Third-order Middle Miocene-Early Pliocene depositional sequences in the prograding delta complex of the Pannonian Basin

TECTONOPHYSICS ELSEVIER

Tectonophysics

240 (1994) 81- 106

Third-order Middle Miocene-Early Pliocene depositional sequences in the prograding delta complex of the Pannonian Basin G. Vakarcs a,b, P.R. Vail ‘, G. Tari ‘, Gy. Pog6cs6s b, R.E. Mattick ‘, A. Szab6 h ” Rice lJnicersi@, Dept. of Gedogy and Geophysics, Houston. Texas 77251- 1892, l&4 ’ MOL. RT Exploration Department, H-10.39 Budapest, Hwzgcrry

’ U.S. Geological Surrqv, Reston, VA 22092. USA Received

3 December

1992: revision

accepted

I I March 1994

Abstract Few studies exist in the geologic literature that show the distribution of seismic facies and depositional scyuences within a lacustrine basin. The Pannonian Basin of Central Europe offers a unique opportunity to evaluate the influence of the eustatic signai on lacustrine deposition. Seismic stratigraphic and sedimentological studies indicate that the Middle Miocene-Early Pliocene infill of the transtcnsional Pannonian Basin was formed by large delta systems. Systematic sequence stratigraphic analysis of 6000 km of reflection seismic data and more than 100 hydrocarbon exploration wells in Hungary allowed the identification of twelve third-order sequence boundaries in the late Neogene sedimentary fill. This number of depositional sequences corresponds to that of the published global eustatic curve for this time period. Furthcrmorc, based on magnetostratig~phic and radiometric data. the ages of these depositional sequences can bc tcntativcly correlated with the global eustatic curve. The Pannonian Basin became isolated from the world sea at the Sarmatian/Pannonian (11.5 Ma) boundary and formed a large lake. The strata1 patterns and sedimentary facies of individual systems tracts within the lacustrinc sequences display the same characteristics as marine depositional sequences. The relatively low rate of thermal subsidence and the high rate of sediment supply resulted in a good sequence resolution. Within the third-order sequences higher-order sequences can be recognized with an average duration of about 0.145 Ma.

1. Introduction

During the last decade significant scientific progress has been made in understanding the Neogene sedimentary fill of the Pannonian Basin. Although the stratigraphy of these sediments is well known, their formation and sequence stratigraphic framework is much less understood. The study area is located in Hungary covering the Central Pannonian Basin. The Pannonian 0040-195 l/94/$07.00 0 1994 Elsevier SSDI 0~~0-195~~94~00101-~

Science

Basin, encircled by the Alps, the Carpathians and the Dinarides, is a typical back-arc depression resulting from the collision of the African promontory, Apulia and the European plate (Fig. 1). The thickness of the Neogene-Quaternary basin fill exceeds 7 km in the deepest subbasins. K&ma&y et al. (1981), Pogacsas (1984, 1985) and Marton (1985) carried out the initial seismic stratigraphic interpretati~~ns of the Late Mio-

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Fig. 1. Tectanic sketch of the Pannonian back-arc basin (Tari, 1993). ‘I’be Pannonian Basin is a Mediterranean type back-arc basin. However, in this basin, the extension did not advance sufficiently to lead to the opening of an oceanic basin (Pannonian type basin, Bally and Snelson. 1980). The inversion of the basin haa already begun (Tari, 1993) and the neotectonic activity can be characterized by uplift of the pre-Tertiary basement rocks in western and northern II:mgary during the Quaternary.

