"The pressures needed to keep CO2 in iiquid form are on the order of 80 atmospheres (or psi, i can't recall which) "
Well it's an order of magnitude difference- atm equal 14.7 psi. And it varys depending temperature (it's mostly cold on Mars). Or:
"The typical standard conditions are a temperature of 68°F (20°C) and a barometric pressure of 1 atmosphere at sea level or 14.696 pounds per square inch (psi)."
http://www.imakenews.com/aristatek/e_article000087864.cfm
Anyways I thought I had a ref, but best I quickly find is this:
http://www.chem.uidaho.edu/~honors/phases.html
"Although carbon dioxide liquid doesn't exist at normal room pressures, it does exist at slightly elevated pressure. A laboratory cylinder of carbon dixoide will contain liquid carbon dioxide at a pressure of about 75 psi at room temperature. On a particularly hot day (above 88 oF) the liquid carbon dioxide will pass though its critical point and the contents of the cylinder will exist as the supercritical fluid."

Anyways roughly speaking a couple feet of solid rock can easily "withstand" this pressure over a short span- if you take the weight of the rock- which is lighter on Mars- you need more rock to hold this pressure back over any span weight. Let's switch to Cu feet instead of Cu inches- 75 psi times 144 equals 10800 pounds per sq ft- the pressure could lower cause the ground temperature on Mars is much colder than "room temperature", but it's in ballpark so we will use it. So the weight of the rock would generally higher than a density of 1- most rocks don't float in water, let's say the rock has average density of 1.8 and since we are on Mars it would be a 1/3 of this weight: .6 (a fairly round number). Now a cubic ft of water weighs 62 lbs, so a cu ft of this rock would weigh 37.2 lbs on Mars. So divide 37.2 into 10800 giving the miminal depth of rock at a density of 1.8 on Mars= 290.3 feet. So with this density of rock around 300' of depth it could hold back hundreds of sq miles in size, pocket of liquid CO2. If the span was shorter (say football field size) you wouldn't this much thickness. But because nature isn't this uniform you probably are looking at a deeper depth- maybe kms in depth with fissures reaching up to near the surface- like how you can have a mountain spring or say a geyser at yellowstone (you need hundreds of psi to make Old Faithful push the water up in the air).
Now if you have a mixture of CO2 and H2O you basically a "Prestone" radiator sealent- the water can freeze solid at surface capping the liquid CO2.