Seems to me it would be fairly easy to date geological events
on the martian surface using a "crater count" IF...IF...IF...
the martian atmosphere had always been more or less
around 5-10 millibars in pressure at the surface.

BUT!...If you throw in a variable such as the early martian atmosphere
possessing a hypothetical pressure around 1000 millibars
[near our own S.L. air pressure] or greater; perhaps even 7000 millibars
[Permitting the formation of Nick's favorite CO2 clathrates?]
then using "crater counts" to date geological events on Mars
becomes more problematic and difficult.

More so if atmospheric loss on Mars over billions of years
by "sputtering" was not always homogeneous and gradual.

EXAMPLE: Imagine if you will Mars 50-100 million-years ago
possessing an atmosphere with a ground-level pressure of perhaps
200 millibars (thirty times what it is today)?

Now imagine then what might happen if a "Rhode Island-sized" asteroid slams
into Mars perhaps where the Hellas Depression is 50-100 million years ago?
Imagine the awesome energy released & the hypersonic shockwave that results
in the martian atmosphere, stripping away 50 to 90 percent of that
martian atmosphere in a matter of hours?

I think such a hypothetical possibility would throw a "spanner in the works"
as far as "crater-count" dating of geo-events on Mars.

Jacob

"We choose to go to the moon, and do those OTHER THINGS, not because they are easy, but
because they are hard."
John F. Kennedy