ABSTRACT

Study of heavy minerals from Upper Cretaceous-Paleocene sandstones of Alberta reveals that imbricate wedge markings, previously successfully reproduced artificially on garnet, are a common feature of naturally etched garnet, sphene and staurolite. Hacksaw terminations are exhibited by naturally etched sphene, staurolite, hornblende, epidote and clinozoisite.

Detailed examination of these etch features with the scanning electron microscope has yielded the following information on hornblende and garnet: Etching and eventual removal of hornblende grains by intrastratal solution begins with the smoothing of the cleavage surfaces and simultaneous etching of the grain terminations. Differential removal of cleavage plates at the terminations results in the hacksawed appearance. Cleavage plates retreating away from the terminations are eventually removed exposing deeper cleavage plates; if the etching process continues the grain will evntually disappear. Garnet etching and removal proceeds by solution sculpture of imbricate wedge markings. With further etching the grain is reduced to a skeletal form. Calcite replacement appears to be an important factor in the removal of many heavy minerals, especially garnet.

Fragile, hacksawed hornblende grains of this study are interpreted to be the result of intrastratal solution. Although it is possible that a large volume of heavy mineral grains in a sandstone may be completely dissolved by chemical etching, it is doubtful that intrastratal solutions have compositions which are regionally consistent and pervasive enough to uniformly dissolve an entire heavy mineral population.