Ground-Penetrating Radar (GPR)

Abstract

Ground-Penetrating Radar (GPR) can be used to combine with an outcrop based facies analysis to analyze a 3D facies picture of subaqueous (meaning formed underwater) fan and delta deposits. These sedimentary systems in this studied location of Northern Germany were deposited in the Middle Pleistocene era. Two genetically distinct depositional environments formed on the northern side in front of a retreating ice sheet. They are Bornhausen deltas with a high-angle deposition on a steep delta slope, and Forest beds made up of massive clast supported gravelly and pebbly sands then shifting to planar-parallel pebbly sand deposited by debris flows as well as turbidity flows.

Whereas the finer-grained sands moved further down the slope and deposited from a lower density turbidity current forming massive ripple cross-laminated sands. Glacifluvial deltas are characterized by a steep forest bed with alternating antidune deposits. Deposits lenticular scours are infilled by cross-stratified pebbly sands and gravels. These are perpendicular and oblique to a palaeoflow direction; depositions appear as troughs with low-angle cross-stratified infills.

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Downflow scours filled into sheet-like cross-stratified pebbly sands deposited by antidunes. These antidunes were formed by surge-type density flows, triggered by major water melt or a slope-failure event. Subaqueous fan and delta deposits from a progradational scour fills, like antidunes and humpback dunes. The Freden ice margin depositional systems are more complex, split into two laterally laid sediment bodies. These gravel-rich deposits pseudo-sheets indicate supercritical flows under a high rate of aggradation. The lower part of Freden section shows signs of fan contact.

As for the upper part represents delta-slope deposits, the GPR shows large structures of sand-rich fans characterized by lobe elements with basal surface erosion associated with scour fills related to deposits of antidunes and humpback dunes. The supply of meltwater sediments transported to the slope of a delta is from consistent seasonal flows. During high energy events dunes formed slopping downward passing into ripple marks. The recurrent facies of elements and prograding, as well as retrograding layering patterns, are related as well as interpreted to autogenic flows.

Introduction

The extent of the transgressing ice sheets with in the lowlands of Central Europe were based mainly on glacial landforms and glacial deposits (Krazyszkowski, 1991; Brodzikowski, 1992; Ehlers and Gibbard, 2004). During the Middle Pleistocene glaciation in North Germany had two major ice advances (Kaltwang, 1992; Ehlers and Gibbard, 2004). Tills from the glacial sediments are separated by lacustrine and fluvial deposits with surprisingly no interglacial sediments found (Caspers et al., 1995; Lippstrew, 1995; Stephan, 1995; Unger and Kahlke, 1995). Bedforms that are related to the supercritical flows even including antidunes and recently shown much importance in identifying deposition environments like deltas (Ventra et al., 2015; Dietrich et al., 2016; Normandeau et al., 2016; Massari, 2017) as well as submarine fans and subaqueous ice contract fans (Russell and Arnott, 2003; Winsemann et al., 2009; Postma et al., 2014, Ventra et al., 2015; Lang et al., 2017). Supercritical flows have been much more understood with the advance of numerical and analogue modelling (Alexander et al., 2001; Fedcle et al., 2017; Vellinga et al., 2017).

Subaqueous ice contact fans were deposited by meltwater jetting downhill, discharging at the ground line of a glacier into an existing body of water. There are characterized by a proximal to distal zonation within the ambient flows of water (Powell, 1990; Hoyal et al., 2003; Russell and Arnott, 2003). A basic example of depositional jet flows is high-energy built fan-shaped mouth bar that develops Downflow flute-like scour (Powell, 1990; Russell and Arnott, 2003; Winsemann et al., 2009). On the mouth bar jet flows then evolve into density flows, allowing sediment transport (Powell, 1990; Lang et al., 2012; Dowdeswell et al., 2015). Deltas of the Glacifluvial Gilbert type are seen to be fed by runoff of meltwater streams shown to be characterized by separated icy margins along with subaerial delta lake plains (Lonne, 1995). Sediments are transferred to the front of the delta by multiple sediment-gravity flow depositions, including debris falls, debris flows, and turbidity currents (Ashley, 1995; Lonne and Nemec, 2004; Winsemann et al., 2009). Palaeogeographic glacial lakes in Germany are based on the occurrences of varves which in a sense are much too small to even be recognized or shown to be existent (Klostermann, 1995; Eissmann, 1995; Junge, 1998; Thome, 2001).

These icy systems are hard to understand and still have a lot of major problems with depositional settings within sedimentological recognition. What is understood is that these specific depositional environments, facies, and glacially influenced subaqueous settings are mainly developed in marine environments (Powell, 1990; Lonne, 1995; Merritt et al., 1995; Benn, 1996; Plink-Bjorklund and Ronnert, 1999). Information of high-resolution facies models of glacionite lacustrine ice deposits and coarse grain glaciolacustrine debris flow subaqueous fans are hard to distinguish from other similar facies (Nemec, 1990; Clemmensen and Houmark-Nielsen, 1981; Postma, 1990; Ashley, 1995; Sohn et al., 2002; Richards, 2002).

