# Fjord network in Namibia: A snapshot into the dynamics of the late Paleozoic glaciation

Pierre Dietrich, Neil P Griffis, Daniel P Le Heron, Isabel P Montañez, Christoph Kettler, Cécile Robin, François Guillocheau

► To cite this version:

Pierre Dietrich, Neil P Griffis, Daniel P Le Heron, Isabel P Montañez, Christoph Kettler, et al., Fjord network in Namibia: A snapshot into the dynamics of the late Paleozoic glaciation. Geology, 2021, 49 (12), pp.1521-1526. 10.1130/G49067.1. insu-03331205

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# Fjord network in Namibia: A snapshot into the dynamics of the late Paleozoic glaciation

Pierre Dietrich<fnref n="1" />,<fnref n="2" />, Neil P. Griffis<fnref n="3" />,<fnref n="4" />, Daniel P. Le Heron<fnref n="5" />, Isabel P. Montañez<fnref n="3" />, Christoph Kettler<fnref n="5" />, Cécile Robin<fnref n="1" /> and François Guillocheau<fnref n="1" />

## ABSTRACT

Fjords are glacially carved estuaries that profoundly influence ice-sheet stability by draining and ablating ice. Although abundant on modern high-latitude continental shelves, fjord-network morphologies have never been identified in Earth's pre-Cenozoic glacial epochs, hindering our ability to constrain ancient ice-sheet dynamics. We show that U-shaped valleys in northwestern Namibia cut during the late Paleozoic ice age (LPIA, ca. 300 Ma), Earth's penultimate icehouse, represent intact fjord-network morphologies. This preserved glacial morphology and its sedimentary fill permit a reconstruction of paleo-ice thicknesses, glacial dynamics, and resulting glacio-isostatic adjustment. Glaciation in this region was initially characterized by an acme phase, which saw an extensive ice sheet (1.7 km thick) covering the region, followed by a waning phase characterized by 100-m-thick, topographically constrained outlet glaciers that shrank, leading to glacial demise. Our findings demonstrate that both a large ice sheet and highland glaciers existed over northwestern Namibia at different times during the LPIA. The fjords likely played a pivotal role in glacier dynamics and climate regulation, serving as hotspots for organic carbon sequestration. Aside from the present-day arid climate, northwestern Namibia exhibits a geomorphology virtually unchanged since the LPIA, permitting unique insight into this icehouse.

## INTRODUCTION

Fjords are long, deep, and narrow glacially carved estuaries that were occupied by outlet glaciers. They play a dramatic role in ice-sheet stability (i.e., drainage and ablation), are sensitive to climate change during icehouse periods, and act as important sediment sinks (Syvitski et al., 1987; Bennett, 2003; Kessler et al., 2008; Briner et al., 2009; Moon et al., 2018). Despite occupying just 0.1% of Earth's modern oceans, fjords account for >10% of Earth's organic carbon burial and may thus significantly impact the global carbon cycle (Smith et al., 2015). Fjord morphology and the sedimentary infill moreover play an instrumental role in assessing ice-sheet dynamics and climate change (Eilertsen et al., 2011; Steer et al., 2012; Normandeau et al., 2019; Bianchi et al., 2020). Despite their present-day abundance across high-latitude continental shelves, fjords from pre-Cenozoic glacial epochs have mostly been inferred from

stratigraphic relationships and geological mapping (e.g., Kneller et al., 2004; Tedesco et al., 2016, and references therein), while the existence of paleo-fjord networks have not been rigorously established. The scarcity of ancient fjord morphologies seemingly reflects the intrinsic transient nature of these large-scale geomorphological features (Bianchi et al., 2020), rendering them prone to erosion over geological time scales. In turn, our ability to fully embrace large-scale dynamics of deep-time ice sheets and to assess their sensitivity to climate change is hindered.

We present a geomorphic and sedimentologic analysis of a pristine paleo-fjord network across northwestern Namibia, which formed during Earths' penultimate and long-lived icehouse, the late Paleozoic ice age (LPIA, 360–260 Ma; Montañez and Poulsen, 2013). Based on the geomorphology of the paleo-fjord network and its glaciogenic sediment infill, we

infer the pace of ice-margin fluctuations and the paleo-ice thickness that once occupied these fjords. Furthermore, we estimate glacioisostatic adjustment following deglaciation. This work highlights the dynamic nature of the LPIA in southwestern Gondwana and the potential for an under-appreciated deep-time carbon and sediment sink that, if valid, would have had direct climate implications through the impact of Carboniferous–Permian atmospheric CO₂.

