I would guess it is Oak, but I would make sure I'd get rid of any of the Rot before using any of it.
And while doing that I would Mask up real good, because rotted wood can carry living fungus that can get into your lungs "Alive" and eventually kill you.
It come more frequently from Lathe turning Spalted Wood, but I'd be very careful of Chainsawing any Rotted wood.
I worked most of my life with wood, and rarely wore a mask, but if anything had been rotted or Spalted, I would wear a good quality mask.
Just My 2 Piasters,
Bear