TENTACULITIDS IN SUBVERTICAL (LIFE) POSITION IN THE MIDDLE DEVONIAN ARKONA FORMATION, SOUTHERN ONTARIO, CANADA

Abstract

Abstract A horizon of aperture-up, subvertically oriented shells of Tentaculites in the Middle Devonian Arkona Formation (Hamilton Group) near Arkona, Ontario, Canada, is investigated with respect to burial processes, tentaculitid life habits, and associated diagenetic features. Field observations of the horizon in situ confirm previous suspicions that thick-walled tentaculitoids were benthic and oriented aperture-up in life. In this biocoenosis, tentaculitids vary from low-density populations to dense clumps, the latter sometimes showing grid-like arrangements. The mutual spacing of individuals reflects space demands of a feeding apparatus. The limited size range of the shells suggest that tentaculitid colonization event was brief, involving no more than two generational growth cohorts. Sedimentary features associated with the subvertical shells indicate that the seafloor mud inhabited by the tentaculitids was soft, but sufficiently cohesive to preserve microtopographic features, and prone to disturbance by storms. Preservation of their shells in (subvertical) life position necessitated rapid burial (via mud blanketing) without significant scouring. The emanation of sulfidic decay products from the tentaculitid shell apertures led to the local inhibition of later-precipitated calcareous concretionary cement. Preferential erosion of this material resulted in the development of circular pockmarks on concretion surfaces. The concretions themselves formed along a thin zone of alkalinity that developed below the sediment-water interface at the sulfate-methane boundary during a depositional hiatus sometime after the burial of the subvertical tentaculitids. Variations in the vertical positions of radially tilted tentaculitid shells apertures show undulations that, in turn, imply tentaculitids mutually adjusted their growth directions to maximize living space and/or food acquisition.