Songliao Basin of Northeast China and its stratigraphic and paleoclimate implications

The ?oating astronomical time scale for the terrestrial Late Cretaceous Qingshankou Formation from the Songliao Basin of Northeast China and its stratigraphic and paleoclimate implications

Huaichun Wu a ,b ,?,Shihong Zhang a ,Ganqing Jiang c ,Qinghua Huang d

a

State Key Laboratory of Geological Processes and Mineral Resources,China University of Geosciences,Beijing,100083,China b

School of Marine Science,China University of Geosciences,Beijing,100083,China c

Department of Geoscience,University of Nevada,Las Vegas,NV 89154-4010,U.S.A.d

Exploration and Development Research Institute of Daqing Oil Field Corporation Ltd.,Daqing,Heilongjiang,163712,China

a b s t r a c t

a r t i c l e i n f o Article history:

Received 19May 2008

Received in revised form 14October 2008Accepted 8December 2008

Available online 21January 2009Editor:P.DeMenocal

Keywords:

Late Cretaceous Milankovitch cycles

astronomical time scale (ATS)lacustrine anoxic event 1(LAE1)Songliao Basin

The Upper Cretaceous Qingshankou Formation (K 2qn)in the Songliao Basin (SLB)of Northeast China consists of up to 550m thick,lacustrine mudstone and shale that constitute one of the most important source rocks of the Daqing oil ?eld.A high-resolution cyclostratigraphic analysis of the natural gamma-ray logging from 10wells of the Qingshankou Formation (K 2qn)reveals orbital cycles of precession (20ka),obliquity (40ka)and eccentricity (100ka and 405ka),providing strong evidence for astronomically driven climate changes in the Late Cretaceous terrestrial environments.Floating astronomical time scales (ATS)are established for all sections,which demonstrate variable durations of K 2qn across the basin (1.09Ma –5.20Ma)and strong diachroneity of the lacustrine strata.Four periods of high depositional rates can be identi ?ed in the central parts of the basin,possibly recording deposition during times of sustained wet climate and high chemical weathering.An ATS established from well M206in the central depression zone of the basin,where the most complete and stable Milankovitch cycles are present,suggests that the maximum duration of the K 2qn is 5.20Ma (from 94.27Ma to 89.07Ma;Late Cenomanian to Early Coniacian).The lacustrine anoxic event 1(LAE1)at the Cenomanian –Turonian boundary lasted ~210–310ka,during which the most proli ?c petroleum source rocks in SLB were deposited.The onset (~94.21–94.18Ma)and duration (~210–310Ka)of LAE1in SLB are comparable to those of the oceanic anoxic event 2(OAE2;onset at 94.21Ma and duration of ~320–900ka),suggesting that the same trigger mechanism,such as increased atmospheric CO 2from large-scale igneous activity,may have initiated high primary productivity and organic carbon burial in both marine and terrestrial systems.

?2009Elsevier B.V.All rights reserved.

1.Introduction

The Cretaceous represents one of the warmest periods of the Phanerozoic eon and a time with signi ?cant ocean anoxic events (OAEs).Establishing a high-precision chronostratigraphic framework is fundamental for a better understanding of the global climate change under the Cretaceous supergreenhouse conditions.Owing to the limitations of radiometric ages and paleontological data in general,the astronomical time scale (ATS),established by tuning astronomical forcing signals recorded in sedimentary strata,plays critical roles for de ?ning and correlating the Cretaceous paleoclimatic/paleoceano-

graphic events including OAEs (e.g.Fiet et al.,2006;Li et al.,2008;Locklair and Sageman,2008;Mitchell et al.,2008).With the progresses made in the last decade,it is anticipated that an ATS covering the entire Cretaceous Period will be completed in a few years (Hinnov and Ogg,2007).The existing ATS,however,is restricted to marine strata.There is an immediate need to establish the Cretaceous ATS from terrestrial sedimentary basins so that the marine and terrestrial records can be compared and a better understanding of the Cretaceous Earth system change can be achieved.

Continental rift basins are unique repositories for long-term palaeoclimate records (Olsen and Kent,1999).Lacustrine mudstone and shales deposited in continental rift basins,due to their sensitivity to changes in precipitation –evaporation ratios and/or water-level changes,are particularly suitable for high-resolution cyclostratigraphic studies (e.g.,Olsen and Kent,1999;Prokoph and Agterberg,2000)and for establishing astronomical time scales with resolution potentially down to 0.02to 0.40Ma (e.g.,Hinnov,2004;Hinnov and Ogg,2007).

Earth and Planetary Science Letters 278(2009)308–323

?Corresponding author.School of Marine Science,China University of Geosciences,Xueyuan Road 29,Haidian District,Beijing 100083,China.Tel.:+861082334699;fax:+861082320065.

E-mail addresses:whcgeo@https://www.360docs.net/doc/909542160.html, ,whccugb@https://www.360docs.net/doc/909542160.html, (H.

