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On the Durability of Shale Lithics — Experimental Constraints, Modern Examples and Implications for Interpreting the Rock Record

Abstract

Detailed petrographic studies of modern muds as well as shales show that they consist of a range of components, including a wide spectrum of composite particles that were contributed in the form of high-water-content suspended floccules, bedload floccules, rip-up intraclasts, pedogenic aggregates, and fully lithified shale clasts. Experimental studies show that shale clasts of sand to silt size (shale lithics), derived from weathering of shale outcrops, are likely to survive multi-year transport periods, and can travel for hundreds to thousands of kilometers of bedload transport, and for multiple thousands of km's in suspension. Shale lithics experience rounding and disaggregation in early transport history and then appear to be stable for extended periods of transport. Observations of modern river and shelf muds reveal the common presence of shale lithics in these sediments, ranging from sand to fine silt size. SEM examination of ion milled samples allows positive identification of shale fragments as small as 10 microns, depending on grain sizes in the original rock. These observations suggest that a significant portion of ancient shale formations could potentially consist of reworked shale lithics and not, as commonly assumed, of primary composite particles such as clay floccules and organo-minerallic aggregates. Identification of shale lithics in the rock record presents challenges, but careful petrographic examination (using SEM and ion-milled samples) and case studies will be essential to develop robust criteria for recognition. The presented observations have manifold implications for the interpretation of many aspects of shales: mud transport and accumulation, sediment compaction and basin-fill modeling, and geochemical proxies. It emphasizes the essential need for petrographic examination of shale samples before more advanced analyses are undertaken.