Soft-sediment deformation at the base of the Neoproterozoic Puga

Soft-sediment deformation at the base of the Neoproterozoic Puga
cap carbonate (southwestern Amazon craton, Brazil): Confirmation
of rapid icehouse to greenhouse transition in snowball Earth
Afonso César Rodrigues Nogueira* Departamento de Geociências, Universidade Federal do Amazonas, Av. Gal. Rodrigo
O. J. Ramos 3000, Manaus, AM 69.077-000, Brazil, and Programa de Pós-Graduação
em Geologia Sedimentar, Instituto de Geociências, Universidade de São Paulo, Rua do
Lago, 562, São Paulo, SP 05508-080, Brazil
Claudio Riccomini* Departamento de Geologia Sedimentar e Ambiental, Instituto de Geociências, Universidade de São
Paulo, São Paulo, SP 05508-080, Brazil
Alcides Nóbrega Sial* Núcleo de Estudos de Granitos–Laboratório de Isótopos Estáveis, Departamento de Geologia,
Universidade Federal de Pernambuco, Recife, PE 50732-970, Brazil
Candido Augusto Veloso Moura* Laboratório de Geologia Isotópica-Pará-Isso, Centro de Geociências, Universidade
Federal do Pará, Belém, PA 66.075-900, Brazil
Thomas Rich Fairchild* Departamento de Geologia Sedimentar e Ambiental, Instituto de Geociências, Universidade de São
Paulo, São Paulo, SP 05508-080, Brazil
ABSTRACT
Stratigraphic and isotopic data identify the lower 45 m of the
Araras Group, on the southwest margin of the Amazon craton, as
a Neoproterozoic platform cap carbonate deposited below wave
base upon Varanger glacial diamictites of the Puga Formation. The
basal beds consist of moderately deep water pinkish dolomudstone
with stratiform to wavy fenestral microbialites locally cut by tubelike structures and fenestral nonmicrobial planar laminites with
tepee-like features. Above the basal carbonates are deep-water bituminous lime mudstones with alternating thin calcite crusts and
lime mudstone laminae commonly disrupted by calcite crystal fans
(pseudomorphs after aragonite). The basal contact of the Puga cap
exhibits soft-sediment deformational structures (principally load
casts) that are here attributed to rebound-induced seismicity acting
upon both recently deposited carbonate sediments and underlying
unconsolidated diamictite. These features constitute the first clearly
recognized sedimentological evidence for the rapid change from
icehouse to greenhouse conditions as postulated in the snowball
Earth model of Neoproterozoic glaciation.
Keywords: Neoproterozoic, Amazon craton, carbonate rocks, glaciation, isotopes, deformation.
INTRODUCTION
Neoproterozoic cap carbonates are generally pinkish dolostones
with pronounced negative d13C signatures and anomalous lithofacies
that overlie glacial deposits with no evidence of a hiatus in deposition
(Kennedy, 1996; Hoffman et al., 1998; Kennedy et al., 2001; Hoffman
and Schrag, 2002). Putatively synchronous successions of this sort have
been described from almost every Precambrian craton, which suggests
some sort of global process for their origin. The favored hypothesis is
one of quick glacial melting and equally rapid carbonate precipitation
during the essentially instantaneous transition from global icehouse to
greenhouse conditions (e.g., Hoffman et al., 1998; Hoffman and
Schrag, 2002).
South America is one of the last continents for which Neoproterozoic cap carbonates have yet to be documented in detail, even though
several authors have described carbonates overlying glacial diamictites
in Neoproterozoic successions in central Brazil (e.g., Maciel, 1959;
Almeida, 1964; Alvarenga and Trompette, 1992), and others have in*E-mail addresses: [email protected]; [email protected]; [email protected];
[email protected]; [email protected].
ferred the presence of cap carbonates in this region on the basis of
aragonite crystal fans and negative carbon isotope values (Peryt et al.,
1990; Santos et al., 2001). However, the lack of detailed stratigraphic
studies has hindered confident correlation with the anomalous deposits
associated with the so-called Neoproterozoic snowball Earth episodes
(Hoffman and Schrag, 2002).
Our recent stratigraphic, sedimentological, and isotopic analyses
of carbonate rocks near Mirassol d’Oeste, Mato Grosso, Brazil, corroborate their identification as a cap carbonate on the southwestern
margin of the Amazon craton (Fig. 1). The cap composes the basal 45
m of the Araras Group, which we interpret to have been deposited on
a deep platform. The identification of soft-sediment deformational features at the base of the cap is strong evidence that carbonate deposition
immediately followed Puga glaciation. Penecontemporaneous seismicity, here interpreted as induced by postglacial rebound, caused this
deformation.
