Tracing the Evolutionary Path in the Genesis of Upper Cambrian Lamotte Sandstone, Missouri

Prodip K. Dutta, Kayle Harbour, Sanjay Karnik, Ximing Li, Robert Watts

The fluvial Lamotte Sandstone of Upper Cambrian age in Missouri rests mostly on the Precambrian granite. In two-well exposed sections, saprolite horizons are observed between the granite and sandstone. Field evidence suggests that the saprolite is of pre-Lamotte age. Taking the saprolite horizons to be the representative of source materials, an evolutionary trend in the genesis of Lamotte Sandstone is reconstructed.

Thirty samples, equally divided among granite, saprolite, and sandstone, were examined petrographically. Clay fractions in the saprolite as identified by XRD are predominantly kaolinite with minor illite.

A sequential change in average mineralogical composition expressed in Feldspar/Quartz ratio, from 2.31 in granite, 1.51 in saprolite, to 0.01 in sandstone, is observed. On an average, the saprolites contain about 59 percent altered feldspar grains, but they are almost absent in sandstones.

The supermaturity of the Lamotte Sandstone with nearly 100 percent quartz content is related to a two-step process:

1. Chemical weathering of granite in a humid climate where the feldspars and other unstable minerals are partially altered to clay while a high percent of weathered feldspars is still retained in the source area.

2. Feldspars in the saprolite horizons were too fragile to withstand the rigor of fluvial transport and were totally disintegrated into finer fractions.

The first-cycle quartz arenite is usually interpreted to be the result of chemical destruction of unstable minerals into fine-grained secondary minerals and their subsequent hydrodynamic separation. This work, however, demonstrates that the physical process of transport may also be an important factor in shaping the maturity of first-cycle sand and sandstones.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995