## RESULTS: FJORD NETWORK IN NAMIBIA

The Kaokoland region of northwestern Namibia is characterized by a prominent bimodal topography where staircase-like plateaus (at ~500 m, ~1000 m, and ~1500 m) separated by NNW-SSE-oriented escarpments are deeply dissected by a network of valleys in which modern rivers flow (Fig. 1A). These valleys are 1–5 km in width and 80–130 km in length and have steep, subvertical flanks defining U-shaped cross-profiles (Figs. 1B and 1C) whose depths range between 400 and 1200 m; interfluves occur up to 1.7 km above the thalweg of the Kunene Valley (Fig. 1A). These valleys are carved within hard Archean to Proterozoic lithologies of the Pan-African and Congo craton basement (Goscombe and Gray, 2008).

The valley floors display abundant hardbed glacial erosion features such as striae, scratches, grooves, and crescentic gouges (Fig. 2A) superimposed on whalebacks and roches moutonnées (see also Martin, 1953, 1981; Martin and Schalk, 1959). Scratches and striations are also developed on subvertical walls that flank the U-shaped valleys as well as on the subvertical westernmost (Purros; Fig. 1)

<footnote n="1">¹Géosciences Rennes, Université de Rennes, CNRS, UMR 6118, 35000 Rennes, France</footnote>

<footnote n="2">²Department of Geology, University of Johannesburg, Johannesburg 2006, South Africa</footnote>

<footnote n="3">³Department of Earth and Planetary Sciences, University of California, Davis, California 95616, USA</footnote>

<footnote n="4">⁴Berkeley Geochronology Center, Berkeley, California 94709, USA</footnote>

<footnote n="5">⁵Department für Geologie, Universität Wien, Althanstraße 14, 1190 Wien, Austria</footnote>

<footnote>CITATION: Dietrich, P., et al., 2021, Fjord network in Namibia: A snapshot into the dynamics of the late Paleozoic glaciation: Geology, v. 49, p. https://doi.org/10.1130/G49067.1</footnote>
<figure><img src="image_3.png" /></figure>

Figure 1. (A) Digital elevation model of Kaokoland (northwestern Namibia) highlighting glacial valleys (white dashed lines). Circle labeled "B & C" represents geographic position of panels B and C. (B) U-shaped Gomatum valley. Note subvertical valley walls at background above modern alluvial fans, and Karoo Supergroup outliers. Valley is ~2 km wide. (C) Topographic transect of U-shaped Gomatum valley, showing position of sedimentary and Etendeka Volcanics successions. (D) Longitudinal profile of Hoarusib valley and its interfluve (dashed line), and Karoo and Etendeka outliers. Note bedrock structures (Goscombe and Gray, 2008). v.e.—vertical exaggeration. Background topography is from SRTM3 (USGS 2006), Shuttle Radar Topographic Mission, 3 Arc Second scene SRTM_n47w053_n54w074, Filled Finished B 2.0, Global Land Cover Facility, University of Maryland, College Park, Maryland, February 2000) available at https://www2.jpl.nasa.gov/srtm/. Map was generated using GlobalMapper® GIS software.

escarpment, indicating westward and north-
ward paleo-ice flows, respectively (Fig. 1A).
These valleys ubiquitously preserve remnants
of partly eroded glaciogenic sediments of the
Dwyka Group, forming the base of the Karoo
Supergroup (Figs. 1B, 1C, and 2), whose age
in the (restored) neighboring Paraná Basin of
Brazil and Aranos and Karasburg Basins of
Namibia is bracketed between ca. 299 Ma and
ca. 296 Ma (Griffis et al., 2021). These glacio-
genic sediments consist here of boulder beds
(Fig. 2B) encompassing numerous exotic, fac-

eted, and/or striated clasts and discontinuous
ridges, patches, and lenses of poorly sorted
conglomerates commonly affected by syn-
sedimentary ductile deformation (Figs. 1D
and 2C), interpreted as ice-contact morainal
banks or ridges that experienced gladiotectonic
folding (Dowdeswell et al., 2015). Impor-
tantly, glaciogenic sediments encompassing
numerous and large (1–3 m) glacial erratics
are plastered to the sides of these U-shaped
valleys and along the escarpment, in associa-
tion with scratched and striated valley walls

(Figs. 2D and 2E). Glaciogenic deposits occur
consistently at a height of 100 m above the
valley bottom, which are interpreted as mar-
ginal moraines.

