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Internal structure and mode of growth of elongate calcite concretions: Evidence for small-scale, microbially induced, chemical heterogeneity in groundwater

2005; Geological Society of America; Volume: 117; Issue: 11 Linguagem: Inglês

10.1130/b25618.1

1943-2674

Peter S. Mozley, J. Matthew Davis,

Building materials and conservation

Research Article| November 01, 2005 Internal structure and mode of growth of elongate calcite concretions: Evidence for small-scale, microbially induced, chemical heterogeneity in groundwater Peter S. Mozley; Peter S. Mozley 1Department of Earth and Environmental Science, New Mexico Tech, Socorro, New Mexico 87801, USA Search for other works by this author on: GSW Google Scholar J. Matthew Davis J. Matthew Davis 2Department of Earth Sciences, University of New Hampshire, Durham, New Hampshire 03824, USA Search for other works by this author on: GSW Google Scholar GSA Bulletin (2005) 117 (11-12): 1400–1412. https://doi.org/10.1130/B25618.1 Article history received: 02 Apr 2004 rev-recd: 09 Mar 2005 accepted: 01 Apr 2005 first online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Peter S. Mozley, J. Matthew Davis; Internal structure and mode of growth of elongate calcite concretions: Evidence for small-scale, microbially induced, chemical heterogeneity in groundwater. GSA Bulletin 2005;; 117 (11-12): 1400–1412. doi: https://doi.org/10.1130/B25618.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Elongate calcite concretions are thought to form parallel to the direction of groundwater flow at the time of their precipitation, and are potentially very useful in determining the nature of paleogroundwater flow and geological variables that influence flow. This study focuses on the mode of growth of such concretions, which has not been previously investigated in detail. Most of the concretions examined are from the Oligocene to Pliocene-Pleistocene, Santa Fe Group of New Mexico, which is the synrifting basin fill of the Rio Grande rift.Based upon their macroscopic characteristics, elongate concretions can be classified into three main types: uniform, composite, and zoned. The uniform and zoned concretions have similar, relatively smooth exteriors, but differ internally, with the zoned variety displaying internal concentric zonation on broken surfaces. Uniform concretions consist of mixtures of early micrite/microspar and later sparite that have different elemental chemistry (mainly Mg variation) and are present throughout the concretion. Petrographic examination reveals significant porosity (up to 15%) in the interiors of some uniform concretions. Zoned concretions consist mainly of sparite, which commonly varies both texturally and in elemental composition in a concentric manner. The composite variety has a rough "warty" exterior and consists of an amalgamation of pea-sized, spherical poikilotopic crystals of calcite. Uniform and composite concretions appear to have formed from the simultaneous precipitation of calcite throughout the volume of the concretions (i.e., early and later stage cements are present throughout). Zoned concretions appear to have formed from concentric growth in which early calcite precipitated at several interior sites (core zones) and was covered by successive layers of younger calcite. Most of these sites of inferred initial precipitation occur preferentially near the up-gradient ends of the concretions, which suggests that the zoned concretions grew in part through preferential precipitation in the direction of flow.The observed petrographic and geochemical characteristics indicate that the elongation is the result of growth within elongate zones of favorable pore-water chemistry (e.g., zones of elevated carbonate alkalinity). Such zones probably formed down-gradient of fragments of organic matter undergoing microbial decay. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.