Wu).0012-821X/$–see front matter ?2009Elsevier B.V.All rights reserved.doi:

10.1016/j.epsl.2008.12.016

Contents lists available at ScienceDirect

Earth and Planetary Science Letters

j o u r n a l h o m e p a g e :w ww.e l s e v i e r.c o m /l o c a t e /e p s l

The Songliao Basin in northeastern China is one of the largest Cretaceous continental rift basins in the world(Fig.1).Well-preserved Cretaceous lacustrine deposits in this basin provide a unique opportunity for the construction of terrestrial Cretaceous ATS. Particularly,the Upper Cretaceous Qingshankou Formation(K2qn) consists of up to550m thick,black mudstone,shale and oil shale that have attracted considerable attention regarding their potential linkage with Milankovitch climate forcing(e.g.,Wu et al.,2007;Cheng et al., 2008).The basal interval of this unit is enriched in oil shale and was considered as deposits during a lacustrine anoxic event(Huang et al., 1998,2007).This event,referred to as lacustrine anoxic event1 (LAE1),has been suspected to be time equivalent to the oceanic anoxic event2(OAE2),but the lack of age controls prevents a chronostrati-graphic correlation with the marine record.

In this paper,we present a cyclostratigraphic study based on the high-resolution natural gamma-ray logging of the Qingshankou Formation from10wells in an east–west transect across the Songliao Basin(Fig.2).The major objective of this study is to establish an astronomically calibrated time scale(ATS)for the Late Cretaceous lacustrine strata.With the new?oating ATS,we discuss(1)the stratigraphic correlation across the basin and temporal variations in sedimentary rates,(2)the duration of the ostracod biozones constrained by the ATS that may be useful for stratigraphic correlation between terrestrial and marine successions,and(3)the duration of the lacustrine anoxic event1(LAE1)and its correlation with the oceanic anoxic event2(OAE2).

2.Geological setting

2.1.Tectonics and stratigraphy

Geographically,the Songliao Basin(SLB)in northeastern China is surrounded by the Great Xing'an Mountains in the west,the Lessar Xing'an Mountains in the north and the Zhangguangcai Mountains in the east(Fig.1a–c).The southern margin of the basin is separated from the North China plate by the Late Paleozoic Chifeng–Kaiyuan fault zone (Sun et al.,2007).The tectonic evolution of the SLB can be divided into (1)a pre-rift phase,(2)a syn-rift phase,(3)a post-rift phase,and(4)a compression phase(Cheng et al.,2006)(Fig.1d).According to the characteristics of rises and depressions,the SLB can be divided into six ?rst-order tectonic units:central depression zone,north plunge zone, west slope zone,northeast uplift zone,southeast uplift zone,and southwest uplift zone(Gao et al.,1994)(Figs.1b and2).

The basement of the SLB consists of Precambrian to Paleozoic metamorphic and igneous rocks and Paleozoic to Mesozoic granites (Wang et al.,2006;Pei et al.,2007).Unconformably overlying the basement,up to7000m thick Mesozoic and Cenozoic terrestrial strata are unevenly distributed across the basin(Gao et al.,1994).A simpli?ed lithostratigraphy of the Songliao Basin is presented in Fig.1d.

The Upper Jurassic and Cretaceous strata in SLB is commonly subdivided into two sequences separated by a regional unconformity at the top of the Yingcheng Formation(K1y).The lower sequence includes the Upper Jurassic Huoshiling Formation(J3h)and the Lower Cretaceous Shahezi(K1s)and Yingcheng(K1y)Formations(Fig.1d). These formations are composed of volcanic–volcaniclastic rocks and alluvial–lacustrine sedimentary rocks.

The upper sequence consists of seven formations,in ascending order including Denglouku(K1d),Quantou(K2q),Qingshankou (K2qn),Yaojia(K2y),Nenjiang(K2n),Sifangtai(K2s)and Mingshui (K2m)Formations(Fig.1d).The Denglouku Formation(K1d)is composed mainly of alluvial and?uvial deposits,while the Quantou Formation(K2q)consists of coarse clastic rocks of?uvial origin in the lower part and lacustrine mudstones in the upper part.The lacustrine sandstone,mudstone,and shale of the Qingshankou (K2qn),Yaojia(K2y)and Nenjiang(K2n)Formations contain the majority of the oil source rocks,reservoir rocks and seals of the Daqing Oil?eld(Li,1995)(Figs.1d and2).Unconformably overlying the K2n,the Sifangtai(K2s)and Mingshui(K2m)Formations are mainly composed of siliciclastic rocks deposited from alluvial and deltaic environments during basin inversion.

The Qingshankou Formation(K2qn)was deposited during the post-rift thermal subsidence stage.The thickness of this unit varies from30to 550m across the basin(Fig.2).It conformably overlies the Quantou Formation(K2q)and is unconformably or conformably overlain by the Yaojia Formation(K2y).On the basis of lithofacies changes,the K2qn can be divided into three members(Fig.1d).Member1(K2qn1)is mainly composed of deep lacustrine black mudstone and shale.The oil shale at the lower part of the K2qn1is the most important marker for regional stratigraphic correlation.Member2and3of the K2qn(K2qn2+3)consist of interbedded black mudstone and gray-to celadon-colored siltstone. In general,the Qingshankou Formation shows a shallowing—upward trend from deep lacustrine deposit in the lower part(K2qn1)to shallow laustrine,coastal and deltaic deposits in the upper part(K2qn2+3)(Gao et al.,1994;Li,1995;Fig.1b and c).

2.2.The age of the Qingshankou Formation(K2qn)

Due to the lack of radiometric ages and dif?culties of de?ning the duration of terrestrial fossils,the age of the K2qn has been debated. Previous age assignments include(1)late Cenomanian-early Tur-onian(Chen,2000),(2)Cenomanian(Stratigraphy Committee of China,2002;Wan et al.,2005;Sha,2007),(3)Cenomanian–Turonian (Ye et al.,2002),and(4)late Cenomanian–early Coniacian(Wang et al.,2007).

