Deltaic sediments in Magong Crater, Mars

Kurzfassung

Xanthe Terra is part of the ancient highlands of Mars and hosts a variety of fan-shaped deposits which are most likely the result of episodic fluvial activity. One of the deposits is located in Magong crater, a highly degraded, flat-floored impact crater with a diameter of 40 km, located at 11.9°N and 313.2°E. The sedimentary deposit is situated at the terminus of Sabrina Vallis, a valley system which extends over 250 km through Noachian highlands west of Magong crater. It covers ~20% of the overall crater area. Earlier observations based on orbital spectroscopy revealed weak signatures linked to Fe-/Mg-phyllosilicates that indicate the past presence of liquid water in Magong crater. The deposit exhibits well-exposed sub-horizontal layering at steep erosional cliffs on its eastern and southern margins. Individual layers can be traced laterally for over 1 km. In contrast to the steep eastern margin, the transition from the deposit to the crater floor is rather shallow in the North. The overall slope of the surface is shallow at ~1°. The top surface of the deposit is covered with dust and aeolian bedforms. Possible channels on top of the deposit are hardly visible. A shallow topographic moat at the foot of the eroded cliffs surrounds the deposit in the south and east. The floor of the moat lies up to 10 m deeper than the crater floor and displays an orthogonal pattern of polygonal fractures. Some fractures can be found outside this moat, but there they are not as well-developed. The cracks are ~2 m deep, 10 m wide and extend for up to 1 km. They are filled with aeolian material. The crater floor is flat, with elevations slightly increasing westwards. Various impact craters are filled with aeolian material, whereas others are filled with a dark material. Dark material also forms elevated patches on the crater floor which can be as large as 13 km2. Smaller patches can also be found on top of the fanshaped deposit and in ejecta blankets. These patches are not limited to Magong crater, but are widely distributed across the surrounding highlands and adjacent impact craters such as Lederberg crater. The thickness of the dark patches varies, and can reach up to 4 m. No such patches can be observed in the polygonised areas. A possible explanation for the dark material is possibly tephra ejected by a volcanic field located in Lederberg crater, although there is no strong independent support for this hypothesis. Another possible magmatic landform is a straight, NE-trending topographic ridge that crosses the crater floor east of the sedimentary deposit and may be an exhumed dike. Other possible dikes were identified near the southern crater rim. According to crater counts, Sabrina Vallis formed at 3.8 Ga, whereas the delta represents the last stage of fluvial activity at around 3.4 Ga. An episode of erosion at ~2.2 Ga exhumed the polygonised areas and eroded the dark patches. We will present observations that suggest that the sediments formed in a prograding delta in a crater lake.