Depositional framework of the East Baltic Tremadocian black shale revisited

Abstract

This article presents a centimetre-to micrometre-scale study of sedimentary fabrics from Lower Ordovician metalliferous black shale from the Baltic palaeobasin. Two sections of the Türisalu Fm. NW and NE Estonia were analysed with light microscopy and scanning electron microscopy. This rock unit is characterised by mostly thin bedding (,10 mm), common occurrence of minor erosional features, and a large variety of sedimentary fabrics, including graded, cross-laminated and massive fabrics. Based on this, we suggest that dynamic sedimentation events, rather than commonly assumed slow net sedimentation, may be the dominant mechanism behind the accumulation of these beds. The storm-related near-bottom flows and the bed-load transport of mud particles were likely common distribution agents of organic-rich mud. The mud (re)distribution, mainly via near-bottom flows and controlled by flat seafloor topography and general clastic starvation, might explain the present lateral distribution and diachronous character of the Türisalu Fm. Documented traces of microbial mat growth and siliceous sponges in the NW Estonia indicate that in more sheltered settings, biogenic factors played a vital role in developing primary mud characteristics. The geochemical palaeoredox proxies, and high trace metal and organic matter content suggest that mud sedimentation could occur under anoxic conditions. The observed sedimentary fabrics and traces of bioturbation, however, favour prevailing oscillating redox conditions in the lower water column. The recorded heterogeneity of microfabrics indicates that dynamic transport and intermittent deposition together with biogenic factors likely forced the development of an array of unique (bio)geochemical microenvironments for syngenetic trace element sequestration.

Centimetre-scale variability of redox-sensitive elements in Tremadocian black shales from the eastern Baltic Palaeobasin

“…The cyclicity of V abundance might be linked to factors such as alteration of thermohaline circulation or water depth fluctuations and related cyclic changes in dissolved oxygen availability near the sea bottom. On the other hand, Hints et al (2014) suggested that high primary productivity rates driven by phosphate loading and effective recycling in the shallow water column (sea bottom within the photic zone) characterized the accumulation of the muds of the Türisalu Fm. The recorded V pattern might thus be connected to cyclic changes in nutrient loading and palaeoproductivity.…”

“…Based on the biostratigraphic framework and a recent time scale calibration (Cooper & Sadler 2012), one could argue that the primary muds of the Türisalu Fm. should have been deposited within a maximum time frame of ~ 5 My (Hints et al 2014). Whereas the long-term accumulation rates apparently stayed very low during sedimentation of the primary organic-rich muds of the Türisalu Fm., the sedimentary fabrics in the black shales suggest dynamic deposition, frequent reworking of sea bottom and changes in deposition rates (Hints et al 2014).…”

“…should have been deposited within a maximum time frame of ~ 5 My (Hints et al 2014). Whereas the long-term accumulation rates apparently stayed very low during sedimentation of the primary organic-rich muds of the Türisalu Fm., the sedimentary fabrics in the black shales suggest dynamic deposition, frequent reworking of sea bottom and changes in deposition rates (Hints et al 2014). The studies of modern organic-rich muds have revealed usefulness of high-resolution sampling within a limited geographic range (e.g.…”

“…The interpretations rely on enrichment models compiled mostly on the basis of data from anoxic deposits from relatively deep-water settings. Nevertheless, black shales can also be found in shallow-water proximal sedimentary environments (Schieber 1994;Hints et al 2014). The high-resolution case studies of such assemblages help to test the usefulness of trace-metal proxies for palaeoenvironments that are likely prone to a much higher influence of short-term (including annual oscillations) environmental changes compared to deep-water settings.…”

“…Recent findings suggest that the Türisalu Fm. exemplifies transgressive, but rather shallowwater reducing marine muds with spatially heterogeneous properties (Voolma et al 2013;Hints et al 2014). According to biostratigraphic data, the unit is diachronous and varies in thickness from less than half a metre in northeast Estonia to more than 6 m in northwest Estonia (Heinsalu et al 2003) (Fig.…”