Recent radiometric ages from volcanic rocks of the Yingcheng Formation(K1y)and paleontological data from Denglouku(K1d)and Quantou(K2q)Formations support a late Cenomanian age for the Qingshankou(K1qn)Formation.The SHRIMP zircon U-Pb age of111–113Ma(Zhang et al.,2007a)and K/Ar ages of113–136Ma(Wang et al., 2002)from the upper Yingcheng Formation(K1y)suggest that the age of the K1y is Aptian and the age of the K1d should be Albian(Fig.1d). The palynological assemblages of Quantonenpollenites crassatus–Cranwellia striatella and Trilobosporites–Cyathidites–Tricolpopollenites in the upper and lower part of the Quantou Formation(K1q)are of early Turonian and late Cenomanian,respectively(Li,2001).This is consistent with the palynological data from the K1d that have the ages of Early to Middle Albian(Li and Li,2005).Recently,Wang et al.(2007) estimated an age of~94Ma for the K2qn/K2q boundary by extrapolating the stratal thickness of the basin.This is con?rmed by their latest,yet to be published radiometric age from the K2qn(Wang, P.J.,personal communications,2008).

https://www.360docs.net/doc/909542160.html,custrine anoxic events(LAEs)in the SLB

Previous studies suggested that two great lacustrine anoxic events (LAEs)happened in SLB during the deposition of the K2qn1and K2n1+2, and it was proposed that they were related to two great marine transgressions that may have created warm and wet climate conditions (Fig.1d;Gao et al.,1994;Hou et al.,2000;Wang et al.,2001;Li and Pang, 2004).These lacustrine anoxic events was considered crucial for the formation of proli?c petroleum source rocks in SLB(Hou et al.,2000; Wang et al.,2001;Li and Pang,2004;Huang et al.,2007).Evidence supporting the lacustrine anoxic event1(LAE1)in K2qn1include(1) deposition of?nely laminated,organic and pyrite—rich black shale and oil shale with a positiveδ13C org spike(Wang et al.,2001;Huang et al., 2007;Huang,2007),(2)extinction of deep water-fauna during LAE1and ?ourish of diversi?ed fossil assemblage after LAE1(Huang et al.,1998; Hou et al.,2000),and(3)appearance of28,30-bisnorhopane which originates from a typical anoxic bacteria and preservation of complete C35 hopanes series,lower diasteranes content in mudstone(Hou et al.,2003).

The sedimentary sequence of the red terrigeneous sediments of the K2q,K2y and K2s and the black sediments of the K2qn1and K2n1+2

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may record the alternation of anoxic and oxic lacustrine depositional environments,similar to the redox cycles recorded in Cretaceous marine strata (Fig.1d;Huang,2007).The two great lake anoxic events (LAE1and LAE2in Fig.1d)in K 2qn 1and K 2n 1+2have been suspected to be correlatable with the ocean anoxic events (Hou et al.,2000;Huang,2007;Huang et al.,2007;Wang et al.,2007),but reliable time constraints are lacking.2.4.Biostratigraphy of the K 2qn

Fossils are abundant in the K 2qn and intensive paleontological studies have been conducted since 1959(e.g.,Gao et al.,1999;Ye et al.,2002).Fossils of the K 2qn include abundant ostracods,conchostraca,?sh,gastropods,bivalves,sporopollen,algae,and vertebrates (Hou et al.,2000;Wan et al.,2007).The identi ?cation of these fossils has played an important role in the stratigraphic correlation and division and the reconstruction of paleoecology,paleoenvironment and paleoclimate in the SLB.However,the biostratigraphic resolution is low and many fossils are restricted to either K 2qn 1or K 2qn 2+3.Fossil abundance and diversity also vary among sections across the basin and thus it has been dif ?cult to establish a reliable biostratigraphic framework correlatable with that of the marine successions (Wan et al.,2005).Recently,a detailed ostracod biostratigraphy of the K 2qn with 19ostracod biozones was established,based on collections from ?ve recovered wells (Zhang et al.,2007b ).These ostracod biozones are going to be calibrated by the new ?oating

ATS so that they can be correlated with the marine biozones in the future.

3.Data processing

Natural gamma-ray logging records the intensity of the gamma ray emitted during the decay of atomic nuclei of radioactive elements contained in sedimentary rocks.The intensity of gamma ray relates to the amount of 40K,232Th and 238U in rocks.Clay and organic particles have strong capacity of absorbing radioactive elements.The gamma-ray logging curves can therefore re ?ect changes in the amount of clay and organic materials in sediments,both of which are sensitive to controlling factors such as precipitation –evaporation ratios and lake-level ?uctuations induced by climate changes (e.g.,Serra,1984;Hinnov,2004).

In this study,natural gamma-ray logs from 10wells penetrating the Qingshankou Formation (K 2qn)across the basin were selected for cyclostratigraphic analysis (Figs.1b,c and 2).The shortest and the longest gamma-ray logs are J32(101m)and G692(544m),respectively (Figs.3–6).In well F64and J32of the west slope zone (Figs.1b and 2),K 2qn unconformably overlies the Upper Jurassic strata,while in the other 8wells K 2qn conformably overlies the Quantou Formation (K 2q).The sampling spacing is from 0.05m to 0.1524m for different logs.

