ABSTRACT

Crinoid grains from Mississippian skeletal packstones and grainstones from the Lake Valley and Kelley Formations, New Mexico, have retained most of their original Mg, which now resides in 1 to 10 µm microdolomite inclusions. The mean microdolomite content of these crinoids is about 9.3% ± 1.1% (95% confidence limit) by volumes, as determined by petrography. The microdolomite contents show regional variations being lowest in the northern part of the study area (about 6%), and highest (about 11%), 170 km to the south. There are no consistent stratigraphic gradients in microdolomite contents.

The average apparent Mg content of the Mississippian crinoids is estimated from the 9.3% microdolomite by assuming 50% original intraskeletal porosity. This estimate of 10.4 mole % MgCO₃ is lower than the average 14.5 mole % of modern low-latitude crinoids. In spite of this, the higher values of Mg contents of Mississippian crinoids fall within the range of Mg contents of modern, low-latitude crinoids. We interpret these data to mean that original Mg contents of Mississippian crinoids were comparable to those of modern crinoids and that the relatively low mean Mg content of Mississippian crinoids is due to diagenetic loss of Mg. We rule out lower temperatures or Mg contents of Mississippian sea waters and biological differences of modern and Mississippian crinoids as causin the low-Mg contents of Mississippian crinoids. Mg loss from Mississippian crinoids is supported by the decrease in microdolomite content toward rims of grains, and by the fact that intragranular variation is lower than intergranular variations of microdolomite content. On the basis of the above evidence we suggest that, on average, Mississippian crinoids have retained about 75% of their Mg.

The southward increase in Mg content of Mississippian crinoids we interpret to reflect a greater diagenetic Mg loss in the north than in the south, rather than a southward increase in sea-water temperature or a southward change in taxa. This interpretation is supported by the southward decrease in intensity of chemical compaction and by the covariance of Mg and ¹⁸O contents of crinoids. Our diagenetic model, based on these data and on previous work, invokes regional, fresh, phreatic groundwater systems that formed in response to late Mississippian-early Pennsylvanian emergence of the shelf. The groundwaters were recharged in the north and flowed, generally, southward. Northern areas contained diagenetically more aggress ve groundwaters which leached more Mg from crinoid grains than did groundwaters farther south.