High-resolution Late Silurian correlations between Gotland, Sweden, and the Broken River region, NE Australia: Lithologies, conodonts and isotopes

Abstract

High-resolution correlations, partly with a precision better than 10 ka, are achieved between late Ludfordian sequences on Gotland, Sweden (on Baltica), and a section (COG) through the Coral Gardens Formation along the Broken River, northeastern Australia (on Gondwana), despite these sections having been on different palaeocontinents facing different oceans. The interval is characterised by rapid, very large faunal, isotopic, and lithologic changes. Lithologies are remarkably similar in the two areas, consisting of, in order from below: marls with thin limestone beds, flaggy limestones, oncoidal crinoidal limestone, oncoidal marls, terrigenous clastics (silty clay, mudstone, sandstone), oolite, and cliff-/gorge-forming limestones. Further, independent correlation dates several of the lithological changes as coeval. The δ 13C excursion in whole rock carbonates is one of the three largest ones known during the Phanerozoic. The increase in δ 13C is very similar in the two areas, from below + 1‰ to c. + 9‰ VPDB though two Gotland samples yielded + 9.71‰ and + 10.54‰ (the corresponding COG interval included fewer carbonate layers than needed to definitely exclude that this difference is due to a lack of suitable rocks for sampling). Compared with the lithologies and the δ 13C curves, the conodont faunas display some divergence, but key taxa permit precise correlations for much of the studied interval. The conodont assemblages change stepwise from very diverse in the Polygnathoides siluricus Zone to a low diversity fauna dominated markedly by a single taxon (Upper Icriodontid Subzone), returning abruptly to a comparatively diverse Ozarkodina snajdri Zone fauna. A method resembling graphic correlation in some respects is used to propose a similarly high-resolution correlation through a longer interval for future testing.