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Episodic late quaternary palaeogully erosion in northern KwaZulu-Natal, South Africa
CATENA ELSEVIER
Catena
23 (1994) 327-340
Episodic late Quaternary palaeogully erosion in northern KwaZulu-Natal, South Africa Greg
A. Bothaa,
Ann
G. Wintleb,
John
C. Vogel’
“Geological Survey, P.O. Box 900. Pietermaritzhurg. 3200, South Africa bInstitute of Earth Studies, University College of Wales. Aherystq th, SY23 3DB. Lk: ‘QUADRU-EMATEK, CSIR, P.O. Box 395, Pretoria, 0001, South Africu
Abstract Hillslope deposits in northern KwaZulu-Natal, South Africa are incised by gullies (dongas) which expose sequences of buried palaeosols developed within a succession of discontinuitybounded colluvial sedimentary units. Infilled and buried palaeogully topography attests to at least four geomorphic cycles, comprising gully cut-and-fill and palaeosol formation, on hillslopes in the region during the past 135 ka. Past attempts at dating colluvial stratigraphic units were limited to radiocarbon dating of associated palaeosol organic matter and authigenic soil carbonates. This study presents a preliminary set of Infra Red Stimulated Luminescence (IRSL) age determinations pertaining to deposition of the succession of colluvial units inlilling and burying palaeogullies at several sites of stratigraphic, geochronological and palaeoenvironmental importance in the region, Interpretation of the IRSL and 14C dates in the context of cyclical hillslope processes suggests that past episodes of palaeogully erosion and infill were not synchronous and individual palaeosol landsurfaces were diachronous during the late Pleistocene.
1. Introduction Gullies (dongas) are a conspicuous feature of hillslopes over much of eastern South Africa. In the northern KwaZulu-Natal region (Fig. la) the intensely rilled and fluted sidewalls of dendritic dongas expose up to 21 m (commonly 2 m to 15 m) of predominantly fine-textured colluvium and interbedded buried palaeosols. Past attempts at dating the colluvial succession in the region have utilised materials, including archaeological artefacts, authigenic soil carbonates and palaeosol organic matter (Watson et al., 1984; Dardis, 1989; Botha et al., 1990; Dardis, 1990) which are not related to the age of deposition of the colluvial sediments. Recently, the issue of donga erosion on hillslopes was investigated through 0341-8 162/94/$07.00 ICI 1994 Elsevier Science B.V. All rights reserved SSDI 034 I-8 162(94)00025-5
328
G.A. Botha et al. / Catena 23 (1994) 327-340
(a
100
\“I--
310"
I
Dundeeo
v
Vryheid
*8
??
*l
*3 7t 6**@*2 4 Nqutu
Ulundi 0 s
Idkmith
t
Dated sites
7) 2) 3) 4) 5) 6) 7) 8)
St Paul’s Nqutu Hazeldene Masotcheni The Aloes Matatana Jojosi Voordrag
5* Muden
??
-.
60km
Fig. 1. (a) Locality map showing northern KwaZulu-Natal. Masotcheni Formation colluvial sediments are widespread.
/
/
Stippled zone represents area where thick (b) Localities of dated sites described in text.
geological, palaeopedological and micromorphological research into the late Quaternary, Masotcheni Formation colluvial sediments over an area exceeding 40000 km” in northern Natal, parts of Swaziland and the eastern Orange Free State (Botha et al., 1990; Botha, 1992). A preliminary set of Thermoluminescence (TL) and Infra Red Stimulated Luminescence (IRSL) dates (Li, 1992; Botha, 1992; Wintle et al., 1993, 1995a) has provided insight into temporal aspects of hillslope sedimentation and erosion by dating colluvial units between irregular intraformational unconformity surfaces which define buried palaeobadland topography at six of the eight sites discussed in this paper (Fig. lb). This chronological framework suggests that palaeobadland erosion has been an important slope process during several geomorphic cycles on hillslopes in the region over at least the past 135 ka (Li, 1992; Botha. 1992). The sediments and hillslope evolution processes described here are similar in some respects to those
G.A. Botha et al. i Catena 23 (19941 327-340
?29
described by Meis and Moura (1984) and Oliveira (1989) in southeastern Brazil and Fink1 and Gilkes (1976) in western Australia.
2. The nature and distribution of colluvium in northern KwaZulu-Natal Granulometric and micromorphological comparison of hillslope deposits exposed in dongas and recent colluvial sediments (Botha, 1992) highlighted the textural differences between the pedogenically altered sediment or palaeosol material and the original colluvial parent sediment. The colluvium was apparently deposited as thinly-bedded or laminated sandy sediment with interbedded lensoid pebble gravel, probably through the actions of unconfined sheetwash and localised, ephemeral gully floor processes. The depositional colour and texture of sandy colluvium was related to both the weathered bedrock and that of palaeosol material eroded from palaeodonga sidewalls in the vicinity. Pedogenically preweathered sediment and pedorelicts (petroplinthite pebbles and soil blocks) are common elastic components. Much of the silt and clay-sized component within the palaeosols is authigenic or was illuviated during pedogenesis and is concentrated as ped and root channel coatings and microlaminated void infills. Although colluvial deposits in northern KwaZulu-Natal occur throughout the landscape, there is a particularly strong association with areas which receive 600mm to 800mm of summer dominated rainfall, where the irregular. concave/ convex topography is structurally influenced by the Permian, Vryheid Formation (sandstone and shale) and intrusive Jurassic dolerite. Colluvial cover occurs on low angle bedrock pediments, incised into the argillite-dominated bedrock below the sandstone or dolerite cliffs of isolated hills and low escarpments. The thickest, stratigraphically-complex colluvial deposits occur along the axis of colluvial depressions on the upper part of bedrock pediments and generally become thinner downslope, over steps in the bedrock pediment surface and towards the interfluves which separate colluvial depositories. 2.1. Stratigraphy of the colluviul deposits
The distinctive colour, texture and structure associated with buried palaeosol profiles is the only practical means of subdividing the lithologically homogeneous Masotcheni Formation colluvial succession (Botha, 1992). The sediments bracketed by successive palaeosols were delineated as discontinuity-bounded colluvial depositional units (allostratigraphic units) and the palaeosol profiles developed within colluvial units were characterised as “Pedoderms” (cf. Brewer et al., 1970). Most Pedoderms were truncated prior to burial and structured, illuvial B2 subsoil horizons are preferentially preserved in the succession. Where Pedoderms split into several thin profiles of limited lateral extent, the minor profiles were grouped as a “pedocomplex”. Other pedocomplexes were identified at the Jojosi and Voordrag sites (Fig. 1b) where a succession of palaeogully cut-and-fill deposits and/or unique sequence of palaeosols of similar age to some of the Pedoderms described are
a)
ALLOSTRATIGRAPHY
SOUTH
St Paul’s
Batshe
(
m
~~~,,,
(Alloformatlon / Allomember) Vumankala soil
I/(;$t) H?ze’ dene vertlc
// Magongolozi
II
Dabekazi pedocomplex cm) minor SOIIS
pedocomplex)
PEDOSTRATIGRAPHY (Pedoderm,
? ? Radiocarbon
date, carbonate nodule CkaJ
* IRSL date; 125-l 50 pm K-feldspar fraction (ka) # IRSL date; 4-l 1 pm mixed mineral fraction (ka) * Radiocarbon date, palaeosol organic fraction (years BP)
333
G.A. Botha et al. 1 Catena 23 (19941 327.-340
exposed. The dual allo- and pedostratigraphic subdivision of the Masotcheni Formation in most of northern KwaZulu-Natal, presented in Fig. 2a, synthesises the stratigraphic succession and sediment/palaeosol characteristics exposed in numerous donga-wall sections in the region (Botha, 1992).
