JOURNAL OF SEDIMENTARY RESEARCH,VOL. 70, NO. 6, NOVEMBER, 2000, P. 1341–1344 Copyright  2000, SEPM (Society for Sedimentary Geology) 1073-130X/00/070-1341/$03.00 A FIELD METHOD FOR DETERMINING THE FIRMNESS OF COLONIZED SEDIMENT SUBSTRATES MURRAY K. GINGRAS 1 AND S. GEORGE PEMBERTON 2 1 Department of Geology, University of New Brunswick, Fredericton, New Brunswick, Canada E3B 5A3 e-mail: mgingras@unb.ca 2 Deptartment of Earth and Atmospheric Sciences, 1–26 Earth Sciences Building, University of Alberta, Edmonton, Alberta, Canada T6G 2E3 FIG. 1.—Test configuration of the modified Brinell firmness test. The parameters used to calculate the force exerted by the substrate are the diameter of the indentor (D), and the diameter of the impression (d). ABSTRACT: Substrate firmness influences the erodibility, remobiliza- tion, and topographic expression of that substrate. Sediment distribu- tion patterns, remobilization of sediment, and the architecture of bio- genic sedimentary structures are strongly affected by the firmness and cohesiveness of the sediment. Given the potentially important role sed- iment firmness plays in different depositional settings it is important to have a consistent means of evaluating it. This paper demonstrates that a modified metallurgical technique, the Brinell hardness test, can be used to produce accurate and consistent firmness data in modern depositional settings. In this method a glass or metal sphere (the indentor) is dropped from a fixed height into a cohesive medium; the size of the indent produced is inversely propor- tional to the firmness of the media. Firmness values can be reported as a pressure exerted by the substrate (kPa). This method has some advantages over standard penetrometers, such as: ease of use, porta- bility and simplicity of equipment, testing a large area, and flexibility of calculation. Field tests show that this method is accurate if the in- dentation diameter is between 10% and 80% of the indentor diameter. The method is inappropriate for dry, unconsolidated sand and thixo- tropic mud. It is, however, extremely useful for assessing the firmness of a wide range of soft to firmground sediments that are composed of clay through coarse sand. INTRODUCTION The Sedimentological Significance of Firm Substrates Substrate cohesiveness, or firmness, is difficult to assess from the rock record. Several studies have shown, however, that trace fossil assemblages in softgrounds, firmgrounds, woodgrounds and hardgrounds are distinctly different from each other. This distinctiveness has permitted ichnologists to characterize these substrate-sensitive trace associations as ichnofacies. These include several softground ichnofacies (Seilacher 1964), and the Glossifungites, Teredolites, and Gastrochaenolites ichnofacies respectively, all of which reflect the different boring or burrowing strategies that are utilized to colonize a substrate. Burrows in soft sediment, for example, result from the activities of infauna moving on and through the sediment for diverse purposes. At the other end of the spectrum, hardground fauna live in hollowed-out living spaces that are similar to their body shape. Firmgrounds encompass a range of sediment cohesiveness and generally consist of advected and excavated burrows that include open tubes, tunnels, and living chambers. It is somewhat intuitive that the degree of firmground induration affects the burrow architecture used to colonize a substrate (Frey and Seilacher 1980; Pemberton and Frey 1985). This has been illustrated in the modern (Pemberton and Frey 1985; Gingras et al. 2000) but has not been related to the rock record; a situation attributable to the small database of modern studies and an absence of reported (comparable) firmness mea- surements. The firmness of a substrate in modern depositional settings is related to many factors. These include grain size, pore-water content, drainage, com- paction, and (in carbonates) the potential for early cementation of grains. Notably, physical and biogenic processes are influenced by the overall co- hesiveness of the substrate. For example, cohesive substrates resist the ero- sion and resuspension of grains (Knighton 1984; DeVries 1992; Dade et al. 1992). Also, antecedent topography due to the erosion of surfaces char- acterized by the patchy distribution of firmgrounds (Huang 1993) affects sediment distribution patterns on many scales (Sanford and Halka 1993). Finally, bioturbation itself alters the overall cohesiveness of the substrate (Cade ´e 1998; Gingras et al. 2000). The dependence of the aforementioned processes on substrate firmness suggests that researchers of modern depositional environments would ben- efit from making detailed observations on the distribution of firmness pro- files. Thomas Ronan, a benthic ecologist/paleontologist, repeatedly dem- onstrated the control substrate stability exerts on burrowing animals. He developed a method for evaluating substrate consistency using a large, weighted rod penetrometer (Ronan 1975). Although his work has not been publicized widely, it provides a framework for comparative, analytical stud- ies (Ronan et al. 1981). Most commonly, however, compaction tests are of greater interest to geotechnical researchers. Their studies range from calculating the mechan- ical resistance of soils (Ohnuki et al. 1997) to subsurface (borehole) en- vironmental interpretation using penetrometer soundings (Nelson et al. 1997). Most previous studies have utilized conical and plate penetrometers. The simplest of these is the drop penetrometer (Levacher 1985), which consists of a dropped conical apparatus that invades the substrate to a certain depth. With the exception of the drop penetrometer, the aforemen- tioned devices do not provide the simplest and most portable equipment for use in field applications. Also, measurements derived from such an apparatus are specific, and can be indicative of the sediment firmness over areas less than 1 or 2 mm diameter. This is not necessarily true of mea- surements taken in relatively soft sediments.