Depositional Facies and Diagenetic Alteration in a Relict Siliceous Hot-Spring Accumulation: Examples from Yellowstone National Park, U.S.A.

Abstract

ABSTRACT Siliceous precipitates from portions of an 11-m-long core drilled through a relict Yellowstone hot-spring deposit provide valuable insights into the diagenetic facies in this hydrothermal environment. Most of the core consists of relatively low-temperature siliceous sinter containing abundant plant, diatom, and microbial fossilized remains derived from the distal debris apron and discharge channel/flowpath of the hot spring. A few intervals contain stromatolitic horizons analogous to modern, siliceous pool-margin and discharge channel/flowpath facies. Despite the abundance of silicified microbial remains (e.g., silicified stromatolitic horizons as well as individual filamentous microbes, diatoms), distinctive organic biomarkers are not preserved in the core. The apparent lack of distinctive organic biomarkers is attributable to pervasive diagenetic alteration in the presence of high-temperature, chemically reactive fluids. Numerous diagenetic features have been superimposed on the siliceous sinter, especially considering the relatively shallow depths of burial. The uppermost interval (0 to 1.2 m) consists of highly porous siliceous sinter (opal-A) and interbedded travertine. Amorphous manganese oxides and sparry calcite cements are present and are irregularly distributed throughout this interval. Below is a diagenetic facies (4.8 to 5.3 m) that consists of opal-CT and minor chalcedony. The most pervasive diagenetic alteration in the core is the 6.8 to 11.2 m depth interval, where abundant zeolites (mordenite, heulandite), bladed cristobalite lepispheres, chalcedony cements, and calcite veins are present. Judging by the assemblage of plant and diatom remains, most of this pervasive alteration is superimposed on relatively low-temperature siliceous sinters and is likely the result of migration of hot waters through this interval. The paragenesis of all these intervals is very complex, involving numerous episodes of mineral precipitation and dissolution. This diagenetic alteration is facilitated by the porous nature of the siliceous sinter, thermal convection of superheated fluids or steam, strongly oxidizing spring waters, and fluctuating hydrochemical regimes. Rapid dissolution and/or replacement of organic matter indicates that standard morphological means (e.g., petrographic identification of body fossils) may be the best method by which to identify that organisms were previously present in ancient hot-spring accumulations.