My interpretation at the time was that the intensely burrowed interval suggests slow rates of sedimentation, which allowed organisms sufficient time for bioturbation. Abrupt changes in habitat conditions (e.g. increased sediment content in the water column caused by landward subaerial exposure and erosion) destroyed the animal population. The abandoned burrows were subsequently filled with sediment (e.g. fine sand and silt) during a time when sediment bypass occurred.
I was never able to clearly identify these trace fossils. The closest I came to it was "Lanicoidichna", but I do not think this is correct. A colleague suggested that this may be related to the jackknife clam Tagelus (see pictures here and here).
If you recognize these and can name the type of borrow, please let me know; it would be nice to be able to finally find an answer.