Mario Coniglio
Introduction
Concretions have long been regarded as interesting geologic curios because of their variety of unusual shapes and compositions. In fact some shapes are so bizarre that concretions have been interpreted to be various types of animal and plant fossils (referred to as "pseudofossils"), extra-terrestrial debris, dinosaur eggs, human carvings, etc. The list is endless and appears limited only by the observer’s imagination, (see cover picture). Adding to the confusion is their extreme variability in size, from objects that require a magnifying lens to be clearly visible to huge bodies that weigh several hundred kilograms. Descriptions dating from the 18th century attest to their longevity as objects of interest. The word "concretion" is derived from the Latin "con", meaning "together" and "cresco", meaning "to grow". These "grown together" rocks have a diversity of origins that challenge the sedimentologist (i.e. a geologist specializing in the study of sediments or sedimentary rocks) to routinely integrate information from a variety of disciplines that include biology and medicine, chemical technology, soil science and meteorology with more traditional geological disciplines such as geochemistry, crystal growth studies and microfabric analysis. The uniquely integrative nature of the study of concretions provoked one scientist at the recent (August 1990) International Sedimentological Congress in Nottingham, England to suggest "concretiology" as a new branch of sedimentology! In this article I will discuss the intriguing concretions that outcrop along the Lake Huron shoreline at Kettle Point in southern Ontario. These concretions, locally referred to as "kettles" because of their resemblance to the bottom of a large cooking pot, illustrate many features common to concretions in sedimentary rock sequences. It must be kept in mind, however, that what follows is necessarily an exceedingly abridged profile of how complex concretions can actually be. For example, the concretions at Kettle Point are composed of calcite. Concretions can also be composed of other sedimentary minerals that include dolomite, ankerite, siderite, pyrite, barite and gypsum, to name a few, and the origin of calcite concretions is not necessarily that of barite or gypsum concretions. The genesis of the concretions at Kettle Point can be understood using basic field and microscopic study in conjunction with geochemical analysis. In this article I will discuss how these techniques provide information on how some crystals grow and how the chemical composition of pore waters changes in response to the decay of organic matter in the sediment.
Geologic setting
The famous "kettles" from Kettle Point on Lake Huron (Fig. 1) occur in a modest 2m high shoreline outcrop that extends laterally for approximately 150 m, exposing 5 m of the lower part of the Kettle Point Formation. This outcrop is a provincial historic site in the Kettle Point Indian Reserve and special arrangements were necessary in order to retrieve samples for study. The Kettle Point Formation of southwestern Ontario is part of an extensive black shale sequence that covered much of the eastern United States and parts of central Canada during the Upper Devonian and Early Mississippian. This unit is organic rich, containing up to 15% by weight organic carbon. Studies of the organic matter in the sequence has shown that the organic matter is thermally immature to marginally mature and has neither generated nor lost significant hydrocarbons. The most abundant minerals in the Kettle Point Formation are quartz, averaging 50 weight %, and illite, averaging 22 weight %. Other minerals include feldspar, chlorite, pyrite, dolomite and calcite, the latter of which forms the concretions in the lower part of the formation. Calcareous fossils are generally absent but algal cysts (notably Tasmanites), fragmentary plant material, and minor other fossils such as fish scales and fish teeth may be found. The organic-rich nature of the sediments and low diversity fossil content of these strata has been interpreted to indicate deposition in an oxygen-depleted sea in which water depths may have varied from 50 m to several hundred metres.
Description of "kettles"
The excellent detailed field descriptions made by R.A. Daly in 1900 remain valid to the present day. The kettles are typically spheres to oblate spheroids that are flattened slightly in the plane of bedding. Diameters range from 0.3 to 1.5 m (Fig. 2). Some concretions are complexly shaped, the result of having formed from the coalescence of two or more smaller concretions. The outer surfaces of the concretions may be covered by cm diameter, shallow indentations similar to the surface of a golf ball. Because the concretions are so much harder than the enclosing weakly indurated shale, the "kettles" readily weather out onto the rubble of the shoreline outcrop and the lake bottom adjacent to the outcrop. As many visitors to the area will certify, the local cottage crowd has imaginatively adapted the concretions which readily split into hemispheres, for use as garden and walkway ornaments. Even at the turn of the century Daly lamented "the number now remaining on the shore does not represent the total that could be counted were it not for the deplorable habit of the numerous visitors to the point, who not only carry away the heavy specimens bodily, but break up others with the hope, destined to disappointment, of finding something at the core more interesting than the interior of the already shattered kettles". As noted by Winder (1974), fragmented concretions at Kettle Point all display a similar internal fabric consisting of a massively textured, commonly burrowed inner zone surrounded by an outer zone of radiating fibrous crystals (Fig. 3). Unlike many other examples of concretions described in the literature, those at Kettle Point contain no obvious nuclei, such as macrofossils, at their centres.
