Subglacial deformation and water-pressure cycles as a key for understanding ice stream dynamics: evidence from the Late Ordovician succession of the Djado Basin (Niger)

Abstract

Subglacial deformation is crucial to reconstructing glacier dynamics. Sediments associated with the Late Ordovician ice sheet in the Djado Basin, Niger, exhibit detailed structures of the subglacial shear zone. Three main types of subglacial shear zones (SSZ) are discriminated. The lowermost SSZ, developed on sandstones, displays Riedel macrostructures and cataclastic microstructures. These resulted from brittle deformation associated with strong glacier/bed coupling and low porewater pressure. Where they developed on a clay-rich bed, the overlying SSZ display S-C to S-C 0 fabrics, sheath folds, and dewatering structures. These features indicate high ductile shear strain and water overpressure. On finegrained sand beds, the SSZ exhibit homogenized sand units with sand stringers, interpreted as fluidized sliding beds. The succession of subglacial deformation processes depends on fluid-pressure behavior in relation to subglacial sediment permeability. Fluid overpressure allows subglacial sediment shear strength and ice/bed coupling to be lowered, leading to ice streaming.

Syn-eruptive, soft-sediment deformation of deposits from dilute pyroclastic density current: triggers from granular shear, dynamic pore pressure, ballistic impacts and shock waves

“…This occurs during or shortly after deposition and prior to consequent diagenesis (Van Loon, 2009;. SSD has been documented for subaqueous clastic sediments from the mud to coarse sand range (Van Loon, 2009;, including turbidites (Moretti et al, 2001), subglacial environments (Ghienne, 2003;Denis et al, 2010;Douillet et al, 2012;Pisarska-Jamroży and Weckwerth, 2013), carbonates (Ettensohn et al, 2011;Chen and Lee, 2013), and volcanic ash (Gibert et al, 2011). In subaerial environments, SSD is documented from earthquake-triggered liquefaction and can form sand blows and dykes.…”

Glacial sedimentology interpretation from borehole image log: Example from the Late Ordovician deposits, Murzuq Basin (Libya)

“…The deformation structures within and under the GA are the best diagnosis to identify the glacial erosion surfaces. The deformation is marked by, sheath folds, intraformational shear planes, and step fractures (Figures 7 and 8a; Denis et al, 2010). The sheath folds are extremely good indicators of the ice flow direction.…”

A new ‘microstructural mapping’ methodology for the identification, analysis and interpretation of polyphase deformation within subglacial sediments

“…Micromorphology is increasingly being used by glaciologists and Quaternary geologists as a primary tool for the analysis of diamictons deposited in different sedimentary environments (van der Meer 1987;Harris 1998;Lachniet et al, 1999;Carr et al, 2000;Carr 2001;Lachniet et al, 2001;Menzies and Zaniewski 2003;Carr et al, 2006;Phillips 2006;Menzies et al, 2006;Reinardy and Lukas 2009;Kilfeather et al, 2010), as an aid to understanding the processes occurring beneath glaciers (Menzies and Maltman 1992;van der Meer 1997;Menzies et al, 1997;Khatwa and Tulaczyk 2001;van der Meer et al, 2003;Roberts and Hart 2005;Hiemstra et al, 2005;Baroni and Fasano 2006;Larsen et al, 2006;Hart 2007;Larsen et al, 2007), to *Manuscript Click here to view linked References help unravel the often complex deformation histories recorded by glacigenic sequences (van der Meer 1993;Phillips and Auton 2000;van der Wateren et al, 2000;Phillips et al, 2007;Lee and Phillips 2008;Denis et al, 2010), and investigate the role played by pressurised pore-water/melt water during these deformation events (Hiemstra and van der Meer 1997;Phillips and Merritt 2008;van der Meer et al, 2009;Denis et al, 2010). In the majority of these published studies the terminology used to describe the various microtextures in thin section follows that proposed by van der Meer (1987van der Meer ( ,...…”

“…In a number of studies of polydeformed glacial sediments (e.g. Phillips and Auton 2000;van der Wateren et al, 2000;Phillips et al, 2007;Denis et al, 2010) the successive generations of plasmic fabrics, folds, rotational structures and faults present in thin section have been distinguished by applying the methodologies and nomenclature normally used in structural geological studies (e.g. F1 earliest folds to Fn latest, S1 earliest fabric to Sn latest).…”