2.5 DISCUSSION AND CONCLUSIONS
3.3.2 Sediment architecture
At the Hardwick site, a 586-m-long stratigraphic cross section was developed (Figures 3.5-3.6)
in order to understand the sediment architecture. The transect at the Hardwick site has a
northern trend originating at Trench HWK6, passing through Trenches HWK1, HWK5, HWK4b,
and finishing at Trench HWK4a on the floodplain (Figure 3.5). The highest topographic area of
the transect coincides with the crevasse splay (5.4 m a.s.l.) while the lowest area (4.2 m a.s.l.)
corresponds with the floodplain to the northwest of the crevasse splay.
80
Figure 3. 5 Stratigraphic cross section at Hardwick site based on information from trenches, hand piston cores and seismic CPTs. The black frame highlights the location of Figure 3. 6. Elevation in meters on the vertical axis and Distance in meters on the horizontal axis.
81 Figure 3. 6 Detail of the stratigraphic cross section across the paleochannel at the Hardwick site. Note the trench stratigraphy is very simplified with only some of the units named to match stratigraphic units defined in the text. For full details on the trench stratigraphy and the soil profile the reader can refer to Villamor et al 2014, 2016. The red frame shows the possible liquefied layer according to the grain size analysis (see text). Elevation in meters on the vertical axis and Distance in meters on the horizontal axis.
82
The stratigraphy is divided into units M1, M2a, M2b, M2c, M3, M4 and M5. Units M1, M2a,
M2b, M2c, M3 observed in the trenches and the cores correspond with, or are subdivisions
of, units M1, M2 and M3 of Villamor et al. (2016; 2014). Units M4 and M5, defined here for
the first time, are based on the soil behaviour column (SBT) derived from the sCPT data (
Appendix A1). A summarised description of these units and their origin is presented next.
3.3.2.1 Crevasse splay (M1)
The crevasse splay is characterized by an organic rich topsoil and an alternation of loamy silt
and very fine sandy layers in the subsoil (Taitapu family LCR, 2016). This unit varied in colour
from yellowish to grey with common redox segregation features. The crevasse splay deposits
form the uppermost unit that underlies the higher southern part of the cross-section. The
crevasse splay deposit lower boundary was identified as: an abrupt erosional contact beneath
soil unit 5Cg in Trench HWK6; the lower boundary of soil unit 2bCg in Trench HWK1 (down to
94 cm depth); and the lower boundary of soil unit 6Cr1 in Trench HWK5 (down to 1.59 m
depth) (Figure 3.6) Villamor et al (2016; 2014). This contact could also be identified on the
GPR profile (Figure 3.7) as a strong reflector between 0.7 and 1 m depth corresponding with
an organic rich layer directly underlying the crevasse splay sediment.
Figure 3. 7 500Hz GPR image at Hardwick site. Black rectangles indicate trench positions. Dash black line is the strong soil reflector due to a change in geophysical properties at nearly 1 m depth. The boundary is inferred to represent the base of
83 crevasse splay deposits (M1 macrounit). Depth from the ground on the vertical axis in meters and Distance in meters on the horizontal axis.
3.3.2.2 Abandoned meander channel (M2)
Unit M2 represents sediments associated with the abandoned meander channel. Two types
of deposits were identified: the upper section was characterized by channel fill deposits
(M2a); while the bottom section was characterized by paleochannel floor deposits grading
into point bar deposits on the southern margin (inner part) of the paleochannel meander
bend (M2b).
3.3.2.2.1 Channel-fill deposits; M2a
The upper section of the paleo channel was occupied by fine sediment such as clay and silty
clay. The upper layer of unit M2a is identified as: the sandy layer 3bCg which grades into a
silty loam layer of 4b2Cg of Trench HWK1; and the silt loam layer 6Cr2 of Trench HWK5. The
lower boundary of this unit was identified as: the base of the silty clay and peaty mixtures
recorded from the seismic CPTs 4 and 5; the silty layer identified in Core 2 of Trench HWK6;
the silty clay unit identified in the auger of Trench HWK1 (auger units 4b2CR, 5b3A); and the
peaty clay layer in the first section of the core of Trench HWK5 (unit 7bOh and blue core
portion beneath it, Figure 3.6). The steep inclined edge in Figure 3.5 and 3.6 of unit M2a is
delineated based on the lack of silty and clay layers in Core 1 of Trench HWK6, and also on
the geomorphic location of the abandoned meander channel margins from the geomorphic
map (Fig. 3.3).
3.3.2.2.2 Paleochannel floor deposits; M2b
Unit M2b had a sandier texture than M2a, varying from very fine sand to fine sand. Unit M2b
was identified as: the soil layers 6bA-6Cg at Trench HWK6 (Figures 3.5 and 3.6, and Figure 3.8
for more details on Trench HWK6); the sandy soil units of Tube 1 and Tube 2 of Core 1
84
between 3.8 and 2.0 m a.s.l.; and the second section of Core 1 at HWK5 between ~ 2.8 and
1.5 m a.s.l. .
85
Figure 3. 8 Log of Trench HWK 6 (E, east wall) (from Villamor et al., 2014). Note that the crevasse splay deposit (M1 in text) is composed of units M1 fine sand, M1 medium sand, M1 silt, and M1 silt loam in the figure. Units M2 medium sand, M2 fine sand and M2 silt in this figure correspond to M2a in text. Note modern liquefaction on top of the crevasse splay in red.
86
It was not possible to clearly outline the bottom boundary of unit M2b across the SBT columns
4 and 5, since the seismic CPT profiles do not record subtle changes in texture. Hence the
bottom boundary of the M2b unit is dashed in Figs 3.5 and 3.6. As per unit M2a, the steep
lateral margins are inferred from the channel boundaries delineated in the geomorphic map.
3.3.2.3 Floodplain deposits (M2c)
The channel deposits described above are embedded within floodplain deposits (M2c), the
surface of which forms the lowest elevation parts of the cross-section of Figure 3.5. Unit M2c
comprises fine and medium sand identified in tubes 3 and 4 of Core 1 at Trench HWK6 site,
fining upward as observed in the clay-silt mixtures with thin sandy deposits, in seismic CPT1
and at Trench HWK4a where they were exposed. The soil profile at Trench HWK4a consisted
of: a topsoil (0- 30 cm depth); a strongly gleyed loamy silt layer (30 to 48 cm depth); a peat
layer (48 to 70 cm depth); and bluish-grey clean very fine (liquefiable) sand at the bottom,
down to 3.40 m a.s.l.; (Figure A 14 in Villamor et al., 2014).
3.3.2.4 Macrounit 3 (M3) – pre-channel sand
Macrounit 3 is characterized by sandy sediment identified in the SBT columns of the seismic
CPTs that currently lies at an elevation below sea level. No core reached this depth, therefore
it was not possible to investigate further differences in grain size.
3.3.2.5 Clay layer (M4) – pre-channel clay
Unit M4 is interpreted as a thick clay layer on the basis of the SBT from the seismic CPT data.
The top of the layer shows little relief, being located at 8 m depth under the crevasse splay
and 7 m depth under the floodplain (Figure 3.5). The thickness of this layer was constant along
the whole transect. Unit M4 was tentatively interpreted as Christchurch Formation estuarine
and swamp sediments.
87