Early Diagenetic Controls from Porosity Distribution in Carbonate Mounds: Insights from Challenger Mound (Porcupine Seabight, NE Atlantic)
Assessing the degree to which early diagenetic processes predestine subsequent patterns of alteration in ancient carbonate strata is often difficult because of overprinting by late-stage diagenesis. A unique opportunity to examine early diagenetic processes in carbonate mounds without the complications of later burial alteration is provided by Integrated Ocean Drilling Program Expedition 307, which drilled a giant Plio-Pleistocene cold-water coral (Lopehlia pertusa) mound in the Porcupine Seabight off SW Ireland. Challenger Mound is a large, conical edifice ~155 m in height and ~2 km in width that grew atop a regional unconformity developed on silty sandstone drift deposits of middle Miocene age. The mound is unlithified and comprises dm-to-m-scale interbeds of silty coral floatstone-rudstone and bafflestone. The matrix consists of roughly equal amounts of calcareous nannofossils and terrigenous mud, with admixtures of silt-sized particles of coral aragonite. Pore water profiles of Ca, Mg, Sr, sulfate, alkalinity, and δ13CDIC record major loss of aragonite via dissolution and suggest precipitation of minor amounts of secondary calcite and dolomite. Because this mound sits above the aragonite lysocline, it is proposed that decomposition of organic matter is driving these reactions by producing CO2, which enhances aragonite dissolution. The absence of dissolved sulfide in pore water suggests that the sulfide produced by microbial sulfate reduction is precipitating as iron sulfides. These reactions drive up carbonate alkalinity and thus have the potential to drive the system toward calcite and/or dolomite precipitation. Although petrographic observations, XRD results, and stable isotopic data from bulk sediment and skeletal separates confirm aragonite dissolution, there is little evidence in Challenger Mound for either significant calcite/dolomite cementation or aragonite inversion to calcite. Rather, other processes appear to be keeping this system undersaturated with respect to calcite and dolomite despite continuing dissolution of coral aragonite. Continued aragonite dissolution without cementation is expected to gradually lower the diagenetic potential of the mound and delay lithification until deep burial, when chemical compaction processes take effect. Results predict that the final product will be a low-porosity body of silty mudstone-wackestone in which fossil evidence of aragonitic coral framework builders is largely absent.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009