carbonate cements Archives

This is part of the of How To…series… on carbonate rocks

The diagrams and images of carbonate crystal habits and cements are descriptive and intended to provide essential background to other posts that detail the different diagenetic environments.

Carbonate diagenesis is like a game of two halves: one part involves mineral dissolution, the other precipitation. The two commonly go hand-in-hand; it all depends on the changing fortunes of thermodynamic stability and interstitial fluid flow as the game progresses.

Cements precipitate in available pore space: intergranular, intragranular (like the whorls of gastropods, the septa of corals, or the chambers of foraminifera), larger voids like those developed in reef frameworks, and microporosity such as pore throats between grains. Neomorphism and mineral replacement involve dissolution and precipitation that change existing cement fabrics and sediment frameworks and hence are not confined to pore space.

Carbonate cements are as varied as the diagenetic environments in which they form – the sea floor, meteoric, deep burial and everywhere in between. The crystal shapes of CaCO₃, it’s polymorphs and chemical variants range from needle and whisker-like, to blocky spar. The transitions from one crystal form to another, their growth in open pores, and replacement by stable carbonate phases is what makes carbonate petrography so fascinating. Continue reading →

This is the companion Atlas to the Cool-water carbonates – outcrop images.

New Zealand cool-water carbonates are predominantly bioclastic, consisting of fauna like bivalves, calcareous bryozoa, barnacles, echinoderms, and flora such as rhodoliths (calcareous algae that encrust rock fragments and shells). This is particularly the case on shelves with little terrigenous sediment input, such that there is a diverse epifauna. Good examples of this setting occur around New Zealand. There is no evidence in any of the Oligocene through Pleistocene stratigraphy for aragonite-producing algae like Halimeda and Penicillus.

However there is a range of bioclast compositions, ranging from low- to high magnesian calcites and aragonite, The bioclast compositional variation has a significant impact on cement types (micrite and rhomohedral calcite envelopes) calcite spar, and neomorphic replacement by calcite of original bioclast aragonite. Like cements in more tropical realms, the cement paragenesis in cool-water carbonates reflects complex histories of fluid flow and evolving fluid chemistry through sea floor cementation, burial, uplift and ingress of meteoric water. Some useful references describing the paragenesis of Pliocene cool-water carbonates from the east coast of North Island (Te Aute Group) are given below.

Contributors:

Vincent Caron is a lecturer in geology and researcher in carbonates based at the Université de Picardie Jules Verne in Amiens, France. He is also a member of the Basins Resources Reservoirs research group. Most of the images presented here on Te Aute limestones formed part of his PhD research at Waikato University. A short list of his publications on the Te Aute is shown below.

Cam Nelson, is one of the original adherents of the Cool Water Carbonate paradigm, who set the scene with his studies of the Oligocene Te Kuiti Group. Cam is an Emeritus Professor at Waikato University.

CS Nelson, PR Winefield, SD Hood, V. Caron, A Pallentine, and PJJ Kamp. 2003. Pliocene Te Aute limestones: Expanding concepts for cool-water shelf carbonates. New Zealand Journal of Geology and Geophysics, 46, 407-424.

V Caron, CS Nelson, and PJJ Kamp. 2004. Contrasting carbonate depositional systems for Pliocene cool-water limestones cropping out in central Hawke’s Bay, New Zealand. New Zealand Journal of Geology and Geophysics, 47, 697-717.

B.D. Ricketts , V Caron & C.S. Campbell 2004. A fluid flow perspective on the diagenesis of Te
Aute limestones. New Zealand Journal of Geology and Geophysics, 47:4, 823-838

V Caron and CS Nelson. 2009. Diversity of neomorphic fabrics in New Zealand Plio-Pleistocene limestones: Insights into aragonite alteration pathways and controls. Journal of Sedimentary Research, v. 79, p. 226-246.