GRADATIONAL EPOCHS ON MARS - EVIDENCE FROM WEST-NORTHWEST OF ISIDIS BASIN AND ELECTRIS

Surface morphology and crater statistics from widely separated and gradationally modified regions of Mars provide important constraints on the history of climate-controlled gradation (erosion, transport, and deposition of surface materials) on the planet. Three areas west-northwest of Isidis Basin and an area in Electris record successive epochs of enhanced gradation that generally decreased with time, perhaps even on a global scale. Gradation of at least regional extent affected the areas west-northwest of Isidis in the period following Isidis basin formation until possibly as late as the time of initial Tyrrhena Patera activity (N5 ages or the log number of craters >5 km/106 km2 ∼ 3.5 to ∼2.7). This early period of gradation may have been punctuated by several pulses of increased activity and can be correlated with regional valley network formation. A second period of widespread (perhaps global) gradation affected areas west-northwest of Isidis and resulted in the emplacement and erosion of a volatile-rich unconformable air-fall deposit in Electris. Valley networks formed within the Electris deposits during erosion correlate with a separate later period of valley network production. The end of this second epoch was contemporary with the end of formation and subsequent modification of unconformable deposits within Isidis, Hellas, and Argyre Basins, Sinus Meridiani, and the end of accelerated gradation in Margaritifer Sinus (N5 ages of ∼2.0 to 1.9). Two possible additional periods of gradation affected only one of the areas west-northwest of Isidis: the first (N5 age ∼2.4) related to either the formation of crater Baldet or volcanic activity around Syrtis Major: the second at about the time (N5 age ∼1.6 to 1.7) of early outflow channel formation. Crater destruction during these gradational epochs likely reflects burial by ejecta during nearby impact basin formation followed by reworking and/or intervals of more equable climate created by recycling of endogenic/exogenic volatiles or from juvenile magmatic sources. Climate changes induced deposition of volatile-rich layers in Electris that was contemporary with extensive erosion elsewhere (west-northwest Isidis) and effectively ceased ∼2-3 billion years ago. Gradation during this epoch can be correlated with a late interval of valley network formation that is clearly distinguished from the generally recognized earlier period of dissection. Gradational activity in the study areas since ∼2-3 billion years ago has been minimal. © 1990.