Layered Mantle Deposits in Northeast Arabia Terra, Mars: I. Observations of Noachian-hesperian Sedimentation, Erosion and Terrain Inversion

Introduction and Background: Northeast Arabia Terra, west and north of Syrtis Major, has long been known to be characterized by significant terrain mantling and exhumation [1-3]. There are two widespread geological units at a scale of 1:15M [1]: dissected terrain (Npld; more highly dissected by valley networks (VN) than cratered unit; covers extensive area S and W of Syr-tis Major, interpreted as cratered unit highly eroded by fluvial processes [1]) and etched terrain (Nple; similar to cratered unit but deeply furrowed by sinuous intersecting grooves producing etched or sculpted surface; craters and other depressions filled with smooth deposits; forms me-sas; distributed in a band to the N of the dissected terrain, and N and W of Syrtis Major; interpreted as cratered unit partly mantled and dissected by fluvial processes, ground ice decay and minor fluvial activity [1]). The etched and dissected terrain are mapped as Noachian and laterally equivalent stratigraphically [1]. These units are marked by the widespread presence of a layered mantling material that fills the interior of craters and valleys and forms mesas in intercrater areas; geological relations show that the mantle has undergone stripping and removal of hundreds of meters of material [1,2,3]. In the etched unit this material commonly forms inverted topography – recognizable craters and valleys that now lie above the surrounding plains. It was recognized on the basis of Viking data that the formation of this inverted terrain requires an episode of both infilling of formerly low-lying regions (gradation) and preferential removal of the former surrounding topography (erosion) [2]. Viking observations [3] show that the fill material is regionally extensive and forms large, horizontal beds and largely mantles the preexisting topography rather than only filling depressions. The etched unit has a mapped extent of over a million km 2 , and in many places have been extensively eroded; thus, it forms an excellent study area for consideration of erosion/exhumation processes on early Mars. We have been using new spacecraft data (e.g., MOLA, MOC, THEMIS, GRS/NS) to characterize these deposits and assess previous theories of origin. Here we present our observations, focusing on the transition region (centered at 50 °E, 15 °N) between the dissected unit and the etched unit; in a companion abstract [4], we discuss the implications of these observations for models of deposi-tion and erosion of the mantle material. General Observations and Regional Characteristics: Virtually all of NE Arabia Terra region has …