L._. 1 ’ Recognition of palaeogullv erosion Whereas Watson et al. (1984) recognised cut-and-fill features within Swaziland colluvia (comparable to the KwaZulu-Natal deposits), Dardis (1989, p. 1) stated that, “Colluvial sequences older than late Holocene show little evidence of paleopiping, dissection or related evidence of badland development”. In northern KwaZulu-Natal, however, many donga sidewalls expose cut-and-fill features with irregular surfaces between truncated palaeosol profiles and the overlying colluvial sediment. Buried palaeogully topography is commonly defined along irregular unconformity surfaces by the textural contrast between truncated. clay-enriched palaeosol profiles and the overlying stratified sediment; the three-dimensional expression of these features is best displayed in complex, dendritic donga systems, e.g. at the St Paul’s site (Fig. 2a). The buried palaeogully surfaces have been correlated between widely spaced sites on the basis of the palaeosol-based stratigraphic subdivision shown in Fig. 2a (after Botha. 1992). In some localities pedogenic profile development within colluvium infilling palaeodongas masks the buried unconformity surface. The scale of palaeogully infill also varies from site to site, as can be seen by comparing infilled palaeogullies at Jojosi (Fig. 3) with the large palaeodonga (up to 50 m wide and 9 m deep) incised into palaeosols at the St Paul’s site (Fig. 2a). In most cases the palaeogully topography differs from the steep-sided form of many current gullies: most palaeogully walls were reduced to angles of about 20” prior to burial. Features such as soil pipes, steep rilled sidewalls and cavities, common in currently active gullies, are seldom preserved within the colluvial palaeogully infill succession as these are generally ephemeral features related to active badland development. The decreased efficacy of processes responsible for their formation results in ‘the disappearance of these features as the donga floor and sidewalls stabilise.
_3..1.Dating techniques and materials Palaeosol profile characteristics and cross-cutting relationships between stratigraphic units are the primary method for “relative dating” and correlation of palaeodonga topography between sites, Although evidence of major erosional episodes within the colluvial succession has been presented previously by Watson et al. (1984) and Botha et al. (1990) these authors expressed difficulty in defining erosive and depositional episodes by radiocarbon dating of palaeosol organic matter and authigenic carbonates alone. mean residence time” Estimation of the period of pedogenesis from “apparent (AMRT) (Matthews 1985) dates of finely-disseminated organic matter associated with the palaeosol silt and clay fraction is hindered by the inability to trace the
e6uop auaplazw
I
WOZ UOP ,
&?6uop nlnbN
G.A. Botha et al. I)/Catena 23
i 1994) 327- 340
t‘ig. 3. Sequence of Jojosi pedocomplex colluvial units/palaeosols (< 30.4 ka) showing cut-and-fill related to three episodes of palaeogully erosion at the Jojosi site, situated between the St Paul’s and Nqutu sitea. Local geological and slope conditions are reflected in palaeosol profiles which cannot be correlated directly ulth the sequence described in Fig. ?a-c.
original organic inputs into the system, differential decomposition rates leading to relative concentration of younger carbon, and translocation of mobile organic compounds in soil solutions. Equally, soil carbonates are an uncertain medium for dating pedogenic events and only carbonate nodules placed in pedostratigraphic context by independent dating of “host” sediments were used to construct a geochronologic framework (Botha et al., 1990). The development of luminescence dating techniques which may be applied to colluvial sediment (Li, 1992) presented us with an opportunity to add to the existing soil-organic matter and carbonate-based radiocarbon chronology. Preliminary JRSL dates for four samples from the St Paul’s site (Fig. 2a) were obtained on the fine grain (3-l 1 pm) bulk (mixed) mineral fraction (Wintle et al.. 1993). Although ages of around 36 ka were consistent with the radiocarbon age of 36700 f 1200 BP (Pta-4924). it was thought that the IRSL signal from the silt fraction may not have sufficient long-term thermal stability (Li, 1992). Subsequent studies have concentrated on the K feldspar component of the modal fine sand fraction (125-150 /Lrn), which has been shown to be more thermally stable (Wintle et al., 1993) and is unlikely to have been illuviated (Botha, 1992). Full information on the luminescence measurements leading to the ages are presented elsewhere (Wintle et al., 1995a). A summary of radiocarbon and luminescence dates representing colluviation and palaeosol formation events for stratigraphic units comprising the Masotcheni Formation succession is presented in Table 1.
334
G.A. Boiha et al. / Catena 23
( 1994) 327-340
Table 1 Chronological framework based on Infra Red Stimulated Luminescence (IRSL) and radiocarbon dates (ka) for stratigraphic units comprising the Masotcheni Formation. Distinction is made between IRSL dated palaeogully-infill parent colluvium and radiocarbon dated palaeosol organic matter and pedogenic carbonate ST PAUL’S
STRATIGRAPHY
NQUTU
‘THE ALOES’
HPZELDENE
MASOTCHENI
MATATANA’
Vumankala soil Satshe Afm.
: .x
Matatana Malonjeni
Pdml Amb.
: L
Magongolozi Kwa Vundla
” 0 ??
Hazeldew
4
VI 1
Pdm
Nqutu Amb.
w
41 f 5
U36.7 * 1.2
#24.6 t 0.6
56+ 12 52 * 6
53 t 6
56+
: a
10.9 f 1.6
46 * 4
Pdml Amb.
?? 10.9 IO #14.07 f 0.29
e9.32 #11.74 f 0.14
7
11
1
66 2 7 unnamed
pataeosol
Ndhlamadcda Pdmi Dingaanstad Attn. Dwarsritier Pdm/ St Augustine’s Afm.
#36.8 i
97*
107*
1.1 ?
56 f 6
10
11
96+
135 * 14
10
Afm. = Alloformation; Amb. = Altomember; Pdm = Pedoderm. italics = radiocarbon dates; # = palaeosol organic matter, ?? = septarmn IRSL dales are from the 125 15O~m fraction.
11oi
13
1 7
carbonate nodule.
3. Results The relative dating framework implicit in the stratigraphic subdivision shown in Fig. 2a and Table 1 is supported by absolute age determinations which define episodic palaeodonga erosion and colluvial deposition on hillslopes in the region.
3.1. Dating the St Augustine’s
AlloformationlDwarsrivier
Pedoderm
The St Augustine’s Alloformation sediments bury an unconformity surface which marks widespread stripping of the soil mantle and exposure of bedrock on hillslopes in northern KwaZulu-Natal. Previous age estimates for the deposition of this basal part of the Masotcheni Formation succession were based on the occurrence of Early Stone Age (ESA) handaxes within this unit; and due to the dearth of absolute age determinations any date within the 200 ka to 130 ka range could be construed as an acceptable minimum estimate. The basal St Augustine’s Alloformation yielded IRSL dates of 96 f 10 (HIT-3) at Nqutu and 107 f 11 ka (STP-1) at St Paul’s (Fig. 2a) with the oldest date of 135 f 14 ka (NEW-3) recorded from the complex, split profile exposed in the Masotcheni donga (Fig. 4).