Spectral analysis and continuous wavelet analysis were conducted in order to investigate whether the cyclicity records signal resulting

Fig.1.(a)Location of the Songliao Basin (SLB)in northeastern China.(b)Simpli ?ed paleogeographic reconstruction of the SLB during deposition of Member 1of the Qingshankou Formation (K 2qn 1).Tectonic units in the SLB and locations of the study wells (black dots)are indicated.Lines A –B mark the position of the cross section in Fig.2.(c)Paleogeographic reconstruction of the SLB during deposition of the K 2qn 2+3.(d)Simpli ?ed stratigraphy and geological events of the Upper Jurassic to Cretaceous in the SLB.The sea-level change curve is adopted from Gradstein et al.(2004)and the lake-level curve is from Gao et al.(1994)

.

Fig.2.Cross section of the Songliao Basin (modi ?ed from Wang et al.,2007).Position of sections matches those in Fig.1b.Sections L2and D501are located at the north plunge zone and northeast uplift zone,respectively.Abbreviation N 2t represents the Taikang Formation and the other abbreviations are the same as in Fig.1d.

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from astronomical climate forcing.A band-pass?lter was designed to remove the low(N100m)and high(b1m)frequency variability. Spectral analysis was performed with the REDFIT software package (Schulz and Mudelsee,2002).Wavelet analysis is used to transform depth-related sedimentary signature into wavelengths at distinct depth intervals,useful for detecting and distinguishing abrupt discon-tinuities,cyclicity and changes in sedimentation rate(Prokoph and Agterberg,2000).The wavelet software was provided by Torrence and Compo(1998)and is available at https://www.360docs.net/doc/909542160.html,/research/ wavelets/.

4.Cyclostratigraphy

4.1.Orbital parameters in Late Cretaceous

According to geological and paleomagnetic evidence(Zhao et al., 1990;Chi et al.,2000),the Songliao Basin has been adjacent to the present position in early Late Cretaceous.We used the June21insolation curve at45°N from94to90Ma as a reference(Fig.3a-1),which is calculated using Analyseries2.0(Paillard et al.,1996)and the improved La2004solution(Laskar et al.,2004).Four distinct frequency bands can be observed from the continuous wavelet analysis scalogram(Fig.3a-2), which are consistent with results from spectral analysis.Spectral analysis shows the presence of the principal eccentricity periods of 405ka(E3),125ka(E2)and100ka(E1),principal obliquity periods of 37.5ka(O1)with lesser ones of48ka(O2),56ka and28ka,and principal precession periods of22.5ka(P2),18.4ka(P1)and15.9ka(Fig.3a-3).We will use these periods as the Late Cretaceous“canonical”orbital varia-tions in our study.

4.2.Cycle analysis and cyclostratigraphic interpretation

Parallel bands in the wavelet scalograms show that the spectral power is con?ned to distinct and relatively continuous period bands for all sections(Fig.3b-l).Spectral analyses reveal distinctive spectral power peaks and the ratio of corresponding periods is ~20:5:2:1,which is similar to the ratio of Milankovitch cyclicities of 405ka:100ka:40ka:20ka(long eccentricity,short eccentricity, obliquity,precession)(Fig.3b-l).

We followed the method of Westerhold et al.(2007)to construct a cyclostratigraphy for the K2qn for each section(Figs.4–6).Direct cycle counting and Gaussian band pass?ltering of the data are used.The ?rst peak of gamma-ray logging(precession),extracted short and long eccentricity cycles were used as the starting point(Figs.4–6).Strong shifts in the wavelet spectral bands for sections G692(at2100m),L2 (at1770m),F64(at557m and500m)suggest changes in cycle periodicities due to change in sedimentation rates,so we use different Gaussian band pass?lters to extract orbital forcing cycles(Figs.5and 6).The longest section(M206)in the central depression zone and the shortest section(J32)in the west slope zone were selected as end members for interpreting the results from wavelet and spectral analyses and constructing cyclostratigraphy.

In section M206,the total thickness of K2qn is496.8m(from1286m to1782.8m in the well log;Fig.4).The lower part of K2qn(K2qn1)is composed mainly of gray and black mudstone,with thin calcareous mudstone layers and three black oil shale layers.The upper part of K2qn (K2qn2+3)consists mainly of thick,gray to dark-gray mudstone with calcareous mudstone interbeds,but changes to thinly laminated,red silty sandstone and mudstone towards the top(Fig.4a).The gamma-ray values are in the range of~60–180API.Low values correspond to calcareous mudstone,whereas high gamma-ray values are produced by black shale and mudstones.Signi?cantly low values at the top of K2qn correspond to the red silty sandstones(Fig.4).The density log has values mostly from1.7to2.3g/cm3,but lower values are found at the top, possibly related to increased porosity in sandstones(Fig.4).

Wavelet analysis of both gamma-ray and density logs reveal relatively continuous cycles with periods of68m,39m,13.5–9m,5–3.8m and2.5–1.7m(Fig.3f and g).The lack of a strong shift at periods of39m and13.5–9m in both gamma-ray and density logging suggests that relatively stable sedimentation rate prevailed during the deposi-tion of K2qn,while the variance at shorter periods of5–3.8m and2.5–

1.7m probably records short-term?uctuations in sediment supply

(e.g.,Prokoph and Agterberg,2000).

