Geology & Geophysics Guide to Borehole Core in the Karoo Basin ...

Geology & Geophysics
Guide to Borehole Core
in the Karoo Basin Coalfields
South Africa
(interim report)
Bruce Cairncross (PhD)
Head: Department of Geology
University of Johannesburg

Wh Why? ?
Provide a colour photographic guide of typical
borehole core samples from the Karoo Basin
coalfields to:
:
- Standardize descriptions when logging core
- Assist geologists, geotechnicians, drillers,
samplers samplers, etc in recognising common features
such as rock type and sedimentary structures
in borehole core.

Cross-bedded sandstone
Matrix-supported conglomerate

Bioturbated siltstone/sandstone
(50 core samples photographed to-date) to date)

Engineering Geology 64 (2002) 19–39
“A geomechanical survey of some different facies in relation to
stability at a mine in the Eastern Transvaal Coalfield, South Africa”
F.G. Bell*, C.A. Jermy
DDepartment t t of f Geology G l and dA Applied li d GGeology, l
University of Natal, Durban, 4041, South Africa
“In particular, poor roof and floor conditions were found to be
associated with rocks of the argillaceous facies. The most
stable t bl strata t t were the th fine-grained fi i d arenaceous facies f i whilst hil t
those of the coarse-grained facies were somewhat less
stable”.

Poor roof / floor
conditions
di i

Stable roof / floor conditions

Interstate highway road-cut,
KKentucky, t k USA USA(A (Appalachian l hi coalfields) lfi ld )
Flat bedded sandstone with interlayered siltstone

Carboniferous coalfields, Alabama, USA

Recording Grain size in Sedimentary Rocks
Üdden‐Wentworth Clastic Sediment Grain‐size Chart
mm diameter Name
(above 256) Boulders
256
128
Cobbles
64
Gravel
32
16
(rock equivalent = conglomerate)
8
4
Pebbles
2 Granules Granulestone (“gritstone”)
1 Very coarse sand
0.5 Coarse sand
0.25 Medium sand
0.125 Fine sand
0.063 Very fine sand
00.031 031 Coarse silt
0.0156 Medium silt
0.0078 Fine silt
00.0039 0039 Very fine silt
(less than 0.0039) Clay
Sand
(rock equivalent = sandstone)
Mud
(rock equivalent = mudstone;
includes siltstone and shale)

The borehole core samples illustrated here are
colour coded for easy reference:
GREY = Mudstone (including shale and
siltstone)
RED = interlayered mudstone
and sandstone
YELLOW = Sandstone
BLUE = conglomerate
GREEN = deformed strata
PURPLE PURP = igneous rocks (dolerite)

Academic / Scientific Reasons for a standardized core log book
Interpreting the environments of formation of the rocks containing the
p g g
coal (different coal types and coal distributionsassociated with different
depositional environments)

Mississippi Delta (River (River-dominated dominated delta)

Bedload Fluvial
Facies Assemblage g

Borehole Core Supplied by
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