Silt Injection as a Mode of Fossilization: A Triassic Example

Abstract

Near Salt Lake City, Utah, several limestone ledges in the upper part of the Thaynes Formation contain siltstone replicas of common Triassic pelecypods. Identical siliciclastic silt is abundant in the ledges in the form of narrow dikes, irregular seams, and thin beds. The casts formed when detrital silt filled molds created by dissolution of buried shells. A gravity-controlled process of mold filling is rejected because the casts are not concentrated beneath discontinuity surfaces, and individual specimens lack geopetal fabrics. By contrast, many casts are lateral extensions of dikes that die out upward in the host ledge, and many isolated, convex-up molds are completely filled from end to end, including the highest part of the arch. These features are evidence for injection of liquefied silt as the mold-filling process. The rarity of connections between dikes and thick, inter-ledge siltstone units indicates that layers and lenses of silt within the ledge-forming carbonates were the sources of the injected sediment. The availability of open molds as sites for injection indicates that the carbonate was at least partially lithified and subject to fracturing by the same event that mobilized the silt. The concurrent responses could have been triggered by rapid deposition of overlying strata, by passage of storm waves, or by an earthquake. Lack of sedimentologic evidence for either of the first two phenomena, plus the well-documented association of historical earthquakes with sediment liquefaction, leads to tentative acceptance of seismicity as the most likely of the three triggering mechanisms.