DISCRIMINATION OF REWORKED PYROCLASTICS FROM PRIMARY TEPHRA-FALL TUFFS - A CASE-STUDY USING KIMBERLITES OF FORT A LA COME, SASKATCHEWAN, CANADA

Reworked volcaniclastics are traditionally discriminated from primary tephra-fall pyroclastics by an absence of features such as blanketing, juvenile lapilli, grain welding and poor sorting. Frequently, these f eatures are difficult to identify, especially in small out-crops, anci ent or altered successions, debris flows and surge deposits. Crystal-r ich volcaniclastics, such as kimberlites, have a large proportion of c oarse euhedral crystals, and abrasion leading to rounding can be recog nised and classified with relative ease. A petrographic method of disc riminating reworked material has been devised, based on the degree of grain roundness, and is illustrated using volcaniclastic kimberlite fr om Fort a la Come, Saskatchewan, Canada. Petrographic thin sections of samples at regular intervals throughout the borehole core, and from a nearby crater-facies kimberlite, show that the percentages of rounded , subrounded and euhedral, grains define two distinct groupings. The f irst group contains a higher percentage of euhedral grains and include s all the samples from the basal 4.8 m of the 14.1-m-thick kimberlite section in borehole 004, and all of the crater facies tephra-fall tuff s. A second group contains more rounded, subrounded and fragmental gra ins and includes all the data from the upper 6.3 m, which are interpre ted as reworked strata. Thus, point counting concurs with hand-sample interpretation and may be used as a verification tool in discriminatin g reworked pyroclastic sands from primary tephra-fall tuffs.