To determine whether the observed cycles in sedimentary strata were formed by astronomical forcing,the most commonly used method is to compare the relative ratio of the observed cycles with that of the Milankovitch cycles(Hinnov,2000;Weedon,2003).The ratios of the major periods of~39m,13.5–9m,5–3.8m and2.5–1.7m from natural gamma-ray and density logging of section M206is 20:5:2:1,which matches well with the Milankovitch cyclicities of 405ka:100ka:40ka:20ka(long eccentricity,short eccentricity, obliquity,precession).Spectral analysis on the full time series of the gamma-ray logging reveals four distinct cycles of14.2–8.9m,4.9–3.8m,2.6–2.3m and1.9–1.7m(Fig.3f and g).The ratio for the?ve main peaks of these cycles is13.3m:10.3m:4.05m:2.43m:1.9m (123:95:37.5:22.5:17.6).These ratios are similar to the results of spectral analysis on the June21insolation curve at45°N from94to 90Ma(Fig.3a).We thus consider that the sedimentary cyclicity in section M206was formed by orbital forcing(Weedon,2003).The major periods of~39m,13.5–9m,5–3.8m and2.5–1.7m were formed most likely by long(405ka)and short(100ka)eccentricity, obliquity(40ka)and precession(20ka),respectively.

The small cycle length of the original natural gamma-ray and density logging curves range from1.7m to2.5m,which are interpreted as precession-related sedimentary cycles(Fig.4).Counting from the base of the K2qn,each cycle is numbered and there are~253precession cycles in both gamma-ray and density logging data(Fig.4).Two Gaussian band-pass?lters were used to?lter the signals of the long and short eccentricity cycles of~39m and13.5–9m,respectively.The ?ltering outputs of the gamma-ray and density logging are shown in Fig.4.We counted12.8long eccentricity cycles and51.6short eccentricity cycles from the gamma-ray logging,but12.3long eccentricity cycles and50.1short eccentricity cycles from the density logging(Fig.4).There are about0.5long eccentricity and1.5short eccentricity offsets.The underlying reason for this difference is uncertain,but because the cycle bands in the wavelet scalograms and the?lter outputs of gamma-ray logging demonstrate more stable and constant signals than those of the density logging(Figs.3f,g and4), and because previous studies demonstrated that gamma-ray readings are more sensitive to variations in lithology and clay–mineral content than other logging tools(Serra,1984;Prokoph and Thurow,2000),we chose the gamma-ray logging data for cyclostratigraphic study for all sections in this study.

The total thickness of K2qn in section J32in the west slope zone is 101m(from398m to499m in the well log;Figs.3k and6f).It unconformably overlies the Jurassic volcanic rocks and is

Fig.3.(a)June21insolation curve at45°N from94to90Ma(a-1),wavelet scalogram(a-2)and spectral analysis(a-3)of the theoretical insolation curve,which are calculated using Analyseries2.0(Paillard et al.,1996)and La2004solution(Laskar et al.,2004).The shaded contours in wavelet scalograms are normalized linear variances,with blue representing low spectral power and red representing high spectral power.Regions below curves on both ends indicate the cone of in?uence where edge effects become signi?cant.(b)–(f)and(h)–(l)Wavelet scalogram and spectral analyses of the gamma-ray logging of the K2qn in different tectonic zones of the Songliao Basin.(g)Wavelet scalogram(g-1)and spectral analysis(g-2)of the density logging of M206.Letters“d”mark the possible discontinuities.See Figs.1b and2for location of sections,and Figs.4–6for original logging data.

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Fig.3(continued).

unconformably overlain by sandstones of the second and third member of Yaojia Formation(K2y2+3).In this section,K2qn1is com-posed of purple mudstone and gray muddy siltstone and K2qn2+3 consists mainly of emerald,celadon and purple mudstone with sandstone and muddy siltstone.

The parallel bands in the wavelet scalogram of section J32show three cycle periods of16–9m,5.8m and3.2m.Spectral analysis on the full time series reveals two distinct cycle bands of6.2–4.3m and 3.6–2.8m(Fig.3k).These periods has a ratio of5:2:1,which is correlative to the short eccentricity,obliquity and precession ratio of 100ka:40ka:20ka(Fig.3k).Although spectral analysis for this section does not show the peaks of eccentricity cycles,short and long eccentricity cycles were extracted by the Gaussian band pass?lters according to the wavelet analysis and the variation of gamma-ray logging(Fig.3k).We counted 2.8long eccentricity,10.9short eccentricity and50precession cycles in section J32(Fig.6f).5.Discussion

5.1.Astrochronology of the Qingshankou Formation(K2qn)

The records from the K2qn in the SLB display excellent precession, obliquity,short and long eccentricity signals(Figs.3–6),and theoreti-cally,every band of signals can be used for tuning to the orbital solutions. However,because the precision of the orbital solution for Mesozoic is limited(Laskar et al.,2004),the high frequency cycles may not be suitable for constructing the ATS.Changes in resonance between the orbits of Earth and Mars may have altered the95to128ka eccentricity. Tidal friction in the Earth–Moon system could slow the Earth's rotation rate over time,increasing the periodicities of precession and obliquity (Laskar et al.,2004).Thus a full-spectrum tuning of the Mesozoic records to the La2004solution is impossible at the moment.Because the long eccentricity cycle(405ka)remains very stable at least back to~250

Ma,

Fig.4.(a)Simpli?ed lithology and stratigraphy of the K2qn from well M206.(b)Continuous gamma-ray(red line)and density(blue line)logging of the K2qn from M206with0.125m sampling resolution.The red and blue numbers indicate the number of precession-related cycles.(c)Cycle counting of long eccentricity?lter outputs of gamma-ray(red)and density (blue)logging data of the K2qn from M206.(d)Cycle counting of short eccentricity?lter outputs of gamma-ray(red)and density(blue)logging data of the K2qn from M206.The long and short eccentricity-related cycles are extracted by Gaussian band pass?lters with?lter frequency of0.0225±0.003cycles/m and0.104±0.013cycles/m,respectively.

