Article Volume 12, Number 9 8 September 2011 Q09003, doi:10.1029/2011GC003602 ISSN: 1525‐2027
Spectral analysis and modeling of microcyclostratigraphy in late Paleozoic glaciogenic rhythmites, Paraná Basin, Brazil Daniel R. Franco Department of Geophysics, National Observatory, R. Gal. José Cristino, 77, 20921‐400 Rio de Janeiro, Brazil (
[email protected])
Linda A. Hinnov Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, Maryland 21218, USA
Marcia Ernesto Department of Geophysics, University of São Paulo, 05508‐900 São Paulo, Brazil [1] We investigate the depositional time scale of lithological couplets (fine sandstone/siltstone–siltstone/
mudstone) from two distinctive outcrops of Permo‐Carboniferous glacial rhythmites in the Itararé Group (Paraná Basin, Brazil). Resolving the fundamental issue of time scale for these rhythmites is important in light of recent evidence for paleosecular variation measured in these sequences. Spectral analysis and tuning of high‐resolution gray scale scans of sediment core microstratigraphy, which comprises pervasive laminations, reveal a comparable spectral content at both localities, with a frequency suite interpreted as that of short‐term climate variability of Recent and modern times. This evidence for decadal‐ to centennial‐ scale deposition of these lithological couplets is discussed in light of the ‘varvic’ character, i.e., annual time scale that was previously assumed for the rhythmites. Components: 8600 words, 5 figures. Keywords: Itararé Group; Paraná Basin; Brazil; Permo‐Carboniferous; glacial rhythmites; laminated sediment. Index Terms: 1861 Hydrology: Sedimentation (4863); 3255 Mathematical Geophysics: Spectral analysis (3205, 3280, 4319); 9614 Information Related to Geologic Time: Paleozoic. Received 2 March 2011; Revised 8 July 2011; Accepted 12 July 2011; Published 8 September 2011. Franco, D. R., L. A. Hinnov, and M. Ernesto (2011), Spectral analysis and modeling of microcyclostratigraphy in late Paleozoic glaciogenic rhythmites, Paraná Basin, Brazil, Geochem. Geophys. Geosyst., 12, Q09003, doi:10.1029/2011GC003602.
1. Introduction [2] Climate changes during the Late Quaternary icehouse appear to have been episodic and even abrupt, reflecting a highly nonlinear interplay among different climate system components [e.g., Alley et al., 1999; Broecker, 2000; Alley et al., 2003; Copyright 2011 by the American Geophysical Union
Rial, 2004]. Multiple environmental proxies have indicated millennial‐scale variability as major contributors for these changes [e.g., Alley et al., 1999, 2003; Marchant and Hooghiemstra, 2004; Clemens, 2005; Flückiger et al., 2006]. Is it possible that similar millennial‐scale harmonic patterns occurred throughout geological time as a prevalent feature of 1 of 15
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Figure 1. (a) Location and distribution of stratigraphic supersequences in the Paraná Basin, southeastern Brazil. The Itararé Group is classified as a Gondwana I Supersequence. Locations of the Itu and Rio do Sul quarries are also indicated; the S‐SE to N‐NW line refers to the cross section depicted in Figure 1b. Modified after Souza et al. [2006], Milani [1997], and Souza and Marques‐Toigo [2003]. (b) Stratigraphy of the Paraná Basin Gondwana I Supersequence. Period boundary ages are as follows: Mississippian‐Pennsylvanian, 322.8 Ma [Davydov et al., 2010]; Pennsylvanian‐Permian, 298.7 Ma [Ramezani et al., 2007], and Permian‐Triassic, 252.16 Ma [Shen et al., 2010; Huang et al., 2011]. The cross section (see map in Figure 1a) is redrawn from Souza et al. [2006] and Souza [2006] (copyright 2006, with permission from Elsevier) shows the time‐space distribution of basinal formations; the approximate stratigraphic positions of the Itu and Rio do Sul study sites are indicated, based on palynostratigraphy [Souza et al., 2010]. The depositional environment interpretation is from Eyles et al. [1993] and Souza et al. [2006].
the climate system? To address this question, it is necessary to carry out investigations on high‐ resolution paleoclimatic data sets with broad spatial and time scale coverage from representative time slices throughout Earth history.
