The stratigraphy of the Upper Ordovician carbonate mounds in the subsurface of Gotland

Abstract

In a revision of the stratigraphic classification of the subsurface Upper Ordovician succession of Gotland, the terms Lower and Upper Östersjö Limestone are rejected and replaced by the Slandrom Limestone and the File Limestone, respectively. Two mound generations are recognized, the RakveranNabalan Liste mounds and the Pirguan Klasen mounds, the latter being by far the most common. Both occur not only in the Gotland subsurface but also in submarine areas off Gotland. Moderately diverse conodont collections retrieved from core samples in and around carbonate mounds provide the first direct biostratigraphic dates of these structures. Samples from the Liste mounds contain species of the Amorphognathus superbus Zone. Besides the zonal index and numerous specimens of Panderodus, there are representatives of Amorphognathus complicatus Rhodes, Belodina confluens Sweet, Decoriconus costulatus (Rexroad), Protopanderodus liripipus Kennedy, Barnes & Uyeno, Pseudooneotodus mitratus (Moskalenko), Dapsilodus sp., Besselodus? sp. and a few other taxa. The Klasen mounds have not yet produced biostratigraphically diagnostic conodonts but comparison with non-mound successions indicate that they belong to the A. ordovicicus Zone. Their biostratigraphic age suggests that the Liste mounds may be slightly younger than the Kullsberg mounds, and the Klasen mounds equivalent to the Boda mounds, in the Siljan district.

Carbonate mud mounds, conglomerates, and sea-level history in the Katian (Upper Ordovician) of central Sweden

“…2). The mounds range in thickness from several tens to more than 100 m and have diameters of several hundreds to more than 1,000 m, the Boda mounds being markedly larger than the Kullsberg mounds (Jaanusson 1982, p. 27; see also Bergström et al 2004a). In spite of the impressive size of the mounds, both generations and the intervening bedded limestone can be studied in individual quarries in the Siljan district.…”

Vagrant benthos (Annelida; Polychaeta) associated with Upper Ordovician carbonate mud-mounds of subsurface Gotland, Sweden

“…In terms of conodont biostratigraphy, the Liste mounds belong to the Amorphognathus superbus Zone, whereas the Klasen mounds were inferred to belong to the A. ordovicicus Zone (Fig. No conodonts were recovered that could confirm the latter age, but because the Klasen mounds occur stratigraphically above the Tudulinna Formation, they are believed to be correlated with the A. ordovicicus Zone (Bergström, Löfgren & Grahn, 2004). No conodonts were recovered that could confirm the latter age, but because the Klasen mounds occur stratigraphically above the Tudulinna Formation, they are believed to be correlated with the A. ordovicicus Zone (Bergström, Löfgren & Grahn, 2004).…”

“…1). Subsequently, Bergström, Löfgren & Grahn (2004) studied the recovered conodonts and chitinozoans in closer detail in order to achieve more precise biostratigraphical age determinations of the mud-mounds (see below). Subsequently, Bergström, Löfgren & Grahn (2004) studied the recovered conodonts and chitinozoans in closer detail in order to achieve more precise biostratigraphical age determinations of the mud-mounds (see below).…”

“…This sparked our interest in developing a more indepth study including the scolecodonts, or the jaws of polychaete annelid worms, particularly because these fossils turned out to be the most diverse and one of the most abundant faunal elements associated with these submarine structures. (b) Detail map of northern Gotland showing the location of the six scolecodontyielding drill cores (see also Sivhed et al 2004;Bergström, Löfgren & Grahn, 2004). Moreover, except for a few scolecodonts illustrated by Eisenack (1976) from the island ofÖland, the assemblage described here forms the first record of Ordovician scolecodonts from Sweden.…”

Terminal Ordovician stratigraphy of the Siljan district, Sweden

“…They formed during an interval in Earth history signified by lowstands and cooling events associated with the ensuing end Ordovician glaciations (Tobin et al 2005;Ainsaar et al 2010;Calner et al 2010aCalner et al , 2010b. In Baltoscandia, carbonate formations equivalent to the Boda Limestone are only developed in the subsurface of Gotland (Bergström et al 2004;Sivhed et al 2004), and as mud mounds in the Pirgu Stage of Estonia (Hints & Meidla 1997), while the contemporary Keisley Mound of England and the older (Dapingian) Meiklejohn Peak Mound in the USA are comparable Ordovician structures elsewhere (Jaanusson in Ross et al 1975;Cherns & Wheeley 2007). The excellent preservation, diverse fauna and fairly continuous succession of the Boda Limestone mounds highlight the great importance of these deposits in contributing to the central position of the Baltoscandian region in deciphering global Ordovician climate, ecology and correlation (Ainsaar et al 2010;Kröger & Ebbestad 2014).…”