Sediments immediately covering the coarse
glaciogenic deposits, infilling the valleys
(Figs. 1C and 3) and abutting against the stri-
ated valley walls (Figs. 1B and 2D), are made up
of a 5–20-m-thick shallowing-upward sequence
of medium-grained sandstones with rhythmic
climbing ripples and sand-mud couplets
(Fig. 2F) interstratified with diamictite layers
<figure><img src="image_4.png" /></figure>

Figure 2. Facies. (A) Striae and crescentic gouges superimposed on roches moutonnées; 18°11'00"S, 12°45'40"E, vertical view. (B) Striated boulder bed covering roches moutonnées; 18°10'46"S, 12°45'45"E, vertical view. (C) Discontinuous and deformed ridge of poorly sorted conglomerate; 18°30'00"S, 12°49'10"E, view toward west. Note hammer near top for scale. (D) Striated vertical wall against which Karoo Supergroup strata abut in background; 18°47'56"S, 13°01'49"E, view toward northwest. (E) Marginal moraine plastered on valley wall, encompassing large erratic boulder; 18°47'53"S, 13°01'45"E, view toward northwest. (F) Rhythmites and sand-mud couplets; 18°29'59"S, 12°49'05"E, view toward south. (G) Lonestone encompassed within sand-sized, rippled sediments; 18°29'59"S, 12°49'05"E, view toward south. (H) Intertidal depositional environment (breached wave ripples) reworking underlying ice-contact fan; 18°29'59"S, 12°49'05"E, view toward south.

characterized by scattered outsized clasts and highlighted by impact structures reflecting ice-rafted debris (Fig. 2G). We interpret this succession to represent ice-proximal subaqueous fan deposits (Dowdeswell et al., 2015), implying a glaciomarine depositional environment. Facies characteristic of intertidal processes (truncated, leveled, and breached wave ripples; Fig. 2H) were deposited in a postglacial con-

text, 20 m above the striated floors, well below valley interfluves (Figs. 1D and 3). The fjords were subsequently drowned and accumulated non-glaciogenic sedimentary units of the Permian–Cretaceous Karoo Supergroup, ultimately culminating with the Etendeka basalt flows at 130 Ma, whose remnants occur in valley axes topping both the sedimentary succession and valley interfluves (Fig. 1).

The presented glaciogenic sediments and geomorphic features preserved within and along the sides of these U-shaped valleys and escarpments indicate that they were occupied by ice masses during the LPIA. Ice retreat was accompanied by postglacial marine incursion that therefore turned the valleys into fjords; the observed valley network therefore corresponds to the original fjord network, here mapped for the first time in Figure 1A.
<figure><img src="image_5.png" /><figcaption>**Figure 3.** Stratigraphic log of lower fjord infill from the Hoarusib valley (from Karoo Supergroup outliers of Fig. 1D), depositional environments, and relative sea-level changes. c—clay; s—silt; vf—very fine-grained sandstone; f—fine-grained sandstones; m—medium-grained sandstone; c—coarse-grained sandstone; vc—very coarse-grained sandstone; g—gravel.</figcaption></figure>

## INTERPRETATION: RECONSTRUCTED LPIA DYNAMICS AND ICE THICKNESSES IN NORTHWESTERN NAMIBIA

The glacial (fjord) geomorphology and its associated sedimentary infill record a snapshot

of the dynamics of the LPIA ice masses (Fig. 4). The deep (as deep as 1.7 km) fjord incisions and the presence of numerous and large glacial erratics (e.g., Fig. 2E) transported by ice across major drainage divides imply the presence of an ice sheet largely overflowing the valleys during

