The small, articulated bivalves from your field localities 73AOv-2271 and 73AOv02334 are:

Macoma balthica (Linne)

This species is especially common in the upper part of the intertidal zone, including brackish water environments, in bays. In the eastern Pacific Ocean it ranges from Alaska southward to San Francisco.

In answer to your question, the occurrence at locality 2271 is indigenous; the specimens from locality 2334 may be transported or might represent a brief period of marine conditions.

Field locality 2271 is described as the surface of a contemporary intertidal sand bar in Turnagain Arm., Cook Inlet, near Anchorage (60 degrees, 54 min N., 149 degrees, 11 min. W.).