Early burial mud diapirism and its impact on stratigraphic architecture in the Carboniferous of the Shannon Basin, County Clare, Ireland

Abstract

Delta fronts are often characterized by high rates of sediment supply that result in unstable slopes and a wide variety of soft‐sediment deformation, including the formation of overpressured and mobile muds that may flow plastically during early burial, potentially forming mud diapirs. The coastal cliffs of County Clare, western Ireland, expose Pennsylvanian (Namurian) delta‐front deposits of the Shannon Basin at large scale and in three dimensions. These deposits include decametre‐scale, internally chaotic mudstone masses that clearly impact the surrounding sedimentary strata. Evidence indicates that these were true mud (unlithified sediment) diapirs that pierced overlying strata. This study documents a well‐exposed ca 20 m tall mud diapir and its impact on the surrounding mouth‐bar deposits of the Tullig Cyclothem. A synsedimentary fault and associated rollover dome, evident from stratal thicknesses and the dip of the beds, define one edge of the diapir. These features are interpreted as recording the reactive rise of the mud diapir in response to extensional faulting along its margin. Above the diapir, heterolithic sandstones and siltstones contain evidence for the creation of localized accommodation, suggesting synsedimentary filling, tilting and erosion of a shallow sag basin accommodated by the progressive collapse of the diapir. Two other diapirs are investigated using three‐dimensional models built from ‘structure from motion’ drone imagery. Both diapirs are interpreted to have grown predominantly through passive rise (downbuilding). Stratal relationships for all three diapirs indicate that they were uncompacted and fluid‐rich mud beds that became mobilized through soft‐sediment deformation during early burial (i.e. <50 m, likely <10 m depth). Each diapir locally controlled the stratigraphic architecture in the shallow subsurface and potentially influenced local palaeocurrents on the delta. The mud diapirs studied herein are distinct from deeper ‘shale diapirs’ that have been inferred from seismic sections worldwide, now largely disputed.

Constraining recycled detritus in quartz‐rich sandstones: Insights from a multi‐proxy provenance study of the Mid‐Carboniferous, Clare Basin, western Ireland

“…This provenance model agrees with previous basin infill interpretation from Rider (1974) and Wignall and Best (2000, 2016, with the Tullig Cyclothem sandstones being sourced from the south-southwest. This also agrees with the palaeocurrent dataset from Pulham (1989; Figure 1c) and from palaeocurrent data reported at the studied outcrops (Figure 4; Blanchard et al, 2019;Stirling, 2003). In contrast, sourcing from the northwest, as mentioned by Collinson et al (1991), Martinsen and Collinson (2002), Pulham (1989) and Pyles (2008), cannot be easily reconciled with the provenance data, especially given that these new data show that significant southern-derived recycled components can be explained.…”

“…Paleocurrent data (cross beddings, current ripples) for the three locations were reported at Liscannor and Killard outcrops by Stirling (2003) and at the Kilkee outcrop by Blanchard et al. (2019). At the three locations, data show a main flow towards the northeast, except in the channels at Liscannor (towards the north‐northwest).…”

“…Palaeocurrent data for Liscannor and Killard channels and mouth bars are from Stirling (2003) and palaeocurrent data for Kilkee outcrop are from Blanchard et al. (2019). Outcrop GPS coordinates are as follow: Liscannor: 52°56'14.40"N; 9°25'45.33"W, Killard: 52°45'4.69"N; 9°32'53.22"W and Kilkee: 52°40'49.46"N; 9°40'35.21"W…”

“…The top part of the logged section represents the maximum flooding surface of this transgression, with a marine band containing goniatites (R 1B 3), which corresponds to the boundary between the Tullig and the Kilkee Cyclothems (Best et al, 2016;Blanchard et al, 2019). Paleocurrent data (cross beddings, current ripples) for the three locations were reported at Liscannor and Killard outcrops by Stirling (2003) and at the Blanchard et al (2019). At the three locations, data show a main flow towards the northeast, except in the channels at Liscannor (towards the north-northwest).…”

Basin Evolution and Shale Tectonics on an Obliquely Convergent Margin: The Bahia Basin, Offshore Colombian Caribbean

“…Figure is a schematic cross‐sectional model, which illustrates the formation of down‐building, mud‐withdrawal minibasins in the context of the strike‐slip tectonics of the Bahia Basin. Although sediment loading leading to the formation of salt withdrawal minibasins is well known in salt provinces (e.g., Giles & Rowan, ; Hudec et al, ; Sylvester et al, ), it is increasing being recognized in areas of mobile shale where similar basin geometries have been observed (Blanchard et al, ; Duerto & McClay, ; Dupuis et al, ; Ings & Beaumont, ; Morley, ; Morley & Guerin, ; Soto et al, ).…”