Magnetic mineralogy and fabrics of small-scale glacial flutes, Múlajökull and Breiðamerkurjökull, Iceland

Flutes are low-relief, elongate landforms that form subglacially and parallel to the glacier flow direction. They usually consist of till and have boulders at their heads. Flutes can be pervasive in the forefields of glaciers and contain information about the role of bed deformation in basal slip. There are two leading hypotheses for their formation that involve cavity formation in the presence of a deformable bed: the freeze-on hypothesis, in which till that flowed into a cavity downstream from a boulder is frozen into the glacier sole, transported downstream in ice, and then redeposited at the downstream end of the flute, and the cavity–propagation hypothesis, in which till flows into a cavity that begins in the lee of a boulder but propagates downstream in the lee of the flute as it builds downstream with time. Five flutes were studied at two temperate, surge-type outlet glaciers in Iceland, Múlajökull and Breiðamerkurjökull, by measuring fabrics based on anisotropy of magnetic susceptibility (AMS) and till matrix densities to infer past strain patterns. To increase the robustness of AMS measurements, a new method for characterizing AMS ellipsoids, calibrated to laboratory ring-shear fabrics, was used, and the magnetic mineralogy of the till was determined with a series of geomagnetic tests, many of which have not before been applied to tills. Flute pebble fabrics from the literature were also re-analyzed. The magnetic susceptibility of the Múlajökull and Breiðamerkurjökull tills is dominated by pseudo-single-domain titanomaghemite and magnetite, respectively, allowing for a straight-forward interpretation of AMS fabrics. When referenced to a single flute orientation, both AMS fabrics from this study and pebbles fabrics from the literature show that convergent fabrics dominate flutes. Locally, however, AMS fabric orientations are highly variable and had shapes that indicate low-to-moderate shear strains (less than, or not much in excess of, ~7). More importantly, till matrix densities, which are a proxy for past maximum effective stress, are significantly higher in the middle of flutes than at their sides. The difference in densities across the width of flutes was much larger in a parallel-sided flute than in a tapered one. Together, these data indicate that flutes form by cavity propagation (e.g., Benn (1994b)) that requires unsteady, subglacial water pressures and sliding speeds for fluted till to strengthen sufficiently to propagate a cavity downstream. Flutes are initiated and grow through flow of weak till into cavities during periods of high subglacial water pressure and sliding speed. Newly accreted till at a flute's end is then compacted and hence strengthened during a subsequent period of lower subglacial water pressure and sliding speed when water pressure in the leeward cavity falls and the stress that ice exerts downward on the till increases. This effect accounts for high till density near the middle of flutes where cavities were longest and stress increases largest when water pressure fell, and also accounts, through strengthening of till, for the preservation of transverse fabric components in an environment dominated by flow-parallel shear. The strengthened till provides the rigid take-off point for a cavity during subsequent glacier acceleration and thereby allows the flute to grow downstream. Long, parallel-sided flutes and short, tapered flutes are likely end-members of a continuum and represent a high and low degree of till strengthening, respectively, during periods of low water pressure. Flute formation and growth may be influenced by the hydraulic diffusivity of till, and therefore by its texture, which controls the rate and magnitude of effective stress increases during decreases in basal water pressure.</p

Magnetic mineralogy and fabrics of small-scale glacial flutes, Múlajökull and Breiðamerkurjökull, Iceland

