@Chief Mark,
OK, I got a return email from a geologist at nearby SMU.
Hi Mark,
It’s hard to tell from the pictures, but my best guess is that it is a preserved burrow. Imagine a sandy area of a river point bar or maybe a tidal area of a beach. Some animal, probably an invertebrate like a salt water shrimp or a freshwater crayfish, has a burrow in the sandy substrate. If something eats the shrimp/crayfish, it’s unoccupied burrow will get filled with sediment. In this case it looks like the burrow in the sand was filled with mud. Again, just a guess from the pictures.
I will just go with the local smart guy. I'm happy now! thanks for looking