G.A. Botha et al. i Catenu 23 (1994) 327-340
335
1:ig. 4. The Masotcheni donga section showing the split Dwarsrivier Pedoderm below a bedrock step (left). overlain by red sediment (containing petroplinthite clasts) and associated plinthic Ndhlamadoda PedoJcrm. IJpper unit is the Matatana Pedoderm.
.q._‘. Dating the Dingaanstad
Alloformation~Ndhlamadoda
Pedoderm
Widespread palaeogully formation prior to deposition of this unit eroded the gleyed Dwarsrivier Pedoderm landsurface considerably. The Dingaanstad Alloformation colluvium, within which the associated Ndhlamadoda Pedoderm plinthic profile formed, was deposited around 97 f 10 ka (NEW-2) at St Paul’s and 110 & 13 ka (NEW-l) at the Matatana site (Table 1). At Masotcheni (Fig. 4) the base of the Dingaanstad Alloformation (as identified by the presence of a plinthic palaeosol profile) gave an apparently anomalous age of 56 f 6 ka (NEW-4) which will be discussed in more detail below. 3.3. Dating the St Paul> AlloformationlDabekazi
pedocomp1e.x
The majority of radiocarbon and luminescence dates were concentrated within the distinctive red Nqutu Allomember sedimentary unit and the associated dark grey, vertic Hazeldene Pedoderm (Fig. 2a-c). At the St Paul’s, Nqutu and Hazeldene sites, Nqutu Allomember sediments infilled and buried palaeogully topography incised into the Ndhlamadoda Pedoderm hard plinthic profile. Comparison of the radiocarbon and IRSL based chronology of these sites provides useful insight into the periodicity of palaeodonga erosion and palaeosol diachronism in the region (Table 1). The St Paul’s site reveals two, post-Ndhlamadoda Pedoderm palaeogullies
336
G.A. Botha et al. / Catena 23 (1994) 327-340
(Fig. 2a). The southern palaeogully is infilled with several thin, laterally impersistent colluvial units and palaeosols, grouped as the, Dabekazi pedocomplex (Fig. 2a, section B-C). An adjacent palaeodonga, which incised the hillslope oblique to the course of the current donga (Fig. 2a, section A-B), is infilled with distinctive red Nqutu Allomember sediment with a basal infill of undated yellowish-brown stratified sand (possibly Dabekazi pedocomplex) preserved locally within the gully (not illustrated). The Hazeldene Pedoderm is developed within the Nqutu Allomember, and locally within the Dabekazi pedocomplex material infilling the southern palaeodonga (Fig. 2a). The base of the southern palaeodonga infill yielded a date of 56 f 11 ka (STP-9). similar to dates of 52 f 6 ka (AF-1) and 56 * 12 ka (AF-2) obtained from the stratigraphically overlying Nqutu Allomember material infilling the adjacent palaeodonga. An IRSL age of 46 * 4 ka (AF-3) was obtained from the Malonjeni Allomember sediment burying the Hazeldene Pedoderm, which was dated at 36 700 * 1 200 BP (Pta-4924) using palaeosol organic matter. A similar palaeosol sequence is exposed in the Nqutu donga, 15 km to the south of St Paul’s. An unnamed, organic-rich palaeosol developed within gravel, dated at 36 800 f 1 100 BP (Pta-4928) using palaeosol organic matter, apparently represents the initial infill of the post-Ndhlamadoda Pedoderm palaeodonga (Fig. 2b). An IRSL date from the red Nqutu Allomember suggests that the colluvium which buried the unnamed soil began accumulating at 53 f 6 ka (HIT-l). In the neighbouring donga a radiocarbon date of 24 600 * 580 BP (Pta-49 14) was obtained for organic matter from the Hazeldene Pedoderm. This palaeosol is overlain by a thin unit, the Kwa Vundla Allomember. for which an IRSL age of 41 & 5 ka (HIT-4) was obtained. Exposures of the Hazeldene Pedoderm at the Hazeldene type site (Fig. 2c) provide another view of the post-Ndhlamadoda Pedoderm palaeodonga incision and the Hazeldene Pedoderm landsurface. Here the red Nqutu Alloformation colluvium began accumulating at 68 i- 7 ka (HIT-5) and subsequent incorporation of organic matter in the Hazeldene Pedoderm profile began before 14070 + 290 BP (Pta-4883). Authigenic, septarian carbonate nodules developed within the profile over a period of about 1000 years between 10930 & 110 BP and 9 980 & 80 BP (Pta-4819/8; ages corrected for “dead” carbon by subtraction of 1000 years; Magaritz et al., 1981; Botha et al., 1992). The Hazeldene Pedoderm was truncated by shallow palaeodongas prior to their infill by Malonjeni Allomember sediment dated at 10.9 + 1.6 ka (HIT-2). A similar scenario is envisaged at “The Aloes” site (not shown), approximately 80 km SSW of Hazeldene, where the palaeosol organic fraction of the Hazeldene Pedoderm, developed in red preweathered sediment, was dated at 11740 f 140 BP (Pta5773) and associated septarian carbonate nodules developed around 9 320 * 100 ka (Pta-5346; corrected age) (Botha, 1992). 3.4. Dating Holocene colluviation In northern KwaZulu-Natal several colluvial episodes occurred during the late Pleistocene and continued into the Holocene, as demonstrated at the St Paul’s and Hazeldene sites. The Malonjeni Allomember sediment exposed in the Hazeldene
G.A. Botha et al. 1 Catena -73 (19941 327-340
:37
section probably accumulated during the early Holocene and had been stabilised by the late Holocene as demonstrated by the formation of septarian carbonate nodules in this material between 3 480 f 60 and 2 720 f 60 years BP (Pta-4806/5; corrected ages). Two episodes of colluvial sedimentation, which were probably linked to donga erosion higher on the slope, occurred during the late Holocene at St Paul’s The Ntababomvu Pedoderm (formed within Telezeni Alloformation sediment) exposed in the St Paul’s donga (Fig. 2a, section CD) gave a bulk organic matter radiocarbon date of 1420 * 60 (Pta-4837) (Botha et al., 1990) and a range of IRSL and TL dates which were combined to give an average age estimate of 1.77 * 0.25 ka (STP-13) (Wintle et al.. 1994b). This palaeosol was buried by Batshe Alloformation colluvial fan sediments which were apparently poorly bleached and deposited with high residual luminescence signals and yielded anomalously old age estimates (Wintle et al.. 1993, 1995b) when compared with the date from the Ntababomvu Pedoderm which underlies it. 3.5. Dating the incision
qf current dongas
Rejuvenation of the Late Holocene donga erosion, which resulted in deposition of the Batshe sediments on the lower St Paul’s hillslope, led to incision of this colluvial fan during formation of the current donga. The escalation of agricultural, pastoral and Iron Age industrial practices during the past 1000 years probably accelerated donga erosion within the southeast African region. A piece of rolled wood (170 & 45 BP, Pta 4576) deposited with fluvial gravels in a small cave high on the St Paul’s donga sidewall suggests that the gully had eroded to this level, or had possibly been locally infilled, within the past few centuries. At many of the dongas in the northern Natal region, the current donga system has exploited the colluvial fill close to the palaeodonga wall defined by the buried hard plinthite-capped Ndhlamadoda Pedoderm (Fig. 2a-c).
