Dolomite overgrowths suggest a primary origin of cone-in-cone

Abstract

A long-debated aspect of cone-in-cone structures is whether the mineral aggregates composing the structure precipitated with their conical form (primary cone-in-cone), or whether the cones formed after precipitation (secondary cone-in-cone). A calcite deposit from the Cretaceous of Jordan bears all the defining characteristics of the structure. Trace dolomite within the sample supports the primary cone-in-cone hypothesis. The host sediment is a biosiliceous mudstone containing abundant rhombohedral dolomite grains. Dolomite rhombohedra are also distributed throughout the calcite of the cone-in-cone. The rhombohedra within the calcite locally have dolomite overgrowths that are aligned with calcite fibres. Evidence that dolomite co-precipitated with calcite, and did not replace calcite, includes (i) preferential downward extension of dolomite overgrowths, in the presumed growth-direction of the cone-in-cone, from the dolomite grains on which they nucleate, and (ii) planar, vertical borders between dolomite crystals and calcite fibres. Because dolomite overgrows host-sediment rhombohedra and forms part of the cones, it follows that the host-sediment was incorporated into the growing cone-in-cone as the calcite precipitated, and not afterward. The host-sediment was not injected into the cone-in-cone along fractures, as the secondary-origin theory suggests. This finding implies that cone-in-cone in general does not form over multiple stages, and thus has greater potential to preserve the chemical signature of its original precipitation.

Regional‐scale development of opening‐mode calcite veins due to silica diagenesis

“…SEM images (Figures 10b and 10c) show that cone-in-cone structure is composed of calcite arrayed in conical masses separated by host-rock material. Cone-in-cone locally includes overgrowths of dolomite rhombohedra, suggesting host-rock dolomite formed before the cone-in-cone [Hooker and Cartwright, 2016]. Cone-in-cone appears to expand the sedimentary framework grains [Hooker and Cartwright, 2016], and thus displace the host-rock in a primarily vertical sense.…”

“…Cone-in-cone is present in siliceous mud intervals ( Figure 10). Cone-in-cone appears to expand the sedimentary framework grains [Hooker and Cartwright, 2016], and thus displace the host-rock in a primarily vertical sense. SEM images (Figures 10b and 10c) show that cone-in-cone structure is composed of calcite arrayed in conical masses separated by host-rock material.…”

“…Calcite also fills radiolaria tests ensconced within the cones. Cone-in-cone appears to expand the sedimentary framework grains [Hooker and Cartwright, 2016], and thus displace the host-rock in a primarily vertical sense. Locally the displacive calcite forms strands oblique to bedding (Figures 10a and 10c).…”

“…Locally the displacive calcite forms strands oblique to bedding (Figures 10a and 10c). Cone-in-cone locally includes overgrowths of dolomite rhombohedra, suggesting host-rock dolomite formed before the cone-in-cone [Hooker and Cartwright, 2016].…”

Shale Anisotropy and Natural Hydraulic Fracture Propagation: An Example from the Jurassic (Toarcian) Posidonienschiefer, Germany

“…Networks of host‐rock strands amid vein‐like calcite aggregates are also present in cone‐in‐cone structure (e.g., Cobbold et al, ). Analogous observations – namely, highly contrasting crystallographic orientations between undeformed crystals separated by thin host‐material strands – were used to argue for incorporation of host sediment (Maher et al, ) or host rock (Hooker & Cartwright, ) during cone‐in‐cone growth and not afterward.…”