Athro, Limited Gubik Field Trip Section 20 Karmuk

You are standing about 3/4 of the way up the cliff (dark blue arrow), looking in the direction of the green arrow. This is near the base of your trench (light blue arrow).

Articulated Bivalve in life position

This part of the section is composed of a silty clay containing scattered shells. The enlarged area shows a fossil bivalve, preserved with both valves intact and articulated. This bivalve was preserved in its living position. Indeed, the interior of shell was empty, no clay had infiltrated into it. This is probably a clam that lived in this clay when it was a mud, died there and was buried without ever having been exhumed and transported. This shell, and many of those in this section, still has its original organic coating (the periostracum). It is not clear in this photo, but some shells in this horizon are surrounded by a rusty halo. This halo suggests oxidized iron sulfates formed in a decay microenvironment around the shell. This suggests that the bivalves died where they are found when the overlying sand was deposited on this clay bed.

Taphonomy is the study of what happens to organisms after they die. One of the key questions in taphonomy is how much a fossil was transported after it died. Finding a bivalve with both valves together like this, and vertically in the sediment, in the position that it lived in (rather than on its side as moving water would have left it) is strong evidence that it was not transported, and is found right where it died.

Move up or down to look at:
• Sands higher in section
• Shell bed lower in section
• Shells low in section
• Contact with Folded Sediments at base of section