Biogenic chaos: Cryptobioturbation and the work of sedimentologically friendly organisms
Abstract Biogenic chaos, from an ichnological point of view, can be defined as total bioturbation of sediment by infaunal organisms, such that the original sedimentary fabric and associated sedimentary structures are entirely destroyed. The truth, however, is that not all forms of thorough bioturbation are purely destructive. As improbable as it initially sounds, sediment can be pervasively (potentially 100%) bioturbated, and yet have suffered remarkably little distortion of the original bedding. In nature there exist many different levels of organism communities. Historically, the study of any natural system begins with the most obvious, attention-grabbing elements. Only later do the more innocuous aspects come to be discovered and appreciated. Ichnology shows a similar historical pattern. The most obvious aspects of the science, the discrete burrow or track forms, have been well studied for many years. Only recently have more subtle biogenic disturbances, such as micro borings, bacterial trails, meiofaunal burrows, infaunal tiering, etc., gained attention. Likewise, another level of bioturbation – cryptobioturbation – exists and is becoming more evident in the rock record. Owing to its diminutive and obscure nature, cryptobioturbation has not received widespread attention. As the name implies, cryptobioturbation is characteristically small in scale and exceedingly subtle in expression. What sets it apart from other forms of bioturbation, however is its tendency to preserve rather than to destroy the sediment’s primary physical structure. The fabric that results from cryptobioturbation can have significant influence on resulting permeability and porosity trends. In some systems where both cryptically bioturbated sandstone and generally unburrowed sandstone are present, mini-permeameter measurements show that numerous heterogeneities are retained in the unburrowed sandstone, resulting in a wide range of permeabilities. In contrast, cryptically bioturbated sandstone intervals show that these heterogeneities are less pronounced, and the resulting permeability curve is more uniform. The effect of cryptobioturbation does not result in enhanced permeabilities, but rather diminishes internal heterogeneities, leading to a more uniform permeability distribution.