Formation of the "kettles"
The concretions are composed of calcite but they also contain all of the same components (quartz, clay minerals, feldspar, pyrite, Tasmanites, etc.) as in the surrounding shale. This is clear evidence that their growth, at least initially, took place close to the sediment-water intgerface and the host clay mud was uncompacted. Uncompacted clay muds can have 80% or more porosity that is filled with seawater. The size of most of these pores is on the order to 1 micrometre (1/1000th of a millimetre) or less. If calcite (or some other concretionary mineral) is precipitated into the pore system, a concretion results from the in situ cementation of the once porous host. The approximately spherical shape of the concretions is strong evidence that the diffusive migration of calcium and bicarbonate ions to the site of calcite precipitation took place in a homogeneous medium, i.e. the permeability of the surrounding clay mud was uniform in all directions. This, of course, can only happen in an uncompacted sediment. If the sediment was compacted prior to concretion growth, distinct permeability anisotropy resulting from compactive alignment of clay particles in the plane of bedding would exist. In such a sediment concretions become conspicuously flattened in the plane of bedding, unlike the Kettle Point concretions. The above explanation accounts for the central massive parts of the concretions. Recall, however, that the outer parts of the concretions are composed of fibrous crystals of calcite. The crystals, rather than filling the microporosity of the uncompacted clay mud, instead pushed the surrounding muds away as they grew! The reasons for this change in crystallization behaviour remain unclear but it seems that a high degree of supersaturation in the pore fluids with respect to calcite was necessary. During subsequent burial, the clay muds surrounding the concretions compacted by dewatering but the concretions, because they were not readily compactable, retained their spherical forms. The result of all of this is that concretions preserve original depositional fabrics and particles whereas, in the surrounding shale, particles are clearly deformed (Fig. 4). The manifestation of this in outcrop is a conspicuous shale "drape" of bedding laminations around the concretions (Fig. 5). What caused the precipitation of calcite in the first place? Evidence from stable carbon isotope analysis of the calcite points quite clearly to an origin involving bacterial sulphate reduction of organic matter. Sulphate reduction is one of numerous pathways by which that certain types of bacteria can metabolize organic matter and this appears to have been the dominant process in the formation of the "kettles" (Fig. 6). The process can be represented by the following simplified equation:
2CH2O + SO2- à H2S + 2HCO3-
where CH2O represents organic matter and dissolved sulphate (SO42+) is derived from seawater. The dissolved hydrogen sulphide (H2S) produced may combine with dissolved iron (Fe2+) in the pore waters to produce iron sulphides, now represented by the mineral pyrite. These phases occur dispersed throughout the shale and also in the form of small concretionary masses. The dissolved bicarbonate (HCO3-) produced ultimately combines with calcium ions (CA2+) from seawater or previously dissolved carbonate minerals to precipitate calcite. If calcite precipitation is localized, as it is at Kettle Point, concretions result. The reasons for the eventual super saturation or pore fluids with respect to calcite remain unclear but an increase in alkalinity related to production of ammonia, various possible phosphate reactions and buffering by hydrogen sulphide are some possibilities.
Applications
Why carry out concretion studies? For Kettle Point type concretions, the results of such studies are useful for better understanding the generation of oil and gas. The Kettle Point Formation is considered to be a potential oil shale. The shallow burial alteration of organic matter and the record left behind in the concretions, besides being fascinating study, has economic significance. Residual organic matter that survives destruction by sulphate reduction, perhaps due to rapid burial or refractory composition, could be an excellent source for economic hydrocarbons, providing the strata become sufficiently buried.
References
Coniglio, M. and Cameron, J.S., 1990. Early diagenesis in a potential oil shale: evidence from calcite concretions in the Upper Devonian Kettle Point Formation, southwestern Ontario. Bulletin of Canadian Petroleum Geology, v. 38, p. 64-77.
Daly, R.A., 1900. The calcareous concretions of Kettle Point, Lambton County, Ontario. Journal of Geology, v. 8, p. 135-150.
Winder, C.G., 1974. The "Kettles" at Kettle Point, Ontario. The Science Terrapin, University of Western Ontario Faculty of Science Bulletin no. 2, p. 13.