Deposition, floral composition and sequence stratigraphy of uppermost Triassic (Rhaetian) coastal coals, southern Sweden

Abstract

Two uppermost Triassic (Rhaetian) coal beds (A- and B-bed; Bjuv Member, southern Sweden) deposited at the margin of the Danish Basin were investigated in order to interpret the peat-forming environments in a sequence stratigraphic context and to establish the peat-forming vegetations of the precursor mires. Peat formed close to the coastline and was governed by base-level (watertable) rise linked to relative sea-level rise, which is emphasized by rare occurrences of marine phytoplankton, Micrhystridium, Lunnomidinium scaniense and Rhaetogonyaulax rhaetica, in the B-bed. A maximum flooding surface (MFS7) between the coal beds A and B shows that peat accumulation occurred as a coastal response to the continued transgression of the Danish Basin that culminated with deposition of the coal-bearing Bjuv Member. The lower B-bed accumulated in a transgressive systems tract (TST) and the upper A-bed in a highstand systems tract (HST). The B-bed consists of coal and coaly mudstone intervals showing that peat formation repeatedly was outpaced by relative watertable rise. The huminite-rich coal intervals indicate peat formation under oxygen-deficient stagnant water conditions, where the peat accumulation rate balanced the watertable rise. Overall low TS values indicate a freshwater mire. The base of coal bed B marks the start of a relative decrease in creation of accommodation space, and the surface between the sediments and coal bed B represents a Terrestrialisation Surface (TeS). The palynofloral composition suggests that the initial mire was characterised by dipterid/dicksoniacean tree ferns and marattialean ground ferns, and few canopy trees dominated by cupressacean/taxodiacean wet-loving conifers with subordinate bennettitalean and caytonialean trees. The coal intervals in the B-bed represent Terrestrialisation situations characterised by a reduced ratio of accommodation rate/peat accumulation rate. The palynology of these intervals indicates establishment of canopy trees, primarily cupressacean/taxodiacean conifers accompanied by bennettitaleans and caytonialeans, with an understory of marattialean ground ferns and rarer tree ferns. Flooding Surfaces (FS) and Accommodation Reversal Surfaces (ARS) in the B-bed reflect changes in the rate of watertable rise. The coaly mudstone intervals represent Paludification situations with an upward increasing ratio of accommodation rate/peat accumulation rate. In general the palynology of the B-bed suggests a coal floral composition that was dominated by mid and upper canopy plants consisting of cupcressaceaen/taxodiaceaen trees, probably occupying areas with standing water, while ginkgos, cycads, Bennettiales, and Caytoniales and other seed ferns grew in less wet areas. Understory plants were less abundant, whereas a ground cover of ferns, mainly of the families Marattiales and Osmundales, as well as lycophytes may have been pronounced. The upper surface of the B-bed is a Give-up Transgressive Surface (GUTS) above which peat accumulation was replaced by siliciclastic deposition. The palynofloral composition suggests an open-mire dominated by ground ferns and probable tree ferns, with minor bennettitalean and caytonialean mid canopy trees, and with hardly any upper canopy trees. The A-bed is considerably richer in inertinite compared to the B-bed. The palynofloral and petrographic change correlates with an increase in fire activity across the Triassic–Jurassic boundary and deforestation of the Early Jurassic mires.