The one's I've found that are not 'disarticulated' are almost always closed, but I do not have a huge sample to go on and am not a paleontologist or avid fossil digger. I've also talked to others who collect fossils, and they say the same thing. But all this I freely admit is basically hearsay (just as your claim is), so if you have a study of some sort to support your claim I would be interested in it. If it is not true that most are not closed, I really would like to know about it so that I no longer use the word "most" in my talks.
I never said "most" are not closed; just that clam fossils are found closed, open, and broken apart. I've found them each way; it depends on the depositional environment.
In fact, there is a specific rock type that is made of broken clam and snail shells:
Also, you are exagerating that rapid burial of marine life is happening "all the time all over the world". I would also be interested to see you support this, especially clear evidence of marine life being buried alive since we know this is the way it generally happened in the past.
Here are some links that mention recent (since the 1890's) observed submarine landslides:
Link4 mentions 7 documented submarine landslides in Puget Sound alone since the 1890's. The other links mention submarine landslides off of Newfoundland, Alaska, Hawaii, and New Guinea.
Anything on the seafloor that was in the path of these landslides was buried instaneously.
Submarine landslides are just one mechanism for rapid burial. Large rivers in flood dump huge amounts of sediments into their deltas, and coastal storms erode beaches and build new sandbars overnight. All of these can cause rapid burial, under the right circumstances.