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it was recommended that a Mesozoic astronomically calibrated time scale can be established as a ?oating time scale using long eccentricity cycles (Laskar et al.,2004;Hinnov and Ogg,2007).However,the extracted long eccentricity cycles from most sections of the K 2qn are less stable than the short eccentricity cycles (Figs.4–6),which also show more stable and clear signals than other bands of signals in most

wavelet

Fig.5.Cycle counting of precession (black),long (red)and short (blue)eccentricity of gamma-ray logging data of the K 2qn from sections (a)G692,(b)C503and (c)Z805of the central depression zone.The long and short eccentricity cycles are extracted by Gaussian band pass ?lters.Filter frequency for long eccentricity cycles:G692–0.025±0.003(from 1800to 2100m)and 0.019±0.002(from 2100to 2344m)cycles/m;C503–0.0187±0.003cycles/m;Z805–0.0208±0.003cycles/m.Filter frequency for short eccentricity cycles:G692–0.083±0.026cycles/m;C503–0.075±0.01cycles/m;Z805–0.081±0.02cycles/m.

Fig.6.Cycle counting of precession (black),long (red)and short (blue)eccentricity of gamma-ray logging data of the K 2qn from sections (a)L2,(b)W209,(c)D501,(d)F64,(e)L271and (f)J32.The long and short eccentricity cycles are extracted by Gaussian band pass ?lters.Filter frequency for long eccentricity cycles:L2–0.028±0.005cycles/m;W209–0.0262±0.003cycles/m;D501–0.03125±0.004cycles/m;F64–0.02381±0.003cycles/m;L271–0.01923±0.003cycles/m;J32–0.02439±0.003cycles/m.Filter frequency for short eccentricity cycles:L2–0.108±0.021(from 1449to 1770m)and 0.091±0.01(from 1770to1874m)cycles/m;W209–0.1042±0.015cycles/m;D501–0.1266±0.025cycles/m;F64–0.09375±0.03125cycles/m;L271–0.0746±0.08cycles/m;J32–0.1111±0.033cycles/m.

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scalograms and spectral analyses (Fig.3).Thus we chose to construct the ATS using the short eccentricity cycles.

After cycle analyses of all sections across the SLB,we chose section M206at the central depression zone as the reference section to establish the ATS (Fig.7)because this section records stable Milankovitch cycles,longest accumulation time (Figs.3f,g and 4),and no identi ?able stratigraphic discontinuities.The estimated age of ~94Ma for the K 2qn/K 2q boundary (Wang et al.,2007)was used as a reference for curve matching between the short eccentricity cycles from gamma-ray logging and the short eccentricity curve of La2004solution (Fig.7).

The

Fig.7.(a)Astronomical calibration of short eccentricity cycles from ?lter output of gamma-ray logging (red)compared with short eccentricity (blue)and full eccentricity (gray)of La2004solution (Laskar et al.,2004)for the Qingshankou formation (K 2qn)in M206.Also shown are the comparisons between extracted long eccentricity,obliquity and precession cycles from gamma-ray logging of well M206(red)and those of the La2004solution (black).The Gaussian band pass ?lters for short (long)eccentricity of La2004solution are 0.0093±0.0013(0.002469±0.0003)cycle/ka.(b)Ostracods biozones of the K 2qn (modi ?ed from Zhang et al.,2007b )calibrated by the established astronomical times of M206.

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correlation was performed using the software of Analyseries 2.0 (Paillard et al.,1996).

In order to get a start point,?ltered long and short eccentricity, obliquity and precession curves of M206were slidden back and forth to get the best visual match of peak locations and amplitudes with La2004solution.The tuning was done by assigning ages of short eccentricity maxima of the La2004solution to the corresponding peaks of extracted short eccentricity of gamma-ray logging of K2qn as identi?ed and labeled in the cyclostratigraphy(Figs.4and7).The long eccentricity,however,show a small offset of~100ka with that of the La2004solution.This offset is possibly due to the unstable long eccentricity signal itself or related to uncertainties with the eccen-tricity solutions(Laskar et al.,2004).The precession and obliquity bands derived from the gamma-ray logging show similar variations with those of La2004solution(Fig.7a).

The established ATS from section M206suggests that the duration of the K2qn is about5.20Ma,from94.27Ma to89.07Ma covering Late Cenomanian to Early Coniacian(Fig.7a).The ATS provides time constraints for the ostracod biozones(Fig.7b),which will be useful for future stratigraphic correlations of the ostracod-bearing terrestrial strata and their correlations with marine successions.For example, the duration of the fossil zone of Ziziphocypris rugosa is about90.09–91.69Ma(Fig.7b).

Using the same method,we tentatively constructed the ATS for all the10sections of K2qn(Fig.8).The duration of K2qn varies from 5.20Ma in M206of the central depression zone to1.09Ma in section J32of the west slope zone(Fig.8and Table1).Signi?cant variations in the duration of K2qn indicate internal stratigraphic discontinuities and particularly the stratigraphic truncation at the top of the K2qn.

