Abstract: Woodgrounds, Log Grounds and the Teredolites Ichnofacies
LAVIGNE, JASON M., MURRAY K. GINGRAS, GEORGE S. PEMBERTON, Ichnology Research Group, Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada and DAVID A. EBERTH, Royal Tyrrell Museum of Palaeontology, Box 7700, Drumheller, Alberta, TOJ OYO .
As genetic stratigraphy continues to evolve, the stratigrapher is continually searching for useful criterion to demarcate stratigraphically significant bounding discontinuities. The presence of bored xylic substrates has been extensively used in identifying marine flooding surfaces in marginal marine environments since the Cretaceous. In the case of woodgrounds, exhumed peat deposits formed in coastal swamps are often laterally continuous and colonization of these substrates typically is the result of lowstand erosion and subsequent marine flooding. These surfaces are valuable stratigraphic indicators. In contrast log grounds are essentially facies associations that contain abundant wood. While many of these logs are in fact bored, their stratigraphic significance is diminished due to the fact that they are commonly transported clasts. Examination of Eocene shoreface sediments of the Empire Formation at Fossil Point, Oregon confirms this as a bimodal current orientation and sparse distribution of bored xylic clasts provides evidence of their transported nature. The log ground concept is, however, useful if one can demonstrate that the wood in question is in situ or has undergone minimal transportation. This is the case in Willipa Bay, Washington where forested low-lying swamps are being are being wave-ravined. The remnant root systems axe ultimately being incorporated into the modern beach. Colonization of this wood by teredinid bivalves and isopods is prevalent This assemblage comprises a complex association of both transported wood fragments as well as trunks of trees that have undergone minimal transport. The occurrence of this wood as a function of wave ravinement at the beach horizon would represent a useful stratigraphic horizon (a transgressive surface of erosion) if it were to pass into the rock record.
Trace fossils are regarded as excellent environmental indicators because they are almost exclusively in situ. Log grounds represent a unique substrate condition because the colonized media are fully transportable. This makes environmental and stratigraphic interpretations of these clasts tenuous at best. The observation of bored wood need not be indicative of the environmental conditions where the clast is incorporated into the stratal package, rather it depicts the conditions prevalent at the time the clast was colonized.
Morphologically, the ichnogenus Teredolites is divided into two species; T. clavatus and T. longissimus. A mud-filled incised valley in the Horseshoe Canyon Formation from Alberta, Canada contains a basal surface that bottoms out on an exhumed coal horizon. This surface is extensively colonized by T. clavatus. The basal 75cm of the channel fill comprises a coarse sand with abundant coalified rip ups. These clasts are colonized by T. longissimus.
The morphologic variation is interpreted to be the result of the interplay between the number of individuals available to colonize and the volumetric quantity of substrate available. There is no evidence in this case of two different trace makers resulting in the different morphologies, rather substrate quantity and colonization density appear to be the controlling factors. This relationship also appears to be represented at the base of brackish bay fill sediments that erosively overlay a coal in the upper Dinosaur Park Formation near Iddesleigh, Alberta.
Ichnofacies names are based on predominant traces within a given assemblage. The Teredolites ichnofacies is not exclusively represented by the ichnogenus Teredolites; Thalassinoides, Diplocraterion and Psilonichnus are also common constituents of the ichnofacies. The Teredolites ichnofacies, like other substrate controlled ichnofacies, is very useful in stratigraphic studies. It is imperative, however, that it be used in a tightly constrained context based on a detailed sedimentologic and stratigraphic framework. The presence of isolated bored wood fragments is not directly indicative of marine influence, especially in cases where the xylic material can be demonstrated to be of allocthanous origin.
AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah