Ferruginous casts of bromalites in kaolin beds: microbial ferrihydrite‐goethite transformation as early stage taphonomy in lacustrine and riparian sediments

Abstract

Two spatiotemporally distant deposits in western North America on differing sides of the K-Pg boundary are recognized to have accumulated innumerable ferruginous faecal-like specimens consisting of 3-D casts suspended in kaolin clay fills of lacustrine and riparian areas proximal to fluvial channels. Evidence presented interprets these specimens as organic in origin, consisting of coprolites and other bromalites (cololites), at the Readlyn deposit of the Whitemud Formation (Maastrichtian) in southern Saskatchewan and the Salmon Creek deposit of the Wilkes Formation (Miocene) near Toledo, Washington State. Comparable taphonomic processes at each deposit are suggested by preservation of identical, metre-long, enigmatic casts of the digestive system (cololites) of an unknown animal associated with the innumerable coprolites. Micro-fabrics of specimen interiors suggest that microbial mats entombed faecal and other organic entities in kaolin-rich lacustrine muds. This research proposes that microbial-induced ferrihydrite precipitation rapidly transformed into thin encrustations of goethite, encasing faecal droppings and other organic remains. Ferrihydrite-goethite-hematite biomineral sequencing cemented kaolin and quartz silt grains at the interface with the organic remains, precluding significant decay and thereby preserving external surface morphologies as encrustations. The mm-thick layers of the cemented sediment enveloped the organic droppings as rigid goethite moulds, preventing the collapse of the encasement morphologies as the organic residues were replaced by biominerals. Concentric growth layers of microbial mat-induced ferrihydrite-goethite cement progressed inward, resulting in 3-D casts. Only external morphologies were preserved, but often with finely detailed surface textures. The interiors of the casts preserve evidence of interconnected fabrics of pseudomorphed bacterial cell walls consisting of radially arranged jackets of acicular (Readlyn deposit) or platy (Salmon Creek deposit) goethite crystallites. Concretionary siderite replacement fabrics in some specimens resulted in the obliteration of the earlier microbial induced biomineral fabrics. Previous interpretations that the Wilkes Formation specimens resulted from inorganic processes only, thereby pseudo-coprolites, have been reappraised as faecal droppings comparable to coprolites of the Whitemud Formation.