SIGNIFICANCE OF CONE-IN-CONE STRUCTURES IN MARINE MUDSTONES

Many in the last century have proposed mechanisms for development of cone-in-cone structures.  Surely, there are many likely explanations.  Synsedimentary calcite cements defining cone-in-cone structures within mudstones deposited during marine highstands from the mid-Cretaceous Ashville Formation in Saskatchewan, Canada and the Middle Pennsylvanian Floris Formation in Iowa, USA have d¹⁸O compositions (averaging –8.8‰ and –6. 3‰ VPDB, respectively) indicative of precipitation from meteorically derived pore fluids.  The disparity between highstand sedimentation and synsedimentary precipitation of meteoric calcite cements presents a certain conundrum.  Intervals of cone-in-cone cements differ in thickness between locations; however, are interpreted to have grown at, or just below, the sediment/water interface, as is evident by mud drapes.  In both cases, these data are compared with d¹⁸O compositions of approximately coeval paleosol sphaerosiderite nodules (averaging –5.1‰ and –4.6‰ VPDB, respectively), which have been used to establish the d¹⁸O of local paleoprecipitation.  The lighter d¹⁸O values of cone-in-cone calcite cements are interpreted to record the composition of regionally recharged paleoaquifers that discharged through submarine springs.  Submarine fresh groundwater discharge in coastal oceanic reaches has been documented in several modern regions, for example in coastal Florida, USA, South Carolina, USA, and Rishiri Island, Japan.  In both examples, highstand marine mudstones overlie coarser grained strata, which likely provided a conduit for regionally derived meteoric groundwater.  We propose that groundwater flow from a region of terrestrial recharge fed an artesian coastal aquifer system where the potentiometric surface forced meteoric water basinward.  Submarine discharge of carbonate rich groundwater resulted in precipitation of radial fibrous calcite cements, resulting in the development of cone-in-cone structures within stratigraphically significant horizons.