[3] In this context, the occurrence of undisturbed late Paleozoic laminated rocks, which crop out in the Itararé Group (IG, Paraná Basin, Brazil; Figure 1), have been targeted as an important archive of Gondwanan glacial climate. These laminated rocks, 2 of 15
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Figure 2. Stratigraphy and sedimentology of the target Permo‐Carboniferous rhythmites. (top left) On‐location field photo at Itu Quarry of the Itu rhythmites (Late Carboniferous) showing lithologic couplet stratification. Note pronounced cross stratified bedding at bottom left. (top right) Stratigraphic progression and sedimentary structures. Additional photos are available from Eyles et al. [1993, Figures 16 and 17], in photos 14–17 of Gama et al. [1992b], and from Rocha‐Campos [2002]. (bottom left) On‐location field photo at Itau Quarry of the Rio dol Sul rhythmites (early Permian). The first author, D. Franco (height: 180 cm), is pictured for scale. (bottom right) Stratigraphic progression and sedimentary structures.
or “rhythmites,” consist of, from fine to coarse, stratified mudstone, siltstone and sandstones, [Setti and Rocha‐Campos, 1999] with centimeter‐scale sandstone/siltstone and siltstone/mudstone alternations forming lithological pairs or couplets with some regularity (Figure 2). Early sedimentological and palynological interpretations [Leinz, 1937; Rocha‐ Campos, 1967; Rocha‐Campos and Sundaram, 1981; Rocha‐ Campos et al., 1981] noted a similarity of the successions with Pleistocene varvic clays, although the unusual thickness of the couplets (up to 500 mm) requiring extraordinarily fast deposition rates, always caused some skepticism in the geological community [e.g., Eyles, 1993].
[4] In their detailed study of the Itu exposure based on petrography and macroscopic features of the light layers Caetano‐Chang and Ferreira [2006] rejected the varve designation for the Itu rhythmic sequence. They argued that the rhythmites constitute relatively distal portions of subaquatic fans influenced by underflow during periods of great ice melting, and decantation during winter. The shale laminae that cover the silt layers formed sporadically when more severe winters occurred or when the glacier was closer, in nonannually paced variable time intervals. These authors counted up to tens of massive or graded (coarse or fine silt) laminae within centimetric or decimetric layers. Normally the lamination is parallel, but in thicker layers cross lamination is present, indicating flow energy variations. 3 of 15
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[5] Based on the old assumption that each couplet corresponded to one year of sedimentation Ernesto and Pacca [1981] performed spectral analysis of the couplet thickness sequence and found periodicities of ∼11 and ∼22 units (interpreted in terms of years by the authors), as well as longer cycles. D. R. Franco et al. (Magnetostratigraphy and mid‐paleolatitude VGP dispersion during the Permo‐Carboniferous Superchron: Results from late Paleozoic rhythmites (Paraná Basin, Brazil), submitted to Geophysical Journal International, 2011) have investigated the paleomagnetic variation within each couplet (sliced into specimens of ∼2 cm thickness) in the same IG rhythmite sequences investigated here. The results indicate homogenous magnetic declination/inclination within couplets but large variations (∼20°–50° in declination) between adjacent lithological couplets suggestive of significant missing time between couplets. In modern times, this magnitude of change is associated with centennial time scales [e.g., Courtillot and Le Mouël, 1988]. Additionally, the paleomagnetic data have a distribution indicative of a well‐sampled, full secular variation pattern, which implies that elapsed times between couplets might be of the order of some thousands of years. The calculated paleomagnetic pole is in good agreement with the established Paleozoic apparent polar wander path. Therefore, the paleomagnetic data corroborate the recent reinterpretation for a nonvarvic character for the IG rhythmites. [6] Using cyclostratigraphic techniques Silva and
Azambuja Filho [2005] investigated glacial sediments of the Itararé Group from a borehole near Anitápolis, Santa Catarina state, about 100 km southeast from Rio do Sul (Figure 1). The Anitápolis section (Core 7‐RL‐04‐SC) exhibits 0.1–1.1 m thick silt beds alternating with 0.1–0.2 m thick shale layers with abrupt contacts. The authors concluded that the cyclic sedimentation was influenced by astronomical forcing and other, millennial‐scale phenomena, indicating accumulation rates of 7.5 to 8.4 cm/kyr for the section. Silva and Azambuja Filho [2005] also investigated two smaller intervals within the section at ultrahigh resolution using gray scale scans: (1) 526.70–525.89 m; (2) 525.87–525.47 m. They documented variations interpreted as decadal solar cycles (Schwabe and Hale cycles) and other quasiperiodicities related to solar activity, including Gleissberg and King Hele/Seuss cycles. They also reported other centennial to millennial periodicities at 280 yr and 650–1000 yr. [7] These studies support a much slower sedimentation rate than those assumed for decadal scale phenomena interpreted in earlier studies of Itararé
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rhythmites [Ernesto and Pacca, 1981; Rocha‐Campos et al., 1981]. If this is true it is possible that short‐ period cyclic processes compatible with well‐known environmental mechanisms are embedded within the lithological couplets. To investigate this we performed high‐resolution analysis of the laminations within individual rhythmite couplets using gray scale scans in a manner similar to the procedures of Archer [1994] and Silva and Azambuja Filho [2005]. We consider two hypotheses to explain the laminations and their stratigraphic patterns, in an attempt to decide on the precise timing of the couplets (annual or centennial‐millennial), and to identify the periodicities of the couplet variations.
