P. Decaritat et al., COMPARISON OF THE MINERALOGICAL, AND CHEMICAL-COMPOSITION OF 2 SHALESFROM THE WESTERN CANADA SEDIMENTARY BASIN AND THE UNITED-STATES GULF-COAST, Clays and clay minerals, 45(3), 1997, pp. 327-332
The mineralogy and geochemistry of shales reflect the composition of t
he initially deposited precursor mud, subsequently modified by diagene
tic processes. To see if significant geochemical differences exist bet
ween shales that mainly owe their present-day composition to either de
position or diagenesis, we compare the published mineralogical, bulk a
nd clay-fraction geochemical, and clay-fraction O-isotopic composition
s of 2 shales. One shale is from the Western Canada Sedimentary Basin
(WCSB), and its composition mainly reflects primary (depositional) che
mical and mineralogical variations (smectitic to illitic illite/smecti
te) within this unit. The other shale is from the United States Gulf C
oast (USGC), and its composition mainly reflects mixed-layer illite/sm
ectite (I/S) diagenesis of deposited smectitic clay material. The chem
ical and mineralogical trends of WCSB and USGC shares, including one o
f increasing illite content in I/S with depth or maturity, are essenti
ally indistinguishable, in both bulk shale and clay fraction, despite
the contrasting genetic interpretations for the origin of the containe
d I/S. Thus, similar mineralogical and chemical trends with depth or t
emperature can result either from inherited depositional compositional
heterogeneity of the sediment, from burial metamorphism of shale or a
combination of both. Interestingly, the O-isotopic compositions of th
e clay fractions from the WCSB and USGC are significantly different, a
fact that reflects original clay formation from source material and w
ater of quite different isotopic compositions. The discrimination betw
een depositional and diagenetic contributions to shale composition con
tinues to pose challenges, but a combination of bentonite, illite poly
type, clay isotopic and trace and rare earth elemental analyses togeth
er with illite age analysis holds promise for future work.