Follow the Crabs - Crustacean Burrows as Indicators for Maximum Regressive Surfaces
Ramp successions are often heavily bioturbated, show only subtle vertical grain-size trends and a lack of preserved sedimentary structures. A reconstruction of sea-level history in such seemingly monotonous units can therefore be challenging. Here we present a hitherto not described Lower Paleozoic trace fossil and its recent counterpart from ramp settings and discuss their environmental significance. The occurrence of these structures can help to better define the subaerial unconformity and thereby aid in reconstructing sea-level history.
The ancient structures come from the Ordovician Dalby Formation of southern Sweden, and closely resemble burrows from the coastlines of the modern Persian Gulf along Kuwait and the UAE. Both structures are roundish to oval in shape, one to two decimeters in diameter, with an undulating upper surface forming a radial pattern, and a shallow central hole. The recent structures are restricted to the upper intertidal zone and produced by the genus Scopimera. Using this analog, the Ordovician structures are also thought to have formed in the intertidal. As crabs had not evolved in the Ordovician, a hitherto unknown predecessor with a preference for similar habitats, likely related to the crustaceans, must have produced the structures in the Dalby Formation.
As the structures are restricted to intertidal environments, they provide an excellent criterion to define intertidal conditions in the rock record. In the Ordovician Dalby Formation of Sweden, these structures are always overlain by shales and calcareous shales indicating lower energy conditions, an increase in water depth and therewith a transgression. The occurrence of these structures in the rock record indicates that during their formation this particular area had been exposed prior to the transgression which is not indicated by other facies characteristics. It is speculated here that long-term exposure would probably have altered or destroyed these structures, so that only a short time exposure seems plausible. This makes it likely that these structures most often occur close to the maximum lowstand of sea-level and therefore are an excellent correlative of the maximum regressive surface.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009