2. Geological Setting and Stratigraphic Framework [8] The studied successions of glaciogenic rhythmites are from the eastern outcropping belt and southern portion of the Permo‐Carboniferous Itararé Group (IG), Paraná Basin, Brazil (Figure 1a). The sediments occur within the Gondwana I Supersequence recorded in Gondwanaland during the late Paleozoic [Rocha‐Campos et al., 1997; Ferreira, 1997; Milani and Zalán, 1999; Weinschütz and Castro, 2004; Souza, 2006; Vesely and Assine, 2006]. The IG formed in glaciolacustrine and/or brackish water paleoenvironmental settings, within an upwardly increasing marine influence and increasing facies diversity [Gama et al., 1992a, 1992b; Eyles et al., 1993]. Facies indicate subaqueous gravity flows, resulting in high sedimentation rates and proximity to steep, fault‐bounded basin margins [Eyles et al., 1993]. These aspects, as well as another glaciotectonic features, e.g., roche moutonnée, striated surfaces, diamictites, and regular rhythmites with dropstones, support the hypothesis for an record of ice masses, provided by a large south African ice sheet, advancing from SE to NW into the Paraná Basin [Rocha‐ Campos, 1967; França and Potter, 1991; Eyles et al., 1993; Santos et al., 1996; Rocha‐Campos et al., 2000; Vesely and Assine, 2002; Gesicki et al., 2002; Archanjo et al., 2006]. [9] Setti and Rocha‐Campos [1999] undertook a detailed facies description of the rhythmites, suggesting that they were deposited in a glacial regime, and were seasonally controlled. Declining thicknesses of the couplets up section, along with decreasing grain size, was suggestive of glacier retreat. Santos et al. [1996] concluded that the IG deposits formed in a restricted marine environment, strongly influenced by meltwater sources. 4 of 15
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[10] The ages of these rhythmites have historically
presented a challenge. Souza et al. [2010] provide the most recent chronostratigraphy from detailed palynostratigraphic analysis of drill cores and quarry rocks (Figure 1b). The Itu rhythmites have palynofloras from the Crucisaccites monoletus Interval Zone, which correlates to the Late Pennsylvanian Kasimovian‐Gzhelian stages. The Rio do Sul rhythmites are younger, with palynofloras from the Vittatina costabilis Interval Zone, corresponding to the Early Permian Asselian‐Sakmarian stages. [11] Rocha‐Campos et al. [2006] reported a U‐Pb
SHRIMP age of 298.5 ± 2.6 Ma in the Rio Bonito Formation (above the IG), which is very close to the most recent Carboniferous‐Permian boundary age of 298.9 + 0.3/ −0.15 Ma [Ramezani et al., 2007], and in good agreement with U‐Pb SHRIMP dating (302.0 ± 3.0 and 299.2 ± 3.2 Ma) for the Karoo Supergroup, southern Africa [Bangert et al., 1999]. In the lower IG, a U‐Pb SHRIMP age of 323.0 ± 15.0 Ma has been measured for the youngest detrital zircon ever found in the IG, indicating that the IG cannot be older than Late Mississippian (A. Rocha‐ Campos, personal communication, 2010).
3. Sedimentology of the Study Sites 3.1. Itu Rhythmites [12] Itu (IT) Quarry (23° 16′S; 47° 19′W), in the city of Itu (state of São Paulo), is preserved as a geological park (“Varvite Park”), and is the best known glacial rhythmite exposure in Brazil (Figure 2, top). Here the rhythmites occur in a ∼15 m thick succession, with approximately 260 lithological pairs or couplets [Sinito et al., 1981; Ernesto and Pacca, 1981], each one consisting of a basal, thicker (cm‐dm), light‐colored layer of fine sandstone/siltstone, coupled with an overlying dark thin layer/lamina (mm) of siltstone/mudstone. There is an upward trend of declining thickness and granulometry in the couplets, mainly due to changes in the light layers (from ∼20 cm to 50 cm at the bottom to ∼1.5 cm at the top); the thickness of the dark layers/ laminae is almost constant along the entire sequence (∼5 mm). There are sharp contacts between couplets, and abrupt but transitional contacts between light and dark layers within couplets [Rocha‐Campos et al., 1981; Setti and Rocha‐Campos, 1999]. [13] Sedimentary structures in light layers include
both flat and cross lamination, climbing ripple cross lamination, and an abundance of normally graded submillimeter‐scale laminae. Clasts (mm‐dm) com-
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posed of granite or quartzite occur as dropstones within the sequence [Rocha‐Campos and Sundaram, 1981; Gama et al., 1992a, 1992b; Ferreira, 1997; Setti and Rocha‐Campos, 1999]. The light layers consist of quartz silt (80–100%), mica (
