Skeletal carbonate productivity and phosphogenesis at the lower–middle Cambrian transition of Scania, southern Sweden

Abstract

AbstractThe lower–middle Cambrian transitional interval of Scania is largely represented by condensed limestone beds, lithostratigraphically grouped in the Gislöv Formation (1–5.7 m thick), and the Forsemölla and Exsulans Limestone beds (lower part of the Alum Shale Formation, up to 4 m thick). The strata display a combination of skeletal carbonate productivity, episodic nucleation of phosphate hardground nodules, and polyphase reworking recorded on a platform bordering the NW corner of Baltica. The shell accumulations can be subdivided into three deepening-upward parasequences, separated by distinct erosive unconformities. The parasequences correspond biostratigraphically to theHolmia kjerulfi,Ornamentaspis?linnarssoniandPtychagnostus gibbuszones, the latter two generally being separated by a stratigraphic gap that includes the middle CambrianAcadoparadoxides oelandicusSuperzone. Except for the Exsulans Limestone, the carbonates reflect development of a prolific epibenthic biota, dominated by filter-feeding nonreefal chancelloriid–echinoderm–sponge meadows, rich in trilobites and brachiopods, and which were subjected to high-energy conditions. The absence of microbial mats or veneers encrusting the erosive surfaces of these event-concentration low-relief shoal complexes may be related to long hiatal episodes resulting in microboring proliferation. High levels of nutrient supply resulted in high primary productivity, eutrophic conditions, glauconite precipitation, phosphogenesis (in some case microbially mediated) and microendolithic infestation. An early-diagenetic mildly reducing environment is suggested by the presence of authigenic (subsequently reworked) pyrite, which contrasts with the syndepositional normal oxygenated conditions reflected by macroburrowing and the abundance of benthic fossils.