an early glaciation acme phase (Fig. 4; Martin and Schalk, 1959; Staiger et al., 2005; Kessler et al., 2008; Steer et al., 2012; Livingstone et al., 2017), with ice of at least 1.7 km thick flowing likely westward toward southeastern Brazil. During the subsequent waning phase, an estimated ice thickness of 100 m existed within the valley, as extrapolated directly from the elevation difference between the valley bottom and the highest observed marginal moraine sediments plastered on the valley flanks (Figs. 1 and 3). As indicated by such an ice thickness, topographically constrained, westward-flowing outlet glaciers occupied the valleys, and some (such as the glaciers that occupied the Gomatum and Hoarusib valleys; Fig. 1) bifurcated or fed into a bigger, northward-flowing ice stream (Fig. 4). These outlet glaciers are therefore regarded as having drained a residual ice sheet located to the east of the study area, possibly over the Otavi Range (Fig. 1A) or the Owambo Basin further east (Miller, 1997). Our findings therefore demonstrate that both a large ice sheet and highland glaciers existed over northwestern Namibia during particular intervals of the icehouse. As is the case for the Quaternary period, however, for which only the last cycle is preserved in the sedimentary and geomorphic record, this acme-waning succession may be representative of only an ultimate ice growth and decay cycle, and perhaps the most important one, of a period that encompassed several cycles that were erased by subsequent ice advance.

Throughout this acme-waning cycle, during ongoing deglaciation and ice-margin retreat, the postglacial sea invaded the valleys and turned them into fjords. Ice-contact fans and deformed morainal banks indicate that sedimentation took place at the front of retreating glaciers during periods of episodic ice-margin stillstands or minor readvance (Fig. 4). Throughout this deglacial cycle, ice-contact and glaciomarine sedimentation was strictly confined within the topographic depression (the fjords) in a setting likely devoid of tectonic subsidence.

Considering such a succession of deglacial events, the sedimentary succession observed infilling the fjords is therefore thought to archive the glacio-isostatic adjustment (Boulton, 1990). Given that glacio-isostatic adjustment operates over a short (10⁴ yr) time scale, i.e., well shorter than the temporal resolution available for such a deep-time record, the unravelling of this process is made by analogy between the fjordal deglacial sedimentary succession showcased here and a Quaternary deglacial succession (Dietrich et al., 2018). Thus, we posit that the basal, 20–40-m-thick shallowing-upward parasequence (glaciomarine capped by intertidal facies) is interpreted as a response to falling relative sea level resulting from the glacio-isostatic adjustment. The subsequent drowning (offshore deposits upon intertidal deposits)

<figure><img src="image_6.png" /><figcaption>**Figure 4.** Three-dimensional model of late Paleozoic ice age dynamics on Kaokoland, northwestern Namibia. RSL—relative sea level; GIA—glacio-isostatic adjustment.</figcaption></figure>
therefore likely corresponds to an eustatic rise
during which non-glaciogenic sedimentation
occurred (Fig. 3). The glacial dynamic that we
envisage is largely comparable to the evolution
of post-Last Glacial Maximum ice masses over
Canada, Norway, and Greenland, whose post-
acme recession saw the confinement of outlet
glaciers into fjords, which after glacial demise
were invaded by postglacial seas (e.g., Syvitski
et al., 1987; Dietrich et al., 2018).

DISCUSSION AND IMPLICATIONS

A Preserved LPIA Glacial Landscape:
A Fjord Network Superimposed on
Preexisting Plateaus and Escarpments

The preservation of glaciogenic deposits and
erosion features indicates that the U-shaped val-
ley network forms an intact relict geomorphic
landscape inherited from the LPIA and com-
plements temporally contemporaneous, though
smaller, glacial landscapes and paleo-fjords
preserved and exhumed (or still sealed) across
southwestern Gondwana (e.g., Visser, 1987;
Assine et al., 2018; Le Heron et al., 2019; Fall-
gatter and Paim, 2019). The presence of incised
valleys furthermore implies that the plateaus and
intervening escarpments were in place when the
glacial topography was carved (Fig. 1D), i.e.,
before the Atlantic (Cretaceous) rifting. Ther-
mochronological studies (Krob et al., 2020)
indicate that little denudation occurred after the
LPIA prior to the deposition of the Cretaceous
Etendeka basalts, whose remnants are preserved
on valley interfluves (Fig. 1C). Therefore, mod-
ern valley depths and the whole of the Kaoko-
land landscape, although uplifted (Baby et al.,
2020), are virtually unchanged since LPIA times
(Martin, 1953). The observed network of valleys,
troughs, and escarpments that currently charac-
terizes the Kaokoland therefore corresponds to
an extensive, ~50,000 km² preserved glacial
landscape (Fig. 1A). Our showcased example
is unique because it represents the sole example
of a pristine fjord network and glacial landscape
yet described for a pre-Cenozoic glacial epoch.
Furthermore, our work highlights the compat-
ibility of both large ice sheets and highland gla-
ciation across the LPIA in a single location (cf.
Isbell et al., 2012).