G. Vakarcs et al. / Tectonophysics 240 (1994) 81-106

cene-Pliocene succession of the basin. These early works were based on the seismic stratigraphic principles of Vail et al. (1974, 19771, Mitchurn et al. (1977), Mitchum and Vail (1977) and Sangree and Widmier (1977). More recent seismic stratigraphic (Mattick et al., 1985, 1988, 19941, sedimentological (Berczi and Phillips, 1985; Bkrczi et al., 1988; BCrczi, 1988; Szalay and Szentgyorgyi, 1988; Juhasz et al., 1989; R&&z et al., 1989; Szentgyorgyi, 1989; Juhasz, 1991) and biostratigraphical (Korecz, 1985; Magyar, 1988, 1991; Miller and Ma~ar. 1992a,b; KorpLs-Hodi and Pogacds, 1992) studies of the Pannonian succession in eastern Hungary resulted in a model in which the infill of the Pannonian Basin involved the advance of deltas from the northwestern, northern, northeastern and southeastern basin margins (Fig. 2). According to these authors, deltas developed in two stages. During the first stage deltas prograded into a 800-900”m-deep lake. Progradation in a shallower water (200-400 m) characterized the second stage. Based on seismic and electro-facies patterns, Pogacsas et al. (1988) subdivided the Pannonian succession into basal transgressional, aggradational deep-water, progradat~onal delta and aggradational alluvial sedimentary units. Subsequent integrated interpretation of magnetostratigraphic (Hamor et al., 1980, 1985; Elston et al., 1985, 1990) and reflection seismic data (Horvath and Pogicsas, 1988; Pogacsas et al., 1988, 1989, 1992a,b, 1994; Pogacsas and Seifert, 1991) showed four non-depositional hiatuses near the northern margin of the basin. Based on seismic and well-log analysis, Vdrnai and Vakarcs (1990), Vakarcs and Varnai (1991) and Varnai (1992) constructed detailed models of the sedimentary facies, hydrocarbon geology and paleogeography of eastern Hungary. Using the sequence stratigraphic method of Vail (19871, Vakarcs and Varnai (1991) identified four major and sixteen minor depositional sequences. On the basis of magnetostratigraphic data, the four major sequence boundaries were tentatively correlated with the Haq et al. (1987) eustatic curve (i.e., 10.5, 8.2. 6.3, 5.5-Ma-old sequence boundaries). The major depositional sequences of the Pan-

83

nonian Basin are third-order sequences (Tari et al., 1992a). These third-order sequences can be subdivided further into systems tracts defined by strata1 patterns (Tari et al., 1992a1. The building blocks of third-order systems tracts are fourthorder sequences and/ or parasequences. Using well-log data higher-order parasequences can be found within this framework (Vakarcs et al., 1993). In a recent synthesis of the Late MiocenePliocene sequence stratigraphy of the Pannonian Basin, Vakarcs et al. (1992, 19931, Vakarcs (19931, V~rkonyi et al., (1993) and Ujsz&zi and Vakarcs (1993) further developed the model of Pogacsas et al. (19881, Vakarcs and Varnai (1991) and Tari et al. (1992a,b). They described seven third-order depositional sequences that were correlated within the resolution of available chronostratigraphic data with the eustatic curve of Haq et al. (1987). In this study mainly the third-order sequence boundaries of the prograding delta complex are considered. The most important objective of this paper is to establish a detailed sequence stratigraphic framework, including third- and fourthorder sequence and systems tracts identification and to verify that the method of sequence stratigraphy can be appbed to an isolated, partialiy lacustrine basin. Generally, the investigation is concentrated on the prograding stage of the basin infill, but a short overview of the deep-water sequences is also presented. The second goal was to correlate the isolated brackish-lacustrine depositional sequences with their marine counterparts, within the resolution of available chronostratigraphic data.

2. Geologic setting The Pannonian Basin is an integral part of the Alpine mountain belts of eastern Central Europe and overlies the Mesozoic thrust sheets of the Eastern Alps, the Carpathians and the Dinarides. The Neogene (Middle Miocene to Recent) Pannonian Basin proper is superimposed on an earlier Paleogene (Middle Eocene-Early Miocene) basin complex as a result of back-arc extension (Tari et al., 1992b; Tari, 1993). During the

G. Vakarcs et al. / Tectonophysics NO (1994) RI-106

Miocene-Pliocene, the Pannonian Basin was one of several Mediterranean back-arc basins (Horvath and Royden, 1981; HorvBth and Berckhemer, 1982; Royden et al., 1982; Royden, 1988; Royden and Burchfiel, 1989) associated with continental collision. It is located on the concave side of an A-subduction arc (Pannonian type basins, Bally and Snelson, 1980). Gravitational collapse of the intra-Carpathian domain (Fig. 1) combined with subduction zone roll-back is thought to be the driving mechanism of the Neogene back-arc extension (Tari et al., 199213). In the Pannonian Basin the back-arc extension did not advance to the opening of an oceanic basin, in contrast to the Western Mediterranean basins (Bally and Snelson, 1980). The post-rift syn-sedimentary tectonics in the basin was minimal. A late-stage compressional period (in the sense of Cloetingh et al., 1989b) in the Late Pliocene enhanced subsidence in the deep basin areas and resulted in uplift of local highs. In the young post-rift sediments neotectonic compressional faults can be observed. These indicate that the inversion of the basin has already begun (Tari, 1993). Neotectonic activity can also he