PUGA CAP CARBONATE
Description and Interpretation of Lithological Units
The Araras Group comprises, from the base upward, the Mirassol
d’Oeste (dolostone), Guia (limestone and shale), Serra do Quilombo
(dolostone and dolomitic breccia), and Nobres (dolostone, chert, sandstone, and lime mudstone) Formations and is covered by siliciclastic
coastal fluvial deposits of the Alto Paraguai Group (Figs. 1A, 1B). The
subhorizontal, unmetamorphosed cap carbonate, represented by the
Mirassol d’ Oeste and Guia Formations, best exposed in the Terconi
quarry in Mirassol d’Oeste, overlies Puga glacial deposits (Fig. 1A)
consisting of pebbly diamictite with striated clasts of granite and sandstone (e.g., Alvarenga and Trompette, 1992). To the east and southwest,
the Araras Group and the Puga Formation are deformed within the
northern segment of the Paraguay belt (Fig. 1A).
The Puga cap is characterized by depleted d13C values, anomalous
facies, and a subwave-base depositional setting similar to other cap
carbonates worldwide (Kennedy, 1996; James et al., 2001; Hoffman
and Schrag, 2002). Values for d13C are generally ;25‰ and 87Sr/86Sr
values are ;0.7080 (Fig. 1B), typical of post-Varanger glacial events
(e.g., Jacobsen and Kaufman, 1999). Sharp anomalies in isotopic values (C, O, Sr) are observed at the base of the cap and at the contact
between the dolostone and limestone (Fig. 1B). The lower part of the
cap (the Mirassol d’Oeste Formation, Fig. 1B) is composed of locally
brecciated, pinkish dolomudstone with stratiform to wavy fenestral microbialites locally cut by tube-like structures, and fenestral nonmicro-
q 2003 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected].
Geology; July 2003; v. 31; no. 7; p. 613–616; 4 figures.
613
Figure 2. Features of Puga cap carbonate, Terconi quarry. A: Dolostone breccia. B: Tube-like structures. C: Deformed stromatolite cut
by fault at left. D: Offset tepee-like structures. E: Alternating laminae
of lime mudstone and crusts (light gray) disrupted by crystal fans.
Part of hammer head in C is 8 cm across; pen in D is 15 cm long;
coin in E is 2.5 cm in diameter. See Figure 3 for location of A–E in
profile.
Figure 1. A: Geologic map, southwest Amazon craton and adjacent
Neoproterozoic Paraguay belt (modified from Almeida, 1984). B:
Measured section and isotopic profiles, Puga cap carbonate, Terconi
quarry, Mirassol d’Oeste, Brazil.
bial planar laminites that are locally broken, disrupted, and upthrust to
form low-relief structures resembling tepees (Figs. 2A–2D). The upper
part of the cap (the Guia Formation) is made up of thick-laminated to
thin-bedded bituminous lime mudstone and interbedded shale laminae
(Fig. 1B). Thin calcite crusts alternate with lime mudstone laminae,
and both are commonly cut by calcite crystal fans (pseudomorphs after
aragonite) (Fig. 2E).
Although the presence of fenestral microbialites has been attributed to deposition within the shallow euphotic zone (Shinn, 1983), several lines of evidence suggest that the carbonates formed under low614
energy conditions, below storm-wave base, upon a moderately deep to
deep platform: (1) predominance of laminated micritic facies; (2) lack
of shallow-water features attributable to tides, waves, storms, or exposure within the two units; (3) intervals rich in bitumen indicative of
anoxic conditions; (4) unbroken aragonite crystal fans with intercrystalline micrite, consistent with precipitation in a deep Neoproterozoic
sea supersaturated in CaCO3 (e.g., Sumner, 2002); and (5) laterally
extensive limestone beds interpreted as hemipelagic facies (e.g., Coniglio and James, 1990) (Fig. 1B).
The basal contact of the cap represents a marine flooding surface
generated by postglacial sea-level rise (Fig. 3). The limit within the
cap between the dolostones and limestones is an irregular, scoured
transgressive surface with as much as 50 cm total relief, covered by a
thin iron-hydroxide–stained mudstone layer, representing a break of
unknown duration in sedimentation. Iron sulfide identified in the argillaceous layer may have been deposited during postglacial upwelling
of anoxic deep water (e.g., Hoffman and Schrag, 2002) (Fig. 3).
