Accueil du site
Master
Canada
Sedimentology, diagenesis and hydrochemistry of the saline, alkaline lakes on the Cariboo Plateau, Interior British Columbia, Canada
Titre : Sedimentology, diagenesis and hydrochemistry of the saline, alkaline lakes on the Cariboo Plateau, Interior British Columbia, Canada
Auteur : Hirst, Joanne Frances
Université de soutenance : University of Saskatchewan
Grade : Master of Science (M.Sc.) Geological Sciences 1995
Résumé
Several hundred lakes occupy the
semi-arid glaciated terrain of the
Cariboo Plateau in Interior British
Columbia. Two small (<1 km2), shallow,
closed-basin lakes with Na-C03-S04-Cl
brines, Goodenough Lake and Last Chance
Lake (15-350 gL-1 TDS, pH 9.7-10.7),
have been studied in detail.
Sedimentological examinations of the
lakes aimed to i) investigate the
distribution and nature of the modem
sediments, emphasizing depositional
processes and brine evolution ; ii)
explain the origin of the extensive
recent dolomite ; and iii) identify the
effects of freezing on the sediments and
to seek criteria to allow the
recognition of cold saline lakes in the
geological record. Goodenough Lake is a
hypersaline perennial lake and Last
Chance Lake is ephemeral, desiccating to
a saline pan in autumn. The lakes are
fed primarily by groundwater, and modem
sedimentation in the basins is
predominantly autochthonous, comprising
carbonates, sodium-carbonate evaporites
and organic-rich muds, with a minor
detrital component. Last Chance Lake is
characterized by several hundred brine
pools (<3 m deep ; <25 m wide) within
carbonate muds, and the pool morphology
appears to be a function of basin depth
and proximity to groundwater seepages.
The saline lakes are extremely
temperature-sensitive and freeze over in
winter, which has important consequences
for brine chemistry, mineral formation
and sedimentary processes. The lakes
show a mineralogical zonation of
carbonates from marginal sites of dilute
inflow towards the hypersaline basins,
due to evaporative concentration. Recent
non-stoichiometric, euhedral dolomite
rhombs (0.5-4 Â_m) and anhedral
aggregates (1-10 Â_m) are found
extensively across the catchment area,
including seepage zones, mixed
carbonate-siliciclastic mudflat
deposits, lake muds and microbial mats,
and are commonly intermixed with
sodium-carbonate salts. Influential
factors in dolomite formation are
probably the mixing of ’dilute’ inflow
waters with mudflat pore waters, and
degassing of C02 from microbial mats.
Most dolomite is thought to have a
primary origin, but some may be
replacive of Mg-calcite, magnesite and
hydromagnesite. Other minerals, such as
smectite and sepiolite, possibly formed
from the etching of detrital silicates
by alkaline pore fluids, releasing
silica and other ions into the brine. It
is likely that many lacustrine
environments have experienced low
temperatures in the past, and the
mudflat environment provides the most
easily recognizable evidence of
freezing. Many features previously
thought to have only originated from
desiccation may also be generated by
ice. Such features include diapiric mud
structures, surface veinlets, ice and
salt mounds, and interstitial ice
textures. Criteria have been established
to help to identify mudflat
modifications by ephemeral ice in the
sedimentological record.
Page publiée le 11 avril 2013, mise à jour le 9 février 2018