5.2.Preservation and origin of Milankovitch cycles

Wavelet and spectral analyses on the natural gamma-ray logging of the K2qn shows excellent preservation of astronomically driven sedimentary cycles,including long and short eccentricity,obliquity, and precession cycles in sections from the central parts of the basin where thicker and more complete strata are preserved(Fig.3b–i). However,in thinner sections of the west slope zone(F64,L271and J32), long eccentricity cycles were not detected in the wavelet or spectral scalograms(Fig.3j–l).These sections record only2–3long eccentricity cycles(Fig.6d–f),which are dif?cult to be detected by wavelet and spectral analyses.In section F64(Fig.3l),only precession and short eccentricity cycles can be identi?ed.Potential stratigraphic disconti-nuities can be identi?ed from these sections(Fig.8)on the basis of abrupt jumps in cycle thickness(cf.Prokoph and Agterberg,1999,2000), consistent with the appearance of relatively coarse-grained lithology (silty sandstone)at the discontinuities.Thus the incomplete preserva-tion of Milankovitch cycles in these sections may relate to non-deposition and/or erosion at the margin of the basin.Stratigraphic discontinuities may also exist in sections D501,Z805,G692and L2 (Fig.8),but the internal time-missing at these discontinuities is hard to be evaluated due to the lack of absolute age constraints across the basin. The traditionally de?ned K2qn2+3/K2qn1boundary show signi?cant diachroneity(Fig.8),but the?ne-grained lithologies(dominated by mudstones)across the boundary in the central parts of the basin suggest that the missing time at this level may be limited.The diachroneity at this boundary may record facies changes across the basin.A major stratigraphic unconformity occurs at the top of the K2qn,which may have resulted in signi?cant stratigraphic truncation across the basin.

The identi?cation of all bands of Milankovitch cycles in the central parts of SLB suggests strong astronomical climate forcing in the development of the sedimentary cyclicity in the K2qn.The sedimen-tary cycles recorded by the gamma-ray logging of relatively homo-geneous,?ne-grained lithologies(mudstones)in the K2qn may re?ect changes in clay mineral input in response to wet and dry periods.Wet periods may have enhanced chemical weathering and clay mineral input,resulting in positive gamma-ray peaks;while decreased chemical weathering during dry periods may have formed the negative gamma-ray peaks.The occurrence of four periods of high sedimentary rate(Fig.9)in the K2qn may record times of sustained wet climate and high sedimentary input in the basin.

5.3.The duration of lacustrine anoxic event1(LAE1)in the SLB and its correlation to OAE2

Because availableδ13C org values are very sparse and cannot de?ne a positiveδ13C org‘excursion’,the traditional LAE1in SLB was de?ned by the presence of oil shales in the lower K2qn1.The thickness of the oil shales ranges from17.0m(D501)to27.0m(G692and L2),which records~2–3short eccentricity cycles and~10–16precession cycles (Figs.4–6and Table1).Our new ATS indicate the shortest and the longest duration of LAE1are210ka(C503)and310ka(M206), respectively(Fig.8and Table1).

The LAE1has been suspected to be time equivalent to the oceanic anoxic event2(OAE2)at the Cenomanian–Turonian boundary(Huang et al.,1998,2007).The established ATS for the K2qn con?rms this correlation.The onset of the OAE2has recently been estimated as 94.09Ma(Sageman et al.,2006)and94.21/93.72Ma(Mitchell et al.,

Table1

Estimated age of the geological events in the Qingshankou formation(K2qn)according to?oating astronomical time scales and cyclostratigraphy

Wells Boundary(thickness)(m)Estimated age(duration)(Ma)Sedimentary

rates(cm/ka) K2qn LAE1K2qn K2qn1/K2qn2+3boundary LAE1

Southeast uplift zone

W209285.0–704.0(418.5)672.0–697.0(25.0)89.85–94.25(4.40)93.30Ma/611.0m93.96–94.21(0.25)9.51 Northeast uplift zone

D501576.0–762.5(186.5)739.0–756.0(17.0)91.80–94.27(2.47)93.38Ma/699.0m93.95–94.19(0.24)7.55

Central depression zone

C503687.0–1118.0(431.0)1090.0–1114.0(24.0)90.91–94.23(3.32)92.63Ma/1042.0m93.99–94.20(0.21)12.98

Z8051338.0–1685.0(347.0)1649.0–1674.0(25.0)91.23–94.31(3.08)93.83Ma/1630.0m93.98–94.21(0.23)11.3

M2061286.0–1782.8(496.8)1750.0–1775.0(25.0)89.07–94.27(5.20)93.32Ma/1700.8m93.87–94.18(0.31)9.55

G6921800.0–2344.0(544.0)2305.0–2332.0(27.0)89.67–94.27(4.60)93.65Ma/2270.0m93.95–94.20(0.25)11.83

North plunge zone

L21449.0–1874.0(425.0)1840.0–1867.0(27.0)89.77–94.25(4.48)93.57Ma/1797.0m93.92–94.18(0.26)9.49