Flutes are low-relief, elongate landforms that form subglacially and parallel to the glacier flow direction. They usually consist of till and have boulders at their heads. Flutes can be pervasive in the forefields of glaciers and contain information about the role of bed deformation in basal slip. There are two leading hypotheses for their formation that involve cavity formation in the presence of a deformable bed: the freeze-on hypothesis, in which till that flowed into a cavity downstream from a boulder is frozen into the glacier sole, transported downstream in ice, and then redeposited at the downstream end of the flute, and the cavity–propagation hypothesis, in which till flows into a cavity that begins in the lee of a boulder but propagates downstream in the lee of the flute as it builds downstream with time. Five flutes were studied at two temperate, surge-type outlet glaciers in Iceland, Múlajökull and Breiðamerkurjökull, by measuring fabrics based on anisotropy of magnetic susceptibility (AMS) and till matrix densities to infer past strain patterns. To increase the robustness of AMS measurements, a new method for characterizing AMS ellipsoids, calibrated to laboratory ring-shear fabrics, was used, and the magnetic mineralogy of the till was determined with a series of geomagnetic tests, many of which have not before been applied to tills. Flute pebble fabrics from the literature were also re-analyzed. The magnetic susceptibility of the Múlajökull and Breiðamerkurjökull tills is dominated by pseudo-single-domain titanomaghemite and magnetite, respectively, allowing for a straight-forward interpretation of AMS fabrics. When referenced to a single flute orientation, both AMS fabrics from this study and pebbles fabrics from the literature show that convergent fabrics dominate flutes. Locally, however, AMS fabric orientations are highly variable and had shapes that indicate low-to-moderate shear strains (less than, or not much in excess of, ~7). More importantly, till matrix densities, which are a proxy for past maximum effective stress, are significantly higher in the middle of flutes than at their sides. The difference in densities across the width of flutes was much larger in a parallel-sided flute than in a tapered one. Together, these data indicate that flutes form by cavity propagation (e.g., Benn (1994b)) that requires unsteady, subglacial water pressures and sliding speeds for fluted till to strengthen sufficiently to propagate a cavity downstream. Flutes are initiated and grow through flow of weak till into cavities during periods of high subglacial water pressure and sliding speed. Newly accreted till at a flute\u27s end is then compacted and hence strengthened during a subsequent period of lower subglacial water pressure and sliding speed when water pressure in the leeward cavity falls and the stress that ice exerts downward on the till increases. This effect accounts for high till density near the middle of flutes where cavities were longest and stress increases largest when water pressure fell, and also accounts, through strengthening of till, for the preservation of transverse fabric components in an environment dominated by flow-parallel shear. The strengthened till provides the rigid take-off point for a cavity during subsequent glacier acceleration and thereby allows the flute to grow downstream. Long, parallel-sided flutes and short, tapered flutes are likely end-members of a continuum and represent a high and low degree of till strengthening, respectively, during periods of low water pressure. Flute formation and growth may be influenced by the hydraulic diffusivity of till, and therefore by its texture, which controls the rate and magnitude of effective stress increases during decreases in basal water pressure

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LITHOFACIES, FLOW DIRECTIONS, AND PRELIMINARY DEPOSITIONAL INTERPRETATIONS OF LEDGE-FORMING SANDSTONES AT ALAGNAK, JEZERO CRATER, MARS

International audienceThe Mars 2020 Perseverance rover is exploring a sedimentary deposit interpreted to be theremnants of a delta within Jezero crater, a 45 km diameter Late Noachian-aged crater. During itsexploration of the “lower delta” exposure of the western delta/fan in Jezero crater, Mars (Fig. 1), Perseveranceacquired image and composition data from Alagnak, a ~2 m thick, well-exposed outcrop of clastic sedimentaryrock located at Cape Nukshak (Fig. 1).This outcrop was thoroughly documented from multiple angles at a cm-scale by the Mastcam-Z camerasystem [1] and the Supercam Remote Micro-Imager (RMI) [2] on the Perseverance rover. Dip and strikemeasurements of sedimentary beds were collected from 3D reconstructions of Mastcam-Z stereo-imagescollected using the Planetary Robotics software tools PRoViP and PRo3D [3].In this study, we examine the physical sedimentology and stratigraphic context of the Alagnakoutcrop. The preliminary depositional interpretation of Alagnak is as a prograding, subaqueous fan builtthrough the deposition of many, meter-scale, gravity driven, sediment-rich flows. Flow directions rangedbetween NNE and SW, with principal directions toward SE and E

LITHOFACIES, FLOW DIRECTIONS, AND PRELIMINARY DEPOSITIONAL INTERPRETATIONS OF LEDGE-FORMING SANDSTONES AT ALAGNAK, JEZERO CRATER, MARS

