There's a good chance this is bacterial canker or Eutypa dieback. If there is a dead limb I'd remove it but not now. Wait until the driest part of summer, say August. For further info UC Davis is your best source. Here is a link that might be useful: apricot diseases

"What caused the injury(ies)" Mabel, Montmorency is said to be good to -40F but not under every circumstance. There are so many things that have to be just right for your tree to be healthy enough to survive that limit. For example a tree that has begun to become stressed can become infected easier with bacteria P.S. short for "Psuedomonas syringae", which is an opportunistic disease. Once stressed and established these bacterias then try to kill plant cells in the way diseases do and the P.S. bacterias do "ice nucleate" adjacent plant cells. This means that the cells that are otherwise prepared to survive much colder than below freezing temperatures are less able to because the cells get injected with compounds that favor ice crystallization at closer temperatures to the normal freezing of water, when then the cells will freeze and burst. I would say most extension services are really not prepared to advise all the things you must do (tree health) because this can become a rather involved project in some weather locals and even soil types. You can't always rely on spraying to prevent initial infection sites because many of them have got specialized and resist the chemicals. I would be prepared to tell you to burn the tree after you've eaten the cherries, or at least surgically torch all the open diseased area. And if it is the disease I truly think it is (it happened in the same way to my cherry in Alaska) planting a peach (lower 48 peach experiences) will give you the same maintenance requirements because they are just as susceptible or more, to the same disease as cherries. But if you DO plant peach I've heard about a new peach rootstock bred in N.J. called 'Guardian' that has in trials shown to keep the host tree scaffold in good enough condition to prevent these bacterial infections, for the most part.

there are two reason to remove the bark.. 1-- so bugs cant hide behind it ... 2 --- so moisture wont build up and lead to rot ... but i said leave it.. because.. right now.. you are more likely to peal into the good stuff ... and make it worse than it is ... giving it a little time.. to start compartmentalizing.. allows the wound to start to heal ... so that you can do the job safer ... think of a nice cut on your finger... with a little skin flap .. give the wound a bit to heal over.. then remove the flap a few days later .... or just take some pruners and remove a big hunk of the separated bark.. it isnt going to heal back on. 6 of this.. half dozen of the other. ken

Don't seal or paint. Advice given above is appropriate. This does not look like cat, or even rodent. I don't see gnawing, but it does looked like a flap as been pulled up and then ripped off, judging by she shape of it. Usually humans do 'that' kind of damage, or something like deer, but that's not deer, either. If you do ever have a chunk taken out that forms a flap, carefully trim the flap off flush and never pull it off. I did that a time or two when I first started pruning years ago, and learnt the hard way. I've seen far worse wounds than this heal up just fine if the only damage is what is apparent by the photograph. I plant in heavy clay too on part of my property, and it looks like there were air pockets around the root ball and they collapsed when the wet soil settled with watering and if this is the case, at the right moisture level, you can take a maddock or something with tines and work the surface up and add a bit more soil, watering it in a bit at a time. But if the soil stays so wet after you watered in and didn't percolate out, and made the tree float in it's situation, you have problems staking won't help. You can plant a lot of trees in heavy clay if you work with it, and not against it, but I've found that planting on high spots, or inclines helps the moisture drain off eventually. If you chose low areas where it can't, you're setting yourself up for issues with aeration or rots down the road and you can literally watch them delcline. It's not always about where you want the tree as much as where the tree wants to be.