Palaeoenvironmental significance of cool-water microbialites in the Darriwilian (Middle Ordovician) of Sweden

Abstract

Well‐developed oncoids and centimetre‐sized stromatolites are reported for the first time from the Darriwilian (Middle Ordovician) cool‐water ‘orthoceratite limestone’ at Kinnekulle, Västergötland, Sweden. The characteristics and stratigraphical distribution of these microbialites show an apparent relationship to fluctuations in relative sea level. The most abundant and well‐developed oncoids occur in stratigraphical intervals that are characterized by notable sea‐level lowstands. Stromatolites, which share many compositional characteristics with the oncoids, are essentially confined to a single bed associated with an especially prominent lowstand. Stromatolite‐like lamination also occurs in the uppermost part of the studied succession, but this feature may be of abiogenic origin. The microbialites appear to be originally calcareous, but synsedimentary iron‐ and/or phosphate‐enriched laminae are conspicuous, and secondary substitution by coarse calcite and barite is common. Iron staining is most prominent in poorly preserved specimens. Diagenesis has occluded the identity of the producers of these microbialites, but characteristics of associated endolithic borings suggest that they were formed in photic waters. The laminated fabrics of the documented microbialites record a depositional environment sensitive to high‐frequency environmental change. Most significantly, the microbialites have provided important information about the depositional environment of their enigmatic host limestone, and the collective observations challenge the notion that the studied strata were deposited in a deep shelf to basinal environment – rather, it appears that they are to a large extent, shallow‐water deposits, formed in waters only a few tens of metres deep.

An extraterrestrial trigger for the mid-Ordovician ice age: Dust from the breakup of the L-chondrite parent body

“…S4). Oncoids and stromatolites, both clear shallow-water indicators, have also been reported ( 44 ).…”

“…In particular, the magnitude of a sea level change is often very difficult to quantify. However, the prominent sea level lowstand associated with the Täljsten is a very obvious feature [e.g., [17][18][19][41][42][43][44]. The 1.4-m thick, gray Täljsten in the middle of a ca.…”

Refined Ordovician timescale reveals no link between asteroid breakup and biodiversification

“…Although the host bed is not recognized as a discrete bentonite horizon, we interpret the zircon grains to be the result of an ash fall related to distal active volcanism. The action of burrowing organisms, as is recorded in the ‘orthoceratite limestone' by pervasive bioturbation patterns and numerous ichnofossils 28 , would quickly mix the fallout from ash clouds into the muddy seafloor and prevent the formation of discrete bentonite beds. Still, bentonite beds do occur in the ‘orthoceratite limestone' as well as in coeval shales.…”

From the mid‐Ordovician into the Late Silurian: Changes in the micrometeorite flux after the L chondrite parent breakup

“…The depth at which the Orthoceratite Limestone formed has been intensively debated with some authors arguing for “deep” conditions (one to several hundred meters) (Lindström ; Chen and Lindström ). Others argue that depths were less than 100 m (e.g., Jaanusson ; Nielsen ; Lindskog ). Some of the confusion may reflect that authors base their generalizations on different sections that represent different depth regimes.…”