West slope zone

F64446.0–585.5(139.5)–(1.36)––10.26

L271647.5–759.5(112.0)–(1.25)––8.96

J32398.0–499.0(101.0)–(1.09)––9.27

320H.Wu et al./Earth and Planetary Science Letters278(2009)308–323

2008)in central Colorado and Italy,respectively.These estimations,particularly the onset age of 94.21Ma,are consistent with the onset age of ~94.21–94.18Ma for the LAE1in SLB (Figs.7and 8).The duration of LAE1and OAE2,however,needs further investigation.First,the ~210–310ka duration for LAE1is based on the occurrence of oil shales rather than a chemical anomaly (such as the positive δ13C org excursion)in K 2qn;therefore its duration may have been under-estimated.Second,existing estimations for the duration of OAE2vary signi ?cantly from ~320ka to ~900ka.On the basis of biostratigraphic data and stratigraphic interpolation,Arthur et al.(1988)and Caron et al.(1999)estimated the duration of OAE2as 0.5–0.8Ma and 0.4Ma,respectively.Estimations based on orbital cyclicity include 563–601ka or 847–885ka in central Colorado (Sageman et al.,2006),320ka in western Canada (Prokoph et al.,2001),and 440ka in the Tarfaya Basin in Morocco (Kuhnt et al.,2005).Nonetheless,the ~210–310ka duration of LAE1is,in ?rst order,comparable with the ~320–900ka duration of OAE2,although apparently the duration of both LAE1and OAE2needs to be re ?ned.Widespread oceanic anoxia at OAE2has been proposed to have resulted from the large-scale igneous activity that may have released large quantities of CO 2to the atmosphere and/or reduced hydro-thermal ?uids to the ocean water column (Sinton and Duncan,1997;Kerr,1998;Turgeon and Creaser,2008).The coincidence of an anoxic (high organic carbon burial)event in both marine and lacustrine environments suggest the involvement of atmospheric CO 2.Elevated CO 2level may have enhanced the primary productivity in both marine and terrestrial systems (e.g.,Schlanger and Jenkyns,1976;Schlanger et al.,1987),leading to water column oxygen de ?ciency and increased organic carbon burial.Mitchell et al.(2008)proposed that the Cretaceous OAEs including OAE2may be astronomically driven,resulting from reduced seasonality during protracted period of low insolation variation.The same onset age of ~94.21–94.18Ma and existence of similar nodes of procession,obliquity and eccentricity around the LAE1(Fig.7a)and OAE2(Mitchell et al.,2008)support this hypothesis.However,it is unclear whether such nodes were resulted from reduced seasonality at elevated atmospheric CO 2or directly

from

https://www.360docs.net/doc/909542160.html,parison of sedimentation rates of the K 2qn across the tectonic zones of the Songliao basin.Four periods of high sedimentary rates can be identi ?ed (gray markers)and they may record periods of sustained wet climate during which enhanced chemical weathering increased sediment inputs in the basin.

321

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the in?uences of long-term insolation change.The time range of the data is insuf?cient to evaluate the response of other anoxic events in SLB and therefore the long-term“oceanic anoxic cycles”proposed by Mitchell et al.(2008).

6.Conclusions

Wavelet and spectral analyses on the natural gamma-ray logging of 10wells of the Late Cretaceous lacustrine Qingshankou Formation(K2qn) in the Songliao Basin reveal Milankovitch cycles of precession(20ka), obliquity(40ka)and eccentricity(100ka and405ka).The sedimentary cyclicity may have been formed by cyclic changes from wet to dry climate in response to astronomical forcing.The number of cycles decreases from the central depression zone to the margin of the basin,possibly resulting from non-deposition and/or erosion associated with stratigraphic discontinuities towards the basin margin and paleogeographic highs.

Floating astronomical time scales(ATS)for the sections in the central parts of basin were established by matching the extracted short eccentricity signals to the La2004solution.The accumulation time of K2qn varies from1.09Ma to5.20Ma across the basin.The longest ATS constructed from section M206in the central depression zone constrains the age of the K2qn as from Late Cenomanian to Early Coniacian(94.27–89.07Ma).The ATS also provides time constraints for the ostracod biozones,which may be useful for future stratigraphic correlation among ostracod-bearing terrestrial successions and potentially,between terres-trial and marine successions.The mean sedimentation rates range from 7.55cm/ka to12.98cm/ka.Four periods of high sedimentation rates were detected across the basin,possibly recording periods of sustained wet climate during which enhanced chemical weathering may have signi?cantly increased sediment inputs to the Songliao basin.

Based on the established ATS,the duration of the lacustrine anoxic event1(LAE1)at the basal Qingshankou Formation is estimated as 210–310ka.The onset age of LAE1(~94.21–94.18Ma)is comparable with that of the oceanic anoxic event2(OAE2)at the Cenomanian–Turonian boundary,which has been estimated as93.72/94.21Ma or 94.09Ma.The duration of LAE1(~210–310ka)and OAE2(~320–900ka)is in the?rst order agreement,but further investigations are needed to better constrain the duration of the anoxic events in both marine and terrestrial basins.The occurrence of a time-equivalent anoxic(high organic carbon burial)event in both terrestrial and marine basins suggest a shared trigger mechanism that involves increased atmospheric CO2from igneous activities.Elevated CO2may have facilitated primary productivity and anoxic water column chemistry in both marine and terrestrial basins.

Acknowledgements

The authors are grateful for the help from Chengshan Wang, Xiaoqiao Wan,Pujun Wang,Yanguang Ren and Yimin Dang.We express our sincere appreciation to the journal editor(Prof.Peter B. deMenocal)and four anonymous reviewers for their careful review and constructive suggestions that signi?cantly improved the paper. This study was jointly supported by the National Key Basic Research Development Program of China(Grant2006CB701400),the National Science Foundation of China(Grant40802012),“111project”(B07011) and Program for New Century Excellent Talents(Grant NCET-04-0727) of Ministry of Education of China.

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