Synsedimentary Deformational Features
Four intervals exhibiting synsedimentary deformation are evident
in the succession at the Terconi quarry (Fig. 3). Interval I, at the basal
cap contact, is described in the following. In interval II, vertical tubelike structures cut across and slightly deform microbial laminites (Fig.
2B). In interval III, nonmicrobial laminites with fenestral fabric have
been broken and disrupted to form tepee-like structures (Fig. 2D) with
crests .3 m long, wavelengths of 0.15–2 m, and heights of 10–20 cm,
comparable to structures observed in other postglacial carbonates (Kennedy, 1996; James et al., 2001). In interval IV, limestone beds at the
base of the Guia Formation are locally brecciated, slumped, faulted,
and fractured, with neptunian dikes and multiple generations of cement
(Fig. 3).
GEOLOGY, July 2003
Figure 3. Detailed description of Puga cap carbonate with inferred
paleoenvironment and allocyclic processes. Gray bands mark zones
with deformation (explanations in text). Letters A–E refer to features
illustrated in Figures 2A–2E, respectively.
DEFORMATIONAL STRUCTURES AT THE BASE OF THE
PUGA CAP CARBONATE
Description of the Contact
At the base of the cap, locally brecciated pinkish dolomudstone
directly overlies massive diamictite containing pebble-sized striated
clasts (sandstone, granite) in a sandy argillaceous matrix (Figs. 4A and
4B). The contact is sharp and laterally irregular to slightly or markedly
wavy. Immediately below the contact, the diamictite is faintly laminated roughly parallel to the undulating contact. The dolostone forms
convex downward lobes, 0.5–2 m across and 0.3–0.7 m high, with
small, pustulate basal protuberances (Fig. 4C). Locally, the diamictite
penetrates as acute tongues between closely spaced lobes (Fig. 4D).
Within the first meter of the cap, the dolostone exhibits faint planar
and convolute lamination with local brecciation. Discontinuous fitted
breccias cemented by sparry dolomite comprise layers 4–10 cm thick
that grade laterally through fractured zones into undeformed dolostone
(Fig. 2A).
Mechanisms of Deformation
The irregular features at the base of the cap are interpreted as
synsedimentary load casts formed when the water-laden, plastic substrate (diamicton) moved upward between closely spaced sinking lobes
of dolomitic muds (e.g., Anketel et al., 1970; Lowe, 1975; Plaziat et
al., 1990; Rossetti and Góes, 2000). Despite some minor deformation,
primary lamination is preserved within the dolostone, which is typical
of plastic flow (Lowe, 1975), and except for locally convoluted lamination, there is no evidence of complete liquefaction and fluidization.
The carbonate behaved more viscously than the diamicton, although
both underwent plastic deformation at their contact. Both sediments
were subjected to low shear stress, liquefaction and thixotropism affecting mainly the diamicton, but also possibly reducing the shear
strength of the denser dolomitic layers. Closely spaced sagging load
GEOLOGY, July 2003
Figure 4. Deformed basal contact between diamictites of Puga Formation and dolomudstone of Mirassol d’Oeste Formation. A: Outcrop sketch drawn from photo montage. B: Protuberances on sole
of dolostone (scale height is 5 cm). C: Large dolostone load cast
(hammer handle 28 cm). D: Acute reentrant of diamictite between
sagging load casts (photo montage, scale height is 5 cm).
casts (cf. Alfaro et al., 1997) formed where dynamic viscosity was
greatest within the diamicton.
Although both snowball and nonsnowball Earth models (Hoffman
and Schrag, 2002, and Kennedy et al., 2001, respectively) postulate
cap-carbonate formation immediately following deglaciation, the deformation of unconsolidated diamicton and dolomitic sediments constitutes strong evidence that the transition from icehouse to greenhouse
conditions at this time was very rapid.
SEISMICALLY INDUCED DEFORMATION OF THE BASAL
CONTACT
Field evidence indicates that plastic flow and limited liquefaction
in water-laden sandy argillaceous diamicton and dolomitic muds were
the mechanisms responsible for deformation. Such possible natural
triggers as tectonic stress, gravity flow, and storm-wave impact may
be dismissed. The obviously plastic behavior of the sediments, the lack
of a pervasive tectonic imprint, and the alternation of undisturbed and
deformed layers (intervals I–IV) within the succession indicate that
later tectonism was not the cause. Localized fitted breccias grade to
undisturbed beds and therefore are not allochthonous, thus eliminating
gravity flow as a mechanism. The deposition of the carbonate below
storm-wave base rules out cyclic wave loading as a trigger. It is still
unclear what role depositional overloading by the dolomitic mud may
have played in this process. However, it is doubtful that even the 5
cm/k.y. accumulation rate suggested by Hoffman et al. (1998) for Namibian cap carbonates would have been sufficient to cause the observed deformation.
