Late Ordovician tunnel valleys in southern Jordan

Abstract

The Upper Ordovician glacial record of southern Jordan (Ammar Fm.) essentially consists of palaeovalley infills and of a subordinate time-transgressive fluvial to shallow-marine succession overstepping both the palaeovalleys and interfluvial areas. Valley size (depth, 60-160 m; width, 1-3 km), steep (20-508) margins, internal organization and depositional facies point to an origin as tunnel valleys. The tunnel valleys are infilled by either fluvioglacial sandstones or fluviodeltaic coarsening-upward successions including fine-grained clayey sediments. Re-occupation of previous valleys is evident in places. At least three generations of tunnel valleys are inferred from cross-cutting relationships, although they most probably only reflect temporary standstills and minor re-advances related to the overall recession following the main glacial advance recorded in Saudi Arabia. Petrophysical measurements indicate that higher permeabilities are located in the glacially related strata (1.5-3 darcy in fluvioglacial infills), with a somewhat reduced porosity (22-28%) relative to the preglacial sandstones owing to a higher clay content, probably of diagenetic origin. Sandstone amalgamation, however, gives the fluvioglacial sandstones a high reservoir quality.

Glaciogenic reservoirs and hydrocarbon systems: an introduction

“…Some studies suggest that the tunnel valleys are incised preferentially into low-permeability clays, because high-transmissivity substrates such as sand and gravel with high porosities and permeabilities would retard the incision process (Huuse & Lykke-Andersen 2000a;Janszen et al 2012b;Sandersen & Jør-gensen 2012). This mechanism may also account for observations from the Hirnantian glacial record of Gondwana, where tunnel valleys are typically incised into preglacial siltstones (Douillet et al 2012).…”

“…The Hirnantian glaciation is clearly recorded in the stable isotope signature of low-latitude carbonates (HICE: Delabroye & Vecoli 2010). The glacial record of the Hirnantian maximum is well expressed in parts of the Gondwana supercontinent, particularly in North Africa, Arabia and South Africa, where extensive, laterally continuous outcrops in Saudi Arabia (Keller et al 2011), Jordan (Douillet et al 2012), Algeria (Girard et al 2012;Hirst 2012) and Libya Girard et al 2012) allow detailed investigation of the sedimentology and facies of the glacial succession. Boreholes drilled for petroleum exploration also provide high-quality subsurface data sets in these areas (e.g.…”

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.…”

“…Used here as a generic descriptive umbrella term for structures such as dikes or pillars (e.g., Mills, 1983;Nichols et al, 1994;Owen, 2003;Owen and Moretti, 2008;Douillet et al, 2012). Degassing (or de-watering) structures or injection dikes are interpretative terms of pipes.…”

“…Here, the dark mLA layer is interpreted as fluidized and overpressurized in dynamic pore pressure during flow in order to explain the basal potatoids and intrusions as injection features. Basal dikes in subglacial deposits are indeed usually interpreted as indicating overpressure of the flows and injections (e.g., Douillet et al, 2012). The associated mLA layer would have held part of the overpressure through rapid sedimentation, and subsequently released the gas during deflation and compaction after burial by the overlying layer, further producing degassing pipes.…”