"Massive" is a relative term but, in my case, somewhat smaller.
 
Makua valley -- on the northwest corner of O'ahu -- opens out onto a
sandy beach that is a couple of hundred yards long, bordered by
'rocky' reefs that are handy for getting in and out of the water
when diving (well, maybe only the one on the Waianae side).
 
So... the die-offs occur after heavy rains wash mud and silt down a
creek that splits the beach (known as "Pray for Sex" Beach -- after a
spray-painted phrase on a large block of ancient limestone there) &
reef, then out into the ocean. If an active period of surf is
occurring, the muck gets spread out and has much less of an impact
than...
 
When the outflow sends the silt and debris out over the reef, it
settles in the low spots and suffocates the sand-dwellers. These
hapless creatures get buried under as much as a foot or more of
slit that is the consistency of stiff chocolate pudding. I have
put my hand -- with fingers splayed -- down through this stuff.
The result is a perfect cast of my hand -- all the way down!
The bottom-dwellers are helpless in this stuff because it displaces
the water but is too soft to offer traction for crawling.
 
After a high-surf session remixes this blanket of doom into the
water and spreads it out, the (dead) cones, miters, clams and
terebrids are all over the place!
 
 
Aloha,
 
makuabob (a.k.a. Bob Dayle)