Relative sea-level changes and the development of a Cambrian transgression

Abstract

The Lower Cambrian in northwest Scotland is one example of a Lower Palaeozoic ‘orthoquartzite-carbonate’ succession deposited on a slowly subsiding, peneplained Precambrian basement during a period of relative sea-level rise. This particular setting led to the development of a very wide, low gradient shelf which was extremely sensitive to minor sea-level changes. The basal quartz arenite section (Lower Member-Pipe Rock) is a transgressive, tide-dominated systems tract, but lacks a systematic parasequence architecture because of three factors: a fluvial sediment flux was insufficient to induce shoreline progradation, accommodation space was limited during sea-level falls (which are commonly expressed by widespread erosional surfaces), and sediment yield to the shelf by transgressive reworking was a major contributor towards the preserved stratigraphy. The storm-dominated Fucoid Beds represent a condensed section and also show the effects of rapid and widespread facies belt oscillations because of the low shelf gradient. An overlying highstand systems tract is also lacking, partly due to the absence of a large fluvial sediment yield and also due to lowstand and transgressive reworking. An erosively based tidal sandsheet at the top of the Fucoid Beds, interpreted to be a lowstand systems tract, therefore rests directly on the condensed section of the underlying sequence. This was in turn reworked into linear tidal sandbanks (Salterella Grit) during slow sea-level rise, prior to the next major transgression. The limited accommodation space therefore introduced a preservational bias towards deepening-upward trends on a parasequence and sequence scale. The oscillations in facies belts, episodic subareal exposure and the potential to remove substantial portions of systems tracts suggests that Lower Palaeozoic ‘orthoquartzite’ successions may exhibit regular and abrupt vertical shifts in depositional environment which, given their subtle lithological character, may require detailed analysis to identify. Such successions may also display incomplete development of several components of transgressive-regressive sequence architecture.

Sedimentological–ichnological model for tide‐dominated shelf sandbodies: Lower Cambrian Gog Group of western Canada

“…Most siliciclastic shallow‐marine continental shelf facies of Early Cambrian age share a remarkable similarity in the extraordinarily large quantity of sand by comparison with their younger counterparts (Schumm, 1968; Dalrymple et al. , 1985; Cant & Hein, 1986; McKie, 1993; MacNaughton et al. , 1997).…”

Stratigraphy and sedimentology of the Upper Cretaceous to Paleogene Kilwa Group, southern coastal Tanzania

“…Lateral continuity and thickness uniformity of formations along shelf palaeo-strike on a passive margin is not uncommon. For instance, in the North-West Highlands of Scotland, formations and fades units less than 100 m thick can be followed along strike on the CambroOrdovician palaeo-shelf for nearly 240 km with no significant change (see McKie, 1993). However, despite the postulated lateral persistence of the Kilwa Group as discussed below, an Early Oligocene-Miocene unconformity may have led to partial or complete erosion of some of the formations prior to recommencement of deposition in the Neogene.…”

Skolithospipe rock and associated ichnofabrics from the southern Rocky Mountains, Canada: colonization trends and environmental controls in an early Cambrian sand-sheet complex

“…One important distinguishing feature of shallow‐marine settings at this time was the extraordinarily large amount of sand on continental shelves by comparison with their younger counterparts (Schumm 1968; Dalrymple et al. 1985; Cant & Hein 1986; McKie 1993; MacNaughton et al. 1997).…”