4. Discussion Several interpretations can be made to explain the apparent discrepancies in 1RSL and radiocarbon age estimates obtained for the parent Nqutu Allomember palaeodonga-fill sediment. the prominent Hazeldene Pedoderm “marker” profile and overlying sediment at the sites described (Table 1). One possibility is that differential contamination of the buried Hazeldene Pedoderm profile by “rejuvenating” soluble organic compounds occurred after burial. Contamination with 1% modern carbon could result in a finite radiocarbon age for the unnamed palaeosol at Nqutu and the Hazeldene Pedoderm profile at St Paul’s, but close to 5% contamination with modern carbon would be required for the Hazeldene palaeosol at Nqutu. Another possible explanation for the age discrepancy is that the Hazeldene Pedoderm radiocarbon date at Nqutu is correct and that the Kwa Vundla Allomember represents local reworking. resulting in deposition of sediment whose IRSL signal was insufficiently zeroed.
338
G.A. Botha et al. 1 Catena 23 (1994) 327-340
Although the 110 ka date obtained from Dingaanstad Alloformation sediment at the Matatana site is similar to that from this unit at St Paul’s (Table l), recent investigations have shown the Ndhlamadoda Pedoderm profile to be locally compound, suggesting that the dated base of the profile could represent a truncated Dwarsrivier Pedoderm relict “welded” to the Ndhlamadoda Pedoderm above. At the Masotcheni section the anomalous date of 56 & 6 ka (NEW-4) from the reddish brown sediment (containing abundant petroplinthite clasts) below a well developed plinthic profile identified as the Ndhlamadoda Pedoderm (Fig. 4) is more consistent with dates recorded for similar red, preweathered sediment comprising the Nqutu Allomember at the St Paul’s and Nqutu sites (Table 1). The possibility of confused stratigraphy and IRSL chronology at the Masotcheni section could be explained in terms of the limited exposure revealing only the plinthic profile of a catena which might also include the vertic Hazeldene Pedoderm in other areas of the hillslope. Most of the donga sections on the upper colluvial footslope only expose part of any palaeosol catena which can lead to generalisations, not applicable at donga sites with different topographic situation. This also raises the question of whether, in specific situations, the colour, bedding and texture of colluvial material alone might represent a more reliable basis for correlating stratigraphic units than the palaeosol profiles (as utilised above) which have proved successful over wide areas in northern KwaZulu-Natal? The chronological framework from St Paul’s, Nqutu and Hazeldene suggests that the dark grey, Hazeldene Pedoderm formed in the red Nqutu Allomember sediment at different times at these three sites. At Hazeldene, the Nqutu Allomember deposition began around 68 ka and the associated Hazeldene Pedoderm yielded a date of 14 ka. However, this palaeosol organic matter-derived date does not give an indication as to the period when pedogenesis was initiated. At St Paul’s and Nqutu, the Nqutu Allomember sediments were being deposited around 56-52 ka, with a similar palaeosol forming, and being buried, by 46 ka and 41 ka at the respective sites (Fig. 2a, b; Table 1). Thus, the dates from these three sites could be construed as emphasising the diachronous character of this palaeosol landsurface or highlighting the different response/relaxation times after transgression of geomorphic threshold conditions for the Hazeldene Pedoderm landsurface at different sites. Whereas the St Paul’s and Nqutu hillslope stabilised soon after palaeodonga incision and infill, the Hazeldene slope apparently remained in a “weathering limited” state which precluded progressive pedogenesis. Poor pollen preservation within the truncated palaeosol subsoil horizons comprising the Masotcheni Formation succession described hinders the definition of palaeoenvironmental conditions during palaeosol formation or the interpretation of episodic hillslope conditions conducive to erosion/colluviation. A punctuated palynological record is preserved in the five buried palaeosols comprising the Voordrag pedocomplex, which spans the same chronological period as the St Paul’s Alloformation elsewhere in northern KwaZulu-Natal. The Voordrag palaeosols record evidence of episodic colluviation interpreted as being due to environmental cooling, progressive desiccation and alteration of rainfall seasonality preceding, and
G.A. Botha et al. / Cafena 173 (1994) 327-340
139
including, the Late Pleistocene Hypothermal (Botha et al., 1992). The apparently independent response of individual hillslopes to environmental change during this period suggests that although precipitation and vegetal factors probably had a dominant influence on hillslope processes, local geomorphic threshold conditions could be responsible for the discrepancies between particular palaeosol landsurfaces at different sites.
5. Conclusion Several generations of palaeogully topography are preserved within the late Quaternary, Masotcheni Formation colluvial succession in northern KwaZuluNatal. Early palaeosol landscapes occur as small, truncated remnants which were exhumed, eroded and reburied on several occasions during the late Quaternary period. The luminescence and radiocarbon dating of palaeogully-infill and palaeosols on several hillslopes in southeastern Africa suggests that there was considerable temporal overlap between palaeosol landsurfaces on different hillslopes. The chronological discrepancies between deposition and burial of the same stratigraphic unit on various hillslopes could be interpreted in terms of site specific responses to changing environmental conditions and the effects of local geomorphic threshold factors.
References Botha. G.A., 1992. The geology and palaeopedology of late Quaternary colluvial sediments in northern Natal. South Africa. Unpubl. Ph.D. thesis, Univ. of Natal, Pietermaritzburg, 287 pp. Botha. G.A., De Villiers, J.M. and Vogel, J.C., 1990. Cyclicity of erosion, colluvial sedimentatton and palaeosol formation in Quaternary hillslope deposits from northern Natal. South Africa. Palaeoecol. Afr.. 19: 195-210. Botha. G.A., Scott, L., Vogel, J.C. and Von Brunn, V., 1992. Palaeosols and palaeoenvironments during the Late Pleistocene Hypothermal in northern Natal. S. Afr. J. Sci., 88: 508-512. Brewer. R.. Crook. K.A.W. and Speight, J.G.. 1970. Proposal for soil stratigraphic units in the Australian Stratigraphic Code. J. Geol. Sot. Aust., 17: 103-l 11. Dardis, G.F.. 1989. Quaternary erosion and sedimentation in badland areas of southern Africa. In: A. Yair and S. Berkowitz (Editors). Arid and Semi-arid Environments: Geomorphological and Pednlogical Aspects. Catena Suppl.. 14: l-9. Dardis. G.F., 1990. Late Holocene erosion and colluvium deposition in Swaziland. Geology, 18: 9344937. Fink], C.W. and Gilkes, R.J., 1976. Relationships between micromorphological soil features and known stratigraphic layers in western Australia. Geoderma, 15: 1799208. Li. S.H., 1992. Development and application of stimulated luminescence dating methods for sediments. Unpubl. Ph.D. thesis. University of Wales, Aberystwyth, 170 pp. Magaritz. M.. Kaufman, A. and Yaalon, D.H., 1981. Calcium carbonate nodules in soils: “O/‘“O and ‘sC/“C ratios and 14C contents. Geoderma, 25: 157-172. Matthews, J.A.. 1985. Radiocarbon dating of surface and buried soils: principles, problems and respects. In: KS. Richards, R.R. Arnett and S. Ellis (Editors). Geomorphology and Soils. Allen and Unwin, London. pp. 2699288. Meis. M.R.M. and Moura, J.R.S., 1984. Upper Quaternary sedimentation and hillslope evolution: Southeastern Brazilian Plateau. Am. J. Sci., 284: 241-254.