This research focuses on characteristics of the history of deposits and the paleolocation significance of coarse grained ice deposition in North West Germany. It is thought to be represented by glaciolacustrine subaqueous fan and delta deposits. Their lateral and vertical successions are shown to evolve from formative flow connected to large scale depositional environments. All these are thought to be from subaqueous delta deposits, which formed at the end of the Saalian Scandinavian ice sheet.

Discussion

Deposit Types

Sedimentary facies analysis indicated that the Freden and Bornhausen ice contacts were deposited by subaqueous gravity flows, associated with consistent previous descriptions of subaqueous fan or a coarse grained delta (Clemmensen and Houmark-Nielsen, 1981; Cheel and Rust, 1982; Postma, 1990; Bornhold and Prior, 1990; Nemec et al., 1990; Lonne, 1995; Sohn et al., 1997; Nemec et al., 1999; Plink-Bjorklund and Ronnert, 1999). Large scale beds also form a resulting cross stratification that are typically from Subaqueous fan and delta deposits (McPherson et al., 1988). Subaqueous fans show a prime example of jet flow deposits where the system cannot evolve (Powell, 1990; Hoyal et al., 2003; Russell and Arnott, 2003). The coarse grained deposits being gravel rich as well as sand rich indicate high discharge sediment flows are related to drainage of subglacial reservoirs (Russell and Arnott, 2003; Winsemann et al., 2004; Dowdeswell et al., 2015). Subaqueous fan ice contacts display differences and similarities to submarine fan settings. Submarine settings deposits of densic flows are associated with coarse grained fans (Hoyal et al., 2014; Hamilton et al., 2015), which occur on active continental margins (Pickering and Hiscott, 2015; Lang et al., 2017).

Turbidity Systems

Gravel abundant fan deposits are characterized by scour fills and antidunes bringing on aggradational environments of some turbidity systems as well (Wynn et al., 2002; Postma et al., 2015). This also shows location of rapid flow expansion and declination (Hoyal et al., 2003; Van Wagoner et al., 2003). These Jet flows were derived from glaciers restricted to a certain migration (Hoyal et al., 2003). There was a rise in the surface aggradation as well as deposition (Van Wagoner et al., 2003; Winsemann et al., 2009). An extremely angler bedding with cohesionless sandy debris flows deposits related to fan and delta slopes (Nemec, 1990; Lonne, 1995). These flows stopped when they became frozen stopping the slop from advancing. High density turbidity flows are directly correlated to increasing slop of sand beds and pebbly sand. The smaller grains moved further down slope, as well as getting finer the more it moves downward and outward deposited from both fast and slow-moving turbidity currents. The lack of flow tills and floated ice debris show signs of delta slope environments (Lonne, 1995). The Ice rafted debris have a Palaeoflow direction Northeast and Southeast. Depositional characteristics of antidunes facies indicate which occurs on an active continental margin with sandy subaqueous icy fan contacts forming at the main body of a submarine fan commonly cause by turbidity flows (Ito, 2010). Normally graded beds of gravels and pebbly sands that came from surge like turbidity flows caused by overriding flows or gravity (Nemec, 1990; Plink-Bjorklund and Ronnert, 1999). The tractional deposition and flow regime changed turbidity flows of increasing pebbly sand towards the distal upper fan zone (Plink-Bjorklund and Ronnert, 1999) can as well be a series of debris flows that came from subaqueous flow transformations. As the slop got steeper and steeper a steady current is required to prolong these depositions. Chutes filled with coarse gravel are usually found in this distal upper fan zone. 10W density turbidity flows deposited thin to thick beds with fine to medium grains composed of silt to sand characterizes the mid fan slope (Lonne, 1995). The same types have been seen in different subaqueous fans. Dune formation are rare due to a high suspension fall out even though dune cross stratification can be seen in turbidity currents depositional environments (Ito, 2010; Lang et al., 2017). This can also be related to antidunes. Sand rich delta deposit succession shows increasing discharges from a major flood event, possibly the drawdown of a tide, proven by related measurements of surge type turbidity flows in channel front deltas.

Hyperpycnal Flow

The front of deltas being scoured is shallow as well as shown by occurrences of gravel beds that are lenticular and think. Delta plains can trigger variations of discharged sediments. The brink of a over plunging glacial fluvial systems that are laid sediments will evolve to hyperpycnal flows (Powell, 1990; Russell and Arnott, 2003; Winsemann et al., 2009). Hyperpycnal flows are distinguished by turbidity currents that formed from sediments of flooded rivers. These specific flows seem to only form very low concentrational flows because of mixing sediments. Although, formations of migrating bedforms from hyperpycnal flows can be thrown out due to the lack of suspended sediments in rivers (Talling, 2014).