Load structures similar to those in the Puga cap have been associated with seismic shocks (e.g., Plaziat et al., 1990; Alfaro et al.,
1997), and Pratt (1994) considered earthquakes as a possible cause of
deformation in Mesoproterozoic carbonates of the Altyn Formation. A
sudden, upward-directed hydraulic force of short duration and upwardly decreasing intensity resulting from shaking (Munson et al., 1995;
Obermeier, 1996) could cause vertical adjustments of the sort registered
as load casts and convolute lamination at the base of the cap carbonate.
615
Furthermore, seismic shocks also commonly generate breccias (Pratt,
1994; Plaziat et al., 1990; Kahle, 2002) of irregular or fitted clasts in
incompletely lithified sediments, as well as synsedimentary faults and
fractures in more consolidated material, all of which are observed in
the cap dolomite. We judge this to be the best explanation for these
features. Deformation on the scale observed here would require an
earthquake of moderate to high magnitude, M . 5 (Ambraseys, 1988).
Additional seismic events could explain the deformation in intervals
II–IV (Fig. 3), especially the faults and deep-water tepee-like structures
in interval III (cf. Pratt, 2002), as well as the diverse structures in
interval IV. The origin of the tube-like structures in interval II is unknown, but may have been related to seismic activity (or gas or fluid
escape; see Kennedy, 1996).
POSTGLACIAL REBOUND AS THE TRIGGER FOR
SEISMICITY
Puga diamictites at the margin of the Amazon craton (Fig. 1)
indicate extensive ice cover of central South America in the terminal
Neoproterozoic, when the region was a continental margin soon to be
involved in late Brasiliano–Pan-African collision (Almeida, 1984; Alkmin et al., 2001). At this time, as in the Quaternary, seismic activity
due to crustal tectonics may have been at least partly suppressed by
the sheer weight of the ice sheets (e.g., Johnston, 1987).
As estimated for Quaternary deglaciation, postglacial rebound
may have increased regional susceptibility to earthquake activity with
attendant soft-sediment deformation for as long as 10 k.y. (Quinlan,
1984; Shilts et al., 1992; Wu and Johnston, 2000). Glacial rebound has
been suggested as a possible cause of subaerial exposure in other similarly thick Neoproterozoic cap carbonates exhibiting karst features
(e.g., James et al., 2001). We therefore suggest that glacial unloading
and postglacial rebound induced earthquake activity resulting in the
soft-sediment deformation at the carbonate-diamictite contact. Later
seismic events caused additional deformation (intervals II–IV) during
deposition of the remainder of the cap.
CONCLUSIONS
Our studies extend the record of post-Varanger global carbonate
precipitation to the Amazon craton of South America. Predominantly
microbially laminated dolomicrite of the Mirassol d’Oeste Formation
(Araras Group) accumulated below storm-wave base, and later lime
muds, crusts, and aragonitic crystal fans (Guia Formation) precipitated
in a CaCO3-supersaturated sea in a deep-water setting. Together they
form the Puga cap.
The Puga cap shares many of the unusual sedimentary features
typical of other latest Neoproterozoic cap carbonates, but is the first
for which soft-sediment deformation has been documented at the contact with underlying glacial diamictites. We suggest that soft-sediment
deformation was a consequence of synsedimentary seismicity related
to earthquakes induced by glacial rebound prior to lithification of both
the overlying dolomicrite and underlying diamicton. Hence, the Puga
cap provides clear evidence for the abrupt transition from icehouse to
extreme greenhouse conditions that is a fundamental tenet of the snowball Earth hypothesis for late Neoproterozoic glacial episodes.
ACKNOWLEDGMENTS
We thank the Fundação de Amparo à Pesquisa do Estado de São Paulo for financial
support (grant 00/02903-8 to Riccomini) and the Coordenação de Aperfeiçoamento de
Pessoal do Ensino Superior for a graduate fellowship to Nogueira. We thank Ricardo I.F.
Trindade, Werner Truckenbrodt, Renata Hidalgo, and reviewers P.F. Hoffman and B.R. Pratt
for helpful comments.
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Manuscript received 30 January 2003
Revised manuscript received 4 April 2003
Manuscript accepted 5 April 2003
Printed in USA
GEOLOGY, July 2003