I dont believe there are any exposed middle cambrian formations in mi ans even if there are, a pebble like that seems so worn that placing in a single rock layer without association, with accuracy, is very hard. My best guess is conglomerate, quartz, or maybe even coral. Some structures do appear suspiciously cnidarian. Burgess shale creatures could only be preserved as 2D stone impressions, and laagerstaten fosssilization in crystalline rock in the pebble is impossible. Still a cool rock though ☺