As for the Freden deltas they show deposits from high lake levels while the space of a delta plain was decreasing (Winsemann et al., 2009). Finer silts and muds overlays cannot be found. Whereas the thicker sandy beds from dunes and ripples need sustainable flows (Winsemann, 2009). According to Plink-Bjorklund and Steel (2004) and Ventra et al., (2015) this may also be related to hyperpycnal flows otherwise known as slope failures. They are also related to low rates of aggregations in deltas when space in the delta brink was decreasing, forming a way for sediments to pass through in the front of a delta (Ventra et al., 2015). Whereas coarse step deposits in a finer grained delta beds with a cross-laminated climbing ripple sand indicates a slope failure with river run-outs from a major flood occurrence (Ventra et al., 2015). So as for these hyperpycnal flows they were still enough to form moving humpback dunes, dunes, and climbing ripples (Alexander, 2001).

Gilbert Facies

To best explain a jet subaqueous fan and delta deposits of the Gilbert type facies is formed by a certain combination of depositional environments. Sandy subaqueous fan and delta sedimentation deposits show a time of low water melt run off, over top of intensely scoured gravelly and sandy surfaces belong to a high energy flow; as well as high discharge of sediments which belong to a small flood type even unlike seasonal water melt run off (Powell, 1990; Lonne, 1995). Deep subaqueous environments are constructed by a fast aggradation of deposits at the base level (Russel and Arnott, 2003). Central scours can become filled with low energy flow deposits, to continue aggradation small jet will then form at these mid channel bar which are fill in the lower areas corresponding to the rising depositional surface (Russel and Arnott, 2003). Deep channel converting to a fan or delta is caused by a major drainage event, possibly like a major rapid lake level fall. Therefore, the glacial lakes high water level controlled the surface allowing water to be stored due to it acting as a base level in and under the Saalian ice sheets. This showing would form a delta or fan (Powell, 1990). Rapid lake level fall would form a delta or fan (Powell, 1990). Rapid lake level falls were most likely caused by rapid openings in the ice sheet that were blocking or holding the water melt. Deep water terminated glaciers are unstable so therefore susceptible to catastrophic retreat. This causes depositing pebbly and gravelly sand and silt deposits.

Conclusion

Ground-penetrating radar can reconstruct in a three dimensional large scale facies model of supercritical flows. Subaqueous gravelly basal ice contract fans are made up of a succeeding scour fills, antidunes and dunes deposits. This shows a waring with supercritical flows. Vertically stacked pseudo sheets show high rate of aggradational depositions.

Subaqueously deposited Freden and Bornhausen sediments are located on the front side of the retreating Saalian age ice sheet. Both icy systems have traits of large scale, high angular environments resulting in a forest deposit on the steep side of subaqueous fans and deltas. The Bornhausen deposits are made up of massive gavels, pebbly sands and planar parallel pebbly sand, alternating through the other two deposits. Avalanche debris flows were stopped when freezing occurred diminishing the slope. High bedload water melt deposits relate to the occurrence of debris flows. The Freden deposits are made up of two different laterally stacked sediments. The southeastern part shows a subaqueous fan ice contact with signs of flow tills and ice-rafted debris deposits. Sediment transportation of a fan is dominated by gravity flows, high-density turbidity currents, representing continuous flows of water melt. As the ice was retreating the center of deposition changed to the northwest part and a Gilbert type delta was formed. On higher parts of the delta slope during a less energetic ripple cross-laminated sandy beds formed.

Subaqueous sand-rich fans are deposited by antidunes and humpback dunes on the fan mouth bar where the glacial jet flow transformed to a density flow and temporarily changed flow conditions. Basal erosions correlated with scour flows on the upward and downward flows transitioning to antidune deposits and progressing to humpback dune deposits. Facies of subaqueous fan deposits are very similar to sand rich submarine fans deposited by supercritical density flows. Perpendicular palaeoflow directions of these deposits are troughs with low angle cross stratified infills. Supercritical density flows are laterally thicker and show a small variation in grain size. Slope failure event or hyperpycnal flows trigger surge type supercritical space changes in a delta brink zone. Supercritical density flows were formed by a low space sediment bypass while hyperpycnal flows triggered density flows along with a slow rise in lake levels while high rates of delta aggradation was occurring.

The first type of delta is characterized by a steep coarse grained forest bed being deposited by high or low density turbidity currents and ununiformed debris flows. This shows an indication of steep high energy deposits. Finer grained delta deposits are overlain by an unconformity, representing a shallower as well as lower energy setting forming a large delta plain. When vertical transition occurs from a delta plain to a delta mouth bar the delta plain must be consumed by water melt along with aggradation in-between increase spacing during the new lake level rise.