Facies and geochemistry across the Early-Middle Frasnian transition (Late Devonian) on South China carbonate shelf: Comparison with the Polish reference succession

摘要

Dongcun and Longmen are two important reference sections for the study of the Frasnian carbonate platform of Guangxi (southern South China). At the Dongcun section, conodonts recovered in the present study provide a robust biostratigraphic framework for the Lower-Middle Frasnian interval. The studied interval (uppermost Givetian to Middle Frasnian basal Palmatolepis hassi Zone) is characterized by repeated shallower and deeper water deposits, but the cyclic depositional trend was mainly deepening upward. Carbon isotopic investigation of bulk samples recognizes a major, four-step perturbation across the Lower-Middle Frasnian boundary, which is also evident in the Holy Cross Mountains (Poland). The pattern is reinforced by correlative organic carbon isotopic secular trends from the reference Polish sections: a prominent long-lasting increase in delta C-13(carb) up to 4.5 parts per thousand (event III) is evident in the Palmatolepis punctata Zone, but is preceded and followed by rapid negative shifts (events II and IV, respectively) to ca. 0 parts per thousand; a weak incipient positive excursion (event 1) is recognized in carbonate carbon only in the upper part of the Palmatolepis transitans Zone. In Chinese sections, different background isotopic values indicate decoupled, isotopically dissimilar, hydrothermally affected and mostly oxygenated water masses over the carbonate shelf, as shown by trace and rare earth elements. The shallower water, upslope, diagenetically unaltered Dongcun succession reveals lighter carbon isotopic ratios, on the order of 2 parts per thousand, while the less densely sampled and less certainly dated, mostly muddy-siliceous Longmen downslope succession shows delta C-13(carb) levels similar to those recorded worldwide. The onset of the bipartite event III is marked by C-13 enrichment of 4.3 parts per thousand at Dongcun. Event IV, a light carbon excursion, is especially well recorded in the shallow-water facies by a 2.5 parts per thousand negative shift. The event I excursion in the transitans Zone is obscured, as in some Polish successions, but is probably recorded in organic carbon trends. Slight differences in timing and magnitude of the delta C-13(carb) oscillations may originate from geochemical decoupling between the open ocean and the tectonically active, morphologically differentiated epeiric basins. Nevertheless, a global extent for this major punctata Event is clear from the similar biogeochemical signals recorded on the distantly separated south Laurussian and South China shelves.