'Mummified' would infer that it's something biological that has been preserved by dehydration, protecting it against decomposition by microbial activity. I would be sure that it isn't mummified. It appears to be solid rock or hardened mineralised sediments.
I'm struggling to see it as purely geological, and my best guess would be that it's some kind of ichnofossil... a trace feeding burrow made by something like a marine worm or mollusc that has become infilled by lithified sediments.
When posting an object photo, it's recommended to include a common everyday object (a coin, a ruler, etc.) in the picture for size comparison, along with the dimensions of the find. It's not really clear what size this find is.
All Garrett: Ace 250 (two), AceApex, VX-9. Have owned just about every Garrett ever made. First three detecter I made myself, with the first one in 1962.