Stratigraphy, mineralogy, and origin of layered deposits inside Terby crater, Mars

Research areas:
The 174 km diameter Terby impact crater (28.0 degrees S-74.1 degrees E)
located on the northern rim of the Hellas basin displays anomalous inner
morphology, including a flat floor and light-toned layered deposits. An
analysis of these deposits was performed using multiple datasets from
Mars Global Surveyor, Mars Odyssey, Mars Express and Mars Reconnaissance
Orbiter missions, with visible images for interpretation, near-infrared
data for mineralogical mapping, and topography for geometry. The
geometry of layered deposits was consistent with that of sediments that
settled mainly in a sub-aqueous environment, during the Noachian period
as determined by crater counts. To the north, the thickest sediments
displayed sequences for fan deltas, as identified by 100 m to 1 km long
clinoforms, as defined by horizontal beds passing to foreset beds
dipping by 6-10 degrees toward the center of the Terby crater. The
identification of distinct sub-aqueous fan sequences, separated by
unconformities and local wedges, showed the accumulation of sediments
from prograding/onlapping depositional sequences, due to lake level and
sediment supply variations. The mineralogy of several layers with
hydrated minerals, including Fe/Mg phyllosilicates, supports this type
of sedimentary environment. The volume of fan sediments was estimated as
>5000 km(3) (a large amount considering classical martian fan deltas
such as Eberswalde (6 km(3))) and requires sustained liquid water
activity. Such a large sedimentary deposition in Terby crater is
characteristic of the Noachian/Phyllosian period during which the
environment favored the formation of phyllosilicates. The latter were
detected by spectral data in the layered deposits of Terby crater in
three distinct layer sequences. During the Hesperian period, the
sediments experienced strong erosion, possibly enhanced by more acidic
conditions, forming the current morphology with three mesas and closed
depressions. Small fluvial valleys and alluvial fans formed
subsequently, attesting to late fluvial processes dated as late Early to
early Late Hesperian. After this late fluvial episode, the Terby impact
crater was submitted to aeolian processes and permanent cold conditions
with viscous flow features. Therefore, the Terby crater displays, in a
single location, geologic features that characterize the three main
periods of time on Mars, with the presence of one of the thickest
sub-aqueous fan deposits reported on Mars. The filling of Terby impact
crater is thus one potential ``reference geologic cross-section{''} for
Mars stratigraphy. (C) 2010 Elsevier Inc. All rights reserved.