Fossil Wood from Late Pennsylvanian Sabkhas in Socorro County, New Mexico

Silicified wood is plentiful in gypsiferous strata of the Tinajas Member of the Atrasado Formation (Missourian, Pennsylvanian) east of Socorro, New Mexico. Upright fossil stumps and less common prone logs occur across an area of similar to 15 km(2) in at least four layers in the basal 5 to 15 m of the Tinajas Member and at about 43 m above the base of the member at a single locality. Stumps are rooted in micritic limestone and partly encased in calcareous tufa that has a dense, crudely layered and rooted lower portion and vuggy upper portion. Overlying the tufa is soft, powdery, calcareous gypsum in layers as thick as 4 m that exhibit low-angle laminations and is interpreted as subaqueously deposited wind-blown clastics and possible eolian dunes, with inter-dune ephemeral pond deposits that accumulated in a sabkha setting. We propose that trees grew on previously deposited algal flats during a temporary local change in groundwater or surface water to fresh. Resumption of sabkha conditions and evaporation promoted groundwater mineralization at the surface, encasing the stumps in tufa; subsequently, flooding occurred, and gypsum and carbonate sediment derived from adjacent deflating sabkhas was mobilized by the wind and deposited as subaqueously deposited elastics and dunes. Alkaline, silica-bearing water permeated the wood and silicified it shortly after burial.