Sedimentological and deformational criteria for discriminating subglaciofluvial deposits from subaqueous ice‐contact fan deposits: A Pleistocene example (Ireland)

Abstract

A pit located near Ballyhorsey, 28 km south of Dublin (eastern Ireland), displays subglacially deposited glaciofluvial sediments passing upwards into proglacial subaqueous ice‐contact fan deposits. The coexistence of these two different depositional environments at the same location will help with differentiation between two very similar and easily confused glacial lithofacies. The lowermost sediments show aggrading subglacial deposits indicating a constrained accommodation space, mainly controlled by the position of an overlying ice roof during ice‐bed decoupling. These sediments are characterized by vertically stacked tills with large lenses of tabular to channelized sorted sediments. The sorted sediments consist of fine‐grained laminated facies, cross‐laminated sand and channelized gravels, and are interpreted as subglaciofluvial sediments deposited within a subglacial de‐coupled space. The subglaciofluvial sequence is characterized by glaciotectonic deformation structures within discrete beds, triggered by fluid overpressure and shear stress during episodes of ice/bed recoupling (clastic dykes and folds). The upper deposits correspond to the deposition of successive hyperpycnal flows in a proximal proglacial lake, forming a thick sedimentary wedge erosively overlying the subglacial deposits. Gravel facies and large‐scale trough bedding sand are observed within this proximal wedge, while normally graded sand beds with developed bedforms are observed further downflow. The building of the prograding ice‐contact subaqueous fan implies an unrestricted accommodation space and is associated with deformation structures related to gravity destabilization during fan spreading (normal faults). This study facilitates the recognition of subglacial/submarginal depositional environments formed, in part, during localized ice/bed coupling episodes in the sedimentary record. The sedimentary sequence exposed in Ballyhorsey permits characterization of the temporal framework of meltwater production during deglaciation, the impact on the subglacial drainage system and the consequences on the Irish Sea Ice Stream flow mechanisms.

Three-dimensional hydrostratigraphy of the Orangeville Moraine area, southwestern Ontario, Canada

“…Thin beds of rippled fine sand and silty sand interbedded with cross-bedded sand (Figs. 11D, 11E), and often traceable between gravel and cobble beds, suggest repeated pulses of sediment-laden water rather than a single depositional event (Ravier et al 2014), providing supporting evidence for discharge into a standing water body (Delaney 2002).…”

“…9B, 9E; BH02, BH14, BH15, BH16) suggest a multistage depositional history was common. The upper portion of pit P5 is characterized by normally graded cobble gravel beds, a bed with 2 m high gravel and coarse sand foresets, relatively thinner beds of cross-bedded gravel, and channels infilled with diffusely stratified coarse sand interpreted as proximal high-density turbulent flow deposits (Winsemann et al 2009;Ravier et al 2014). Gravel and sand infilling a shallow channel eroded into rippled sand is interpreted as a distributary channel scoured by the high-density turbulent flows.…”

“…Ripped-up clasts 3. Repeated discharge events into a standing water body (Delaney 2002;Ravier et al 2014 Fig. 5A).…”

“…Thick, irregular, and typically poorly defined beds of cobble gravel, normally graded cobble gravel, crude to well-defined planar and (or) trough cross-bedded cobble gravel, gravel, and sandy gravel (Figs. 11A-11C) are interpreted as high-density turbulent flow deposits (Winsemann et al 2009;Ravier et al 2014) reflecting a proximal subaqueous fan setting (Benn and Evans 1998;Delaney 2002;Thomas and Chiverrell 2006). Irregularly shaped metre-scale ripped-up clasts of fine sand concentrated near the base of a cobble gravel bed (Fig.…”

“…Rhythmically bedded laminated and rippled sand, rippled silty sand, and rarely silty clay 1. Dense turbulent flow ice-proximal deposits in a subaqueous fan setting (Benn and Evans 1998;Delaney 2002;Thomas and Chiverrell 2006;Winsemann et al 2009;Ravier et al 2014) 1000 2. Ripped-up clasts 3.…”