340
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Oliveira, M.A.T., 1989. Erosion disconfonnities and gully morphology: A threedimensional approach. Catena, 16: 413-423. Watson, A., Price-Williams, D. and Goudie, A.S., 1984. The palaeoenvironmental interpretation of colluvial sediments and palaeosols of the Late Pleistocene hypothermal in southern Africa. Palaeogeog. Palaeoclim. Palaeoecol., 45: 225-249. Wintle, A.G., Li, S.H. and Botha, G.A., 1993. Luminescence dating of colluvial deposits from northern Natal, South Africa. S. Afr. J. Sci.. 89: 77782. Wintle, A.G., Botha, G.A., Li, S.H. and Vogel, J.C., 1995a. A chronological framework for colluviation during the last 110 kyr in Natal. S. Afr. J. Sci., in press. Wintle, A.G., Li, S.H., Botha, G.A. and Vogel, J.C., 1995b. Evaluation of luminescence dating procedures applied to Holocene colluvium near St. Paul’s Mission, Natal. Holocene, _‘(I). in press.
Catena
23 (1994) 327-340
Episodic late Quaternary palaeogully erosion in northern KwaZulu-Natal, South Africa Greg
A. Bothaa,
Ann
G. Wintleb,
John
C. Vogel’
“Geological Survey, P.O. Box 900. Pietermaritzhurg. 3200, South Africa bInstitute of Earth Studies, University College of Wales. Aherystq th, SY23 3DB. Lk: ‘QUADRU-EMATEK, CSIR, P.O. Box 395, Pretoria, 0001, South Africu
Abstract Hillslope deposits in northern KwaZulu-Natal, South Africa are incised by gullies (dongas) which expose sequences of buried palaeosols developed within a succession of discontinuitybounded colluvial sedimentary units. Infilled and buried palaeogully topography attests to at least four geomorphic cycles, comprising gully cut-and-fill and palaeosol formation, on hillslopes in the region during the past 135 ka. Past attempts at dating colluvial stratigraphic units were limited to radiocarbon dating of associated palaeosol organic matter and authigenic soil carbonates. This study presents a preliminary set of Infra Red Stimulated Luminescence (IRSL) age determinations pertaining to deposition of the succession of colluvial units inlilling and burying palaeogullies at several sites of stratigraphic, geochronological and palaeoenvironmental importance in the region, Interpretation of the IRSL and 14C dates in the context of cyclical hillslope processes suggests that past episodes of palaeogully erosion and infill were not synchronous and individual palaeosol landsurfaces were diachronous during the late Pleistocene.
1. Introduction Gullies (dongas) are a conspicuous feature of hillslopes over much of eastern South Africa. In the northern KwaZulu-Natal region (Fig. la) the intensely rilled and fluted sidewalls of dendritic dongas expose up to 21 m (commonly 2 m to 15 m) of predominantly fine-textured colluvium and interbedded buried palaeosols. Past attempts at dating the colluvial succession in the region have utilised materials, including archaeological artefacts, authigenic soil carbonates and palaeosol organic matter (Watson et al., 1984; Dardis, 1989; Botha et al., 1990; Dardis, 1990) which are not related to the age of deposition of the colluvial sediments. Recently, the issue of donga erosion on hillslopes was investigated through 0341-8 162/94/$07.00 ICI 1994 Elsevier Science B.V. All rights reserved SSDI 034 I-8 162(94)00025-5
328
G.A. Botha et al. / Catena 23 (1994) 327-340
(a
100
\“I--
310"
I
Dundeeo
v
Vryheid
*8
??
*l
*3 7t 6**@*2 4 Nqutu
Ulundi 0 s
Idkmith
t
Dated sites
7) 2) 3) 4) 5) 6) 7) 8)
St Paul’s Nqutu Hazeldene Masotcheni The Aloes Matatana Jojosi Voordrag
5* Muden
??
-.
60km
Fig. 1. (a) Locality map showing northern KwaZulu-Natal. Masotcheni Formation colluvial sediments are widespread.
/
/
Stippled zone represents area where thick (b) Localities of dated sites described in text.
geological, palaeopedological and micromorphological research into the late Quaternary, Masotcheni Formation colluvial sediments over an area exceeding 40000 km” in northern Natal, parts of Swaziland and the eastern Orange Free State (Botha et al., 1990; Botha, 1992). A preliminary set of Thermoluminescence (TL) and Infra Red Stimulated Luminescence (IRSL) dates (Li, 1992; Botha, 1992; Wintle et al., 1993, 1995a) has provided insight into temporal aspects of hillslope sedimentation and erosion by dating colluvial units between irregular intraformational unconformity surfaces which define buried palaeobadland topography at six of the eight sites discussed in this paper (Fig. lb). This chronological framework suggests that palaeobadland erosion has been an important slope process during several geomorphic cycles on hillslopes in the region over at least the past 135 ka (Li, 1992; Botha. 1992). The sediments and hillslope evolution processes described here are similar in some respects to those
G.A. Botha et al. i Catena 23 (19941 327-340
?29
described by Meis and Moura (1984) and Oliveira (1989) in southeastern Brazil and Fink1 and Gilkes (1976) in western Australia.
