  Unraveling the stratigraphic record is the key to understanding ancient
climate and past climate changes on Mars. River deposits when placed in
stratigraphic order could constrain the number, magnitudes, and durations of
the wettest climates in Mars history. We establish the stratigraphic context of
river deposits in Aeolis Dorsa sedimentary basin, 10E of Gale crater. Here,
wind has exhumed a stratigraphic section of >=4 unconformity-bounded
sedimentary rock packages, recording >=3 distinct episodes of surface runoff.
Early deposits (>700m thick) are embayed by river deposits (>400m), which are
in turn unconformably draped by fan-shaped deposits (<100m) which we interpret
as alluvial fans. Yardang-forming deposits (>900 m) unconformably drape all
previous deposits. River deposits embay a dissected sedimentary-rock landscape,
and comprise >=2 distinguishable units. The total interval spanned by river
deposits is >(1x10^6-2x10^7) yr; more if we include alluvial-fan deposits.
Alluvial-fan deposits unconformably postdate thrust faults which crosscut river
deposits. We infer a relatively dry interval of >4x10^7 yr after river deposits
formed and before fan-shaped deposits formed. The time gap between the end of
river deposition and the onset of yardang-forming deposits is constrained to
>10^8 yr by the density of impact craters embedded at the unconformity. We
correlate yardang-forming deposits to the upper layers of Gale crater's mound
(Mt. Sharp/Aeolis Mons), and fan-shaped deposits to Peace Vallis fan.
Alternations between periods of low vs. high mean obliquity may have modulated
erosion-deposition cycling in Aeolis. This is consistent with results from an
ensemble of simulations of Solar System orbital evolution and the resulting
history of Mars obliquity. Almost all simulations yield intervals of
continuously low mean Mars obliquity that are long enough to match our
unconformity data.
