo in the bed, rise to near +4.0 o/oo where the O. snajdri fauna appears, and fall back to +1.0 o/oo in the overlying resistant wackestones. The Lau Event appears to be truncated, possibly at a sequence boundary marked by the dark shale bed.
The Lau Event may be preserved more completely in eastern Missouri than in Oklahoma. In Missouri, pre-Lau strata comprise mottled red argillaceous wackestones and shales yielding a Panderodus-dominated fauna, with abundant P. recurvatus and less common Ozarkodina confluens and Walliserodus. Above this lies a 0.9 m argillaceous greenish gray carbonate mudstone from which only a few elements of Pseudooneotodus have been recovered. Interbedded reddish shale and thin wackestones reappear above the mudstone, in which an abundant Dapsilodus and Wurmiella excavata fauna appears. This is overlain by a resistant wackestone with Parazieglerodina plodowskii? Values of del13C dip from +1.0 to –3.5 o/oo in the base of the greenish-gray mudstone, rise to near +5.0 o/oo in the top of the mudstone, and fall to +1.0 o/oo in the overlying shale and thin limestones and the upper resistant wackestone unit.