2. The nature and distribution of colluvium in northern KwaZulu-Natal Granulometric and micromorphological comparison of hillslope deposits exposed in dongas and recent colluvial sediments (Botha, 1992) highlighted the textural differences between the pedogenically altered sediment or palaeosol material and the original colluvial parent sediment. The colluvium was apparently deposited as thinly-bedded or laminated sandy sediment with interbedded lensoid pebble gravel, probably through the actions of unconfined sheetwash and localised, ephemeral gully floor processes. The depositional colour and texture of sandy colluvium was related to both the weathered bedrock and that of palaeosol material eroded from palaeodonga sidewalls in the vicinity. Pedogenically preweathered sediment and pedorelicts (petroplinthite pebbles and soil blocks) are common elastic components. Much of the silt and clay-sized component within the palaeosols is authigenic or was illuviated during pedogenesis and is concentrated as ped and root channel coatings and microlaminated void infills. Although colluvial deposits in northern KwaZulu-Natal occur throughout the landscape, there is a particularly strong association with areas which receive 600mm to 800mm of summer dominated rainfall, where the irregular. concave/ convex topography is structurally influenced by the Permian, Vryheid Formation (sandstone and shale) and intrusive Jurassic dolerite. Colluvial cover occurs on low angle bedrock pediments, incised into the argillite-dominated bedrock below the sandstone or dolerite cliffs of isolated hills and low escarpments. The thickest, stratigraphically-complex colluvial deposits occur along the axis of colluvial depressions on the upper part of bedrock pediments and generally become thinner downslope, over steps in the bedrock pediment surface and towards the interfluves which separate colluvial depositories. 2.1. Stratigraphy of the colluviul deposits
The distinctive colour, texture and structure associated with buried palaeosol profiles is the only practical means of subdividing the lithologically homogeneous Masotcheni Formation colluvial succession (Botha, 1992). The sediments bracketed by successive palaeosols were delineated as discontinuity-bounded colluvial depositional units (allostratigraphic units) and the palaeosol profiles developed within colluvial units were characterised as “Pedoderms” (cf. Brewer et al., 1970). Most Pedoderms were truncated prior to burial and structured, illuvial B2 subsoil horizons are preferentially preserved in the succession. Where Pedoderms split into several thin profiles of limited lateral extent, the minor profiles were grouped as a “pedocomplex”. Other pedocomplexes were identified at the Jojosi and Voordrag sites (Fig. 1b) where a succession of palaeogully cut-and-fill deposits and/or unique sequence of palaeosols of similar age to some of the Pedoderms described are
a)
ALLOSTRATIGRAPHY
SOUTH
St Paul’s
Batshe
(
m
~~~,,,
(Alloformatlon / Allomember) Vumankala soil
I/(;$t) H?ze’ dene vertlc
// Magongolozi
II
Dabekazi pedocomplex cm) minor SOIIS
pedocomplex)
PEDOSTRATIGRAPHY (Pedoderm,
? ? Radiocarbon
date, carbonate nodule CkaJ
* IRSL date; 125-l 50 pm K-feldspar fraction (ka) # IRSL date; 4-l 1 pm mixed mineral fraction (ka) * Radiocarbon date, palaeosol organic fraction (years BP)
333
G.A. Botha et al. 1 Catena 23 (19941 327.-340
exposed. The dual allo- and pedostratigraphic subdivision of the Masotcheni Formation in most of northern KwaZulu-Natal, presented in Fig. 2a, synthesises the stratigraphic succession and sediment/palaeosol characteristics exposed in numerous donga-wall sections in the region (Botha, 1992).
L._. 1 ’ Recognition of palaeogullv erosion Whereas Watson et al. (1984) recognised cut-and-fill features within Swaziland colluvia (comparable to the KwaZulu-Natal deposits), Dardis (1989, p. 1) stated that, “Colluvial sequences older than late Holocene show little evidence of paleopiping, dissection or related evidence of badland development”. In northern KwaZulu-Natal, however, many donga sidewalls expose cut-and-fill features with irregular surfaces between truncated palaeosol profiles and the overlying colluvial sediment. Buried palaeogully topography is commonly defined along irregular unconformity surfaces by the textural contrast between truncated. clay-enriched palaeosol profiles and the overlying stratified sediment; the three-dimensional expression of these features is best displayed in complex, dendritic donga systems, e.g. at the St Paul’s site (Fig. 2a). The buried palaeogully surfaces have been correlated between widely spaced sites on the basis of the palaeosol-based stratigraphic subdivision shown in Fig. 2a (after Botha. 1992). In some localities pedogenic profile development within colluvium infilling palaeodongas masks the buried unconformity surface. The scale of palaeogully infill also varies from site to site, as can be seen by comparing infilled palaeogullies at Jojosi (Fig. 3) with the large palaeodonga (up to 50 m wide and 9 m deep) incised into palaeosols at the St Paul’s site (Fig. 2a). In most cases the palaeogully topography differs from the steep-sided form of many current gullies: most palaeogully walls were reduced to angles of about 20” prior to burial. Features such as soil pipes, steep rilled sidewalls and cavities, common in currently active gullies, are seldom preserved within the colluvial palaeogully infill succession as these are generally ephemeral features related to active badland development. The decreased efficacy of processes responsible for their formation results in ‘the disappearance of these features as the donga floor and sidewalls stabilise.
_3..1.Dating techniques and materials Palaeosol profile characteristics and cross-cutting relationships between stratigraphic units are the primary method for “relative dating” and correlation of palaeodonga topography between sites, Although evidence of major erosional episodes within the colluvial succession has been presented previously by Watson et al. (1984) and Botha et al. (1990) these authors expressed difficulty in defining erosive and depositional episodes by radiocarbon dating of palaeosol organic matter and authigenic carbonates alone. mean residence time” Estimation of the period of pedogenesis from “apparent (AMRT) (Matthews 1985) dates of finely-disseminated organic matter associated with the palaeosol silt and clay fraction is hindered by the inability to trace the
e6uop auaplazw
I
WOZ UOP ,
&?6uop nlnbN
G.A. Botha et al. I)/Catena 23
i 1994) 327- 340
t‘ig. 3. Sequence of Jojosi pedocomplex colluvial units/palaeosols (< 30.4 ka) showing cut-and-fill related to three episodes of palaeogully erosion at the Jojosi site, situated between the St Paul’s and Nqutu sitea. Local geological and slope conditions are reflected in palaeosol profiles which cannot be correlated directly ulth the sequence described in Fig. ?a-c.
original organic inputs into the system, differential decomposition rates leading to relative concentration of younger carbon, and translocation of mobile organic compounds in soil solutions. Equally, soil carbonates are an uncertain medium for dating pedogenic events and only carbonate nodules placed in pedostratigraphic context by independent dating of “host” sediments were used to construct a geochronologic framework (Botha et al., 1990). The development of luminescence dating techniques which may be applied to colluvial sediment (Li, 1992) presented us with an opportunity to add to the existing soil-organic matter and carbonate-based radiocarbon chronology. Preliminary JRSL dates for four samples from the St Paul’s site (Fig. 2a) were obtained on the fine grain (3-l 1 pm) bulk (mixed) mineral fraction (Wintle et al.. 1993). Although ages of around 36 ka were consistent with the radiocarbon age of 36700 f 1200 BP (Pta-4924). it was thought that the IRSL signal from the silt fraction may not have sufficient long-term thermal stability (Li, 1992). Subsequent studies have concentrated on the K feldspar component of the modal fine sand fraction (125-150 /Lrn), which has been shown to be more thermally stable (Wintle et al., 1993) and is unlikely to have been illuviated (Botha, 1992). Full information on the luminescence measurements leading to the ages are presented elsewhere (Wintle et al., 1995a). A summary of radiocarbon and luminescence dates representing colluviation and palaeosol formation events for stratigraphic units comprising the Masotcheni Formation succession is presented in Table 1.
334
G.A. Boiha et al. / Catena 23
( 1994) 327-340
Table 1 Chronological framework based on Infra Red Stimulated Luminescence (IRSL) and radiocarbon dates (ka) for stratigraphic units comprising the Masotcheni Formation. Distinction is made between IRSL dated palaeogully-infill parent colluvium and radiocarbon dated palaeosol organic matter and pedogenic carbonate ST PAUL’S
STRATIGRAPHY
NQUTU
‘THE ALOES’
HPZELDENE
MASOTCHENI
MATATANA’
Vumankala soil Satshe Afm.
: .x
Matatana Malonjeni
Pdml Amb.
: L
Magongolozi Kwa Vundla
” 0 ??