In the absence of recent (middle to late
Paleozoic) major tectonic events antecedent
to the LPIA in this region, the preexistence of
plateaus and escarpments exploited by glacial
erosion that carved fjords is interpreted as fol-
lows. The NNW-SSE-oriented escarpments
correspond to basement sutures delineating the
Congo craton to the west and segments of the
Kaoko (Pan-African) orogen to the east (Gos-
combe and Gray, 2008). Because these plateaus
and escarpments had already formed prior to the
LPIA, we suggest that this existing topography
resulted from substantial rejuvenation of the
Kaoko orogenic structures after 200 Ma dur-

ing post-orogenesis exhumation and peneplana-
tion (Krob et al., 2020). A differential response
of the Congo craton and the Kaoko orogen to
vertical tectonic forces that promoted the initia-
tion of Karoo-aged basins over southwestern
Gondwana (Pysklywec and Quintas, 1999) is
tentatively invoked to explain this topographic
rejuvenation. Alternatively, or complementarily,
glacial erosion itself through isostatic uplift may
have generated the mountainous relief required
to create fjords (Medvedev et al., 2008). After
the LPIA and until 130 Ma, this paleo-fjord
network was progressively buried by the Karoo
sediments and Etendeka volcanics, and subse-
quently exhumed until the present (Krob et al.,
2020; Margirier et al., 2019; Baby et al., 2020).
Thus, preservation of a pristine paleo-fjord net-
work, in spite of 130 m.y. of uplift and exhuma-
tion, is remarkable. Determining the reasons for
this exceptional preservation will be a driver of
future research.

Implications for Global Climate Change

The Namibian paleo-fjords have major impli-
cations for understanding the turnover from the
late Paleozoic icehouse to a permanent green-
house state. Delineating the extent and dynamics
of ice masses in northwestern Namibia provides
more realistic boundary conditions for the scale
of glaciation in this region of Gondwana. In par-
ticular, the reconstructed paleo-landscape that
requires an ice sheet during the acme followed
by upland glaciation through the demise of the
LPIA could well be explained by the insertion
of fjords that promoted dramatic ice-mass loss
through drainage and ablation (Bennett, 2003;
Briner et al., 2009), in turn triggering abrupt
climate change and enhanced ice-sheet sensitiv-
ity to climate change, ultimately leading to ice
shrinkage (Kessler et al., 2008). Importantly, the
Namibian paleo-fjord network together with its
South American (Tedesco et al., 2016, and refer-
ences therein) and South African (Visser, 1987)
counterparts could have facilitated the deposi-
tion and long-term sequestration of organic car-
bon analogous to Quaternary fjords (Smith et al.,
2015). If this was the case, then burial of large
amounts of glacially derived organic material
may have contributed to a 10 m.y. nadir in atmo-
spheric CO₂ in the earliest Permian that defines
a paradox, given the loss of major carbon sinks
prior to the close of the Carboniferous (Richey
et al., 2020). The potential for paleo-fjords of
southwestern Gondwana to have played an
important role in atmospheric pCO₂ and climate
regulation is thus a worthwhile area of further
study and carbon-cycle modeling.

ACKNOWLEDGMENTS

P. Dietrich and D. Le Heron acknowledge funding
from the South Africa–Austria joint project of the
National Research Foundation (NRF) of South Africa
and the Österreichischer Austauschdienst (OEAD
project ZA 08/2019). I. Montañez and N. Griffis

acknowledge funding from the U.S. National Science
Foundation (grant EAR-1729882). Julia Tedesco and
Michael Blum are thanked for their thorough and con-
structive reviews that greatly led to the improvement
of the paper.

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