International audienceThe Mars 2020 Perseverance rover is exploring a sedimentary deposit interpreted to be theremnants of a delta within Jezero crater, a 45 km diameter Late Noachian-aged crater. During itsexploration of the “lower delta” exposure of the western delta/fan in Jezero crater, Mars (Fig. 1), Perseveranceacquired image and composition data from Alagnak, a ~2 m thick, well-exposed outcrop of clastic sedimentaryrock located at Cape Nukshak (Fig. 1).This outcrop was thoroughly documented from multiple angles at a cm-scale by the Mastcam-Z camerasystem [1] and the Supercam Remote Micro-Imager (RMI) [2] on the Perseverance rover. Dip and strikemeasurements of sedimentary beds were collected from 3D reconstructions of Mastcam-Z stereo-imagescollected using the Planetary Robotics software tools PRoViP and PRo3D [3].In this study, we examine the physical sedimentology and stratigraphic context of the Alagnakoutcrop. The preliminary depositional interpretation of Alagnak is as a prograding, subaqueous fan builtthrough the deposition of many, meter-scale, gravity driven, sediment-rich flows. Flow directions rangedbetween NNE and SW, with principal directions toward SE and E

INVESTIGATION OF SEDIMENTARY FAN DEPOSITS ALONG THE BEAGLE GAP TRAVERSE AT JEZERO CRATER, MARS

International audienceIn February 2023, the Perseverance rover ascended the Jezero sedimentary fan deposit that has been interpreted as an ancient river delta (~3.6-3.8 Ga). The ~1.2 km traverse up “Beagle Gap” climbed ~35 m in elevation over 10 sols (martian days). The route passed outcrop exposures up to 20 m higher than had been observed along the delta front drive. Image and compositional data add new information on fan sedimentology and stratigraphy, and indicate a complex history of depositional processes and paleoenvironments.At five mounds along the route, 5-10 m thick, thin-bedded horizontal sandstone strata are present in the uppermost exposures. Locally, conglomerate beds are observed within these strata. This unit is interpreted as fluvial deposits in a delta top environment punctuated by episodic high discharge floods that transported cobble to boulder size clasts. Multiple outcrops of inclined strata are present, although these vary significantly in clast caliber, bedding thickness, and dip direction, which may indicate a range of deposit types. With the diversity in observed sedimentary attributes, a variety of depositional models are being evaluated, including deltaic foresets or lobes, and fluvial deposits.Proximity science using the arm instruments was conducted at one location, “Jenkins Gap.” The poorly sorted, pebbly coarse sandstone has extensive porosity evident in WATSON images. Carbonate composition was identified in the matrix by both arm spectrometers. A candidate lake level (-2490 m) identified in the Kodiak outcrop passes through Jenkins Gap, supporting the interpretation of carbonate formation in a shore-marginal environment. Additionally, this outcrop is texturally similar to lacustrine shoreline ‘tufa’ deposits documented at the Provo level of Pleistocene Lake Bonneville in northeast Utah. Further investigation of the fan stratal geometry through correlation with Beagle Gap units will provide constraints on the relative timing and duration of lakes levels at Jezero crater

PAST VARIATIONS OF WATER LEVEL OF JEZERO PALEOLAKE

International audienceThe western fan of Jezero crater displays features interpreted as fluvial and deltaic sedimentary rocks from orbital data [1,2]. Images obtained using the SuperCam Remote Micro-Imager (RMI) and the Mastcam-Z camera provide in-situ observations of Jezero crater’s western fan in various locations along the Perseverance traverse. In the last two years, the rover analyzed the fan front from a distance using these imaging tools and at close range using its entire payload. Then, in 2023, the Perseverance rover explored the top of the western Jezero sedimentary fan. Here we show that fluvial topsets and deltaic foresets dominate sedimentary rocks. Determining the boundary between fluvial and prodelta deposits enables us to draw the evolution of the lake level through time

PAST VARIATIONS OF WATER LEVEL OF JEZERO PALEOLAKE

International audienceThe western fan of Jezero crater displays features interpreted as fluvial and deltaic sedimentary rocks from orbital data [1,2]. Images obtained using the SuperCam Remote Micro-Imager (RMI) and the Mastcam-Z camera provide in-situ observations of Jezero crater’s western fan in various locations along the Perseverance traverse. In the last two years, the rover analyzed the fan front from a distance using these imaging tools and at close range using its entire payload. Then, in 2023, the Perseverance rover explored the top of the western Jezero sedimentary fan. Here we show that fluvial topsets and deltaic foresets dominate sedimentary rocks. Determining the boundary between fluvial and prodelta deposits enables us to draw the evolution of the lake level through time