Hazeldew
4
VI 1
Pdm
Nqutu Amb.
w
41 f 5
U36.7 * 1.2
#24.6 t 0.6
56+ 12 52 * 6
53 t 6
56+
: a
10.9 f 1.6
46 * 4
Pdml Amb.
?? 10.9 IO #14.07 f 0.29
e9.32 #11.74 f 0.14
7
11
1
66 2 7 unnamed
pataeosol
Ndhlamadcda Pdmi Dingaanstad Attn. Dwarsritier Pdm/ St Augustine’s Afm.
#36.8 i
97*
107*
1.1 ?
56 f 6
10
11
96+
135 * 14
10
Afm. = Alloformation; Amb. = Altomember; Pdm = Pedoderm. italics = radiocarbon dates; # = palaeosol organic matter, ?? = septarmn IRSL dales are from the 125 15O~m fraction.
11oi
13
1 7
carbonate nodule.
3. Results The relative dating framework implicit in the stratigraphic subdivision shown in Fig. 2a and Table 1 is supported by absolute age determinations which define episodic palaeodonga erosion and colluvial deposition on hillslopes in the region.
3.1. Dating the St Augustine’s
AlloformationlDwarsrivier
Pedoderm
The St Augustine’s Alloformation sediments bury an unconformity surface which marks widespread stripping of the soil mantle and exposure of bedrock on hillslopes in northern KwaZulu-Natal. Previous age estimates for the deposition of this basal part of the Masotcheni Formation succession were based on the occurrence of Early Stone Age (ESA) handaxes within this unit; and due to the dearth of absolute age determinations any date within the 200 ka to 130 ka range could be construed as an acceptable minimum estimate. The basal St Augustine’s Alloformation yielded IRSL dates of 96 f 10 (HIT-3) at Nqutu and 107 f 11 ka (STP-1) at St Paul’s (Fig. 2a) with the oldest date of 135 f 14 ka (NEW-3) recorded from the complex, split profile exposed in the Masotcheni donga (Fig. 4).
G.A. Botha et al. i Catenu 23 (1994) 327-340
335
1:ig. 4. The Masotcheni donga section showing the split Dwarsrivier Pedoderm below a bedrock step (left). overlain by red sediment (containing petroplinthite clasts) and associated plinthic Ndhlamadoda PedoJcrm. IJpper unit is the Matatana Pedoderm.
.q._‘. Dating the Dingaanstad
Alloformation~Ndhlamadoda
Pedoderm
Widespread palaeogully formation prior to deposition of this unit eroded the gleyed Dwarsrivier Pedoderm landsurface considerably. The Dingaanstad Alloformation colluvium, within which the associated Ndhlamadoda Pedoderm plinthic profile formed, was deposited around 97 f 10 ka (NEW-2) at St Paul’s and 110 & 13 ka (NEW-l) at the Matatana site (Table 1). At Masotcheni (Fig. 4) the base of the Dingaanstad Alloformation (as identified by the presence of a plinthic palaeosol profile) gave an apparently anomalous age of 56 f 6 ka (NEW-4) which will be discussed in more detail below. 3.3. Dating the St Paul> AlloformationlDabekazi
pedocomp1e.x
The majority of radiocarbon and luminescence dates were concentrated within the distinctive red Nqutu Allomember sedimentary unit and the associated dark grey, vertic Hazeldene Pedoderm (Fig. 2a-c). At the St Paul’s, Nqutu and Hazeldene sites, Nqutu Allomember sediments infilled and buried palaeogully topography incised into the Ndhlamadoda Pedoderm hard plinthic profile. Comparison of the radiocarbon and IRSL based chronology of these sites provides useful insight into the periodicity of palaeodonga erosion and palaeosol diachronism in the region (Table 1). The St Paul’s site reveals two, post-Ndhlamadoda Pedoderm palaeogullies
336
G.A. Botha et al. / Catena 23 (1994) 327-340
(Fig. 2a). The southern palaeogully is infilled with several thin, laterally impersistent colluvial units and palaeosols, grouped as the, Dabekazi pedocomplex (Fig. 2a, section B-C). An adjacent palaeodonga, which incised the hillslope oblique to the course of the current donga (Fig. 2a, section A-B), is infilled with distinctive red Nqutu Allomember sediment with a basal infill of undated yellowish-brown stratified sand (possibly Dabekazi pedocomplex) preserved locally within the gully (not illustrated). The Hazeldene Pedoderm is developed within the Nqutu Allomember, and locally within the Dabekazi pedocomplex material infilling the southern palaeodonga (Fig. 2a). The base of the southern palaeodonga infill yielded a date of 56 f 11 ka (STP-9). similar to dates of 52 f 6 ka (AF-1) and 56 * 12 ka (AF-2) obtained from the stratigraphically overlying Nqutu Allomember material infilling the adjacent palaeodonga. An IRSL age of 46 * 4 ka (AF-3) was obtained from the Malonjeni Allomember sediment burying the Hazeldene Pedoderm, which was dated at 36 700 * 1 200 BP (Pta-4924) using palaeosol organic matter. A similar palaeosol sequence is exposed in the Nqutu donga, 15 km to the south of St Paul’s. An unnamed, organic-rich palaeosol developed within gravel, dated at 36 800 f 1 100 BP (Pta-4928) using palaeosol organic matter, apparently represents the initial infill of the post-Ndhlamadoda Pedoderm palaeodonga (Fig. 2b). An IRSL date from the red Nqutu Allomember suggests that the colluvium which buried the unnamed soil began accumulating at 53 f 6 ka (HIT-l). In the neighbouring donga a radiocarbon date of 24 600 * 580 BP (Pta-49 14) was obtained for organic matter from the Hazeldene Pedoderm. This palaeosol is overlain by a thin unit, the Kwa Vundla Allomember. for which an IRSL age of 41 & 5 ka (HIT-4) was obtained. Exposures of the Hazeldene Pedoderm at the Hazeldene type site (Fig. 2c) provide another view of the post-Ndhlamadoda Pedoderm palaeodonga incision and the Hazeldene Pedoderm landsurface. Here the red Nqutu Alloformation colluvium began accumulating at 68 i- 7 ka (HIT-5) and subsequent incorporation of organic matter in the Hazeldene Pedoderm profile began before 14070 + 290 BP (Pta-4883). Authigenic, septarian carbonate nodules developed within the profile over a period of about 1000 years between 10930 & 110 BP and 9 980 & 80 BP (Pta-4819/8; ages corrected for “dead” carbon by subtraction of 1000 years; Magaritz et al., 1981; Botha et al., 1992). The Hazeldene Pedoderm was truncated by shallow palaeodongas prior to their infill by Malonjeni Allomember sediment dated at 10.9 + 1.6 ka (HIT-2). A similar scenario is envisaged at “The Aloes” site (not shown), approximately 80 km SSW of Hazeldene, where the palaeosol organic fraction of the Hazeldene Pedoderm, developed in red preweathered sediment, was dated at 11740 f 140 BP (Pta5773) and associated septarian carbonate nodules developed around 9 320 * 100 ka (Pta-5346; corrected age) (Botha, 1992). 3.4. Dating Holocene colluviation In northern KwaZulu-Natal several colluvial episodes occurred during the late Pleistocene and continued into the Holocene, as demonstrated at the St Paul’s and Hazeldene sites. The Malonjeni Allomember sediment exposed in the Hazeldene
G.A. Botha et al. 1 Catena -73 (19941 327-340
:37
section probably accumulated during the early Holocene and had been stabilised by the late Holocene as demonstrated by the formation of septarian carbonate nodules in this material between 3 480 f 60 and 2 720 f 60 years BP (Pta-4806/5; corrected ages). Two episodes of colluvial sedimentation, which were probably linked to donga erosion higher on the slope, occurred during the late Holocene at St Paul’s The Ntababomvu Pedoderm (formed within Telezeni Alloformation sediment) exposed in the St Paul’s donga (Fig. 2a, section CD) gave a bulk organic matter radiocarbon date of 1420 * 60 (Pta-4837) (Botha et al., 1990) and a range of IRSL and TL dates which were combined to give an average age estimate of 1.77 * 0.25 ka (STP-13) (Wintle et al.. 1994b). This palaeosol was buried by Batshe Alloformation colluvial fan sediments which were apparently poorly bleached and deposited with high residual luminescence signals and yielded anomalously old age estimates (Wintle et al.. 1993, 1995b) when compared with the date from the Ntababomvu Pedoderm which underlies it. 3.5. Dating the incision
qf current dongas
Rejuvenation of the Late Holocene donga erosion, which resulted in deposition of the Batshe sediments on the lower St Paul’s hillslope, led to incision of this colluvial fan during formation of the current donga. The escalation of agricultural, pastoral and Iron Age industrial practices during the past 1000 years probably accelerated donga erosion within the southeast African region. A piece of rolled wood (170 & 45 BP, Pta 4576) deposited with fluvial gravels in a small cave high on the St Paul’s donga sidewall suggests that the gully had eroded to this level, or had possibly been locally infilled, within the past few centuries. At many of the dongas in the northern Natal region, the current donga system has exploited the colluvial fill close to the palaeodonga wall defined by the buried hard plinthite-capped Ndhlamadoda Pedoderm (Fig. 2a-c).
4. Discussion Several interpretations can be made to explain the apparent discrepancies in 1RSL and radiocarbon age estimates obtained for the parent Nqutu Allomember palaeodonga-fill sediment. the prominent Hazeldene Pedoderm “marker” profile and overlying sediment at the sites described (Table 1). One possibility is that differential contamination of the buried Hazeldene Pedoderm profile by “rejuvenating” soluble organic compounds occurred after burial. Contamination with 1% modern carbon could result in a finite radiocarbon age for the unnamed palaeosol at Nqutu and the Hazeldene Pedoderm profile at St Paul’s, but close to 5% contamination with modern carbon would be required for the Hazeldene palaeosol at Nqutu. Another possible explanation for the age discrepancy is that the Hazeldene Pedoderm radiocarbon date at Nqutu is correct and that the Kwa Vundla Allomember represents local reworking. resulting in deposition of sediment whose IRSL signal was insufficiently zeroed.
338
G.A. Botha et al. 1 Catena 23 (1994) 327-340
Although the 110 ka date obtained from Dingaanstad Alloformation sediment at the Matatana site is similar to that from this unit at St Paul’s (Table l), recent investigations have shown the Ndhlamadoda Pedoderm profile to be locally compound, suggesting that the dated base of the profile could represent a truncated Dwarsrivier Pedoderm relict “welded” to the Ndhlamadoda Pedoderm above. At the Masotcheni section the anomalous date of 56 & 6 ka (NEW-4) from the reddish brown sediment (containing abundant petroplinthite clasts) below a well developed plinthic profile identified as the Ndhlamadoda Pedoderm (Fig. 4) is more consistent with dates recorded for similar red, preweathered sediment comprising the Nqutu Allomember at the St Paul’s and Nqutu sites (Table 1). The possibility of confused stratigraphy and IRSL chronology at the Masotcheni section could be explained in terms of the limited exposure revealing only the plinthic profile of a catena which might also include the vertic Hazeldene Pedoderm in other areas of the hillslope. Most of the donga sections on the upper colluvial footslope only expose part of any palaeosol catena which can lead to generalisations, not applicable at donga sites with different topographic situation. This also raises the question of whether, in specific situations, the colour, bedding and texture of colluvial material alone might represent a more reliable basis for correlating stratigraphic units than the palaeosol profiles (as utilised above) which have proved successful over wide areas in northern KwaZulu-Natal? The chronological framework from St Paul’s, Nqutu and Hazeldene suggests that the dark grey, Hazeldene Pedoderm formed in the red Nqutu Allomember sediment at different times at these three sites. At Hazeldene, the Nqutu Allomember deposition began around 68 ka and the associated Hazeldene Pedoderm yielded a date of 14 ka. However, this palaeosol organic matter-derived date does not give an indication as to the period when pedogenesis was initiated. At St Paul’s and Nqutu, the Nqutu Allomember sediments were being deposited around 56-52 ka, with a similar palaeosol forming, and being buried, by 46 ka and 41 ka at the respective sites (Fig. 2a, b; Table 1). Thus, the dates from these three sites could be construed as emphasising the diachronous character of this palaeosol landsurface or highlighting the different response/relaxation times after transgression of geomorphic threshold conditions for the Hazeldene Pedoderm landsurface at different sites. Whereas the St Paul’s and Nqutu hillslope stabilised soon after palaeodonga incision and infill, the Hazeldene slope apparently remained in a “weathering limited” state which precluded progressive pedogenesis. Poor pollen preservation within the truncated palaeosol subsoil horizons comprising the Masotcheni Formation succession described hinders the definition of palaeoenvironmental conditions during palaeosol formation or the interpretation of episodic hillslope conditions conducive to erosion/colluviation. A punctuated palynological record is preserved in the five buried palaeosols comprising the Voordrag pedocomplex, which spans the same chronological period as the St Paul’s Alloformation elsewhere in northern KwaZulu-Natal. The Voordrag palaeosols record evidence of episodic colluviation interpreted as being due to environmental cooling, progressive desiccation and alteration of rainfall seasonality preceding, and
G.A. Botha et al. / Cafena 173 (1994) 327-340
139
including, the Late Pleistocene Hypothermal (Botha et al., 1992). The apparently independent response of individual hillslopes to environmental change during this period suggests that although precipitation and vegetal factors probably had a dominant influence on hillslope processes, local geomorphic threshold conditions could be responsible for the discrepancies between particular palaeosol landsurfaces at different sites.
5. Conclusion Several generations of palaeogully topography are preserved within the late Quaternary, Masotcheni Formation colluvial succession in northern KwaZuluNatal. Early palaeosol landscapes occur as small, truncated remnants which were exhumed, eroded and reburied on several occasions during the late Quaternary period. The luminescence and radiocarbon dating of palaeogully-infill and palaeosols on several hillslopes in southeastern Africa suggests that there was considerable temporal overlap between palaeosol landsurfaces on different hillslopes. The chronological discrepancies between deposition and burial of the same stratigraphic unit on various hillslopes could be interpreted in terms of site specific responses to changing environmental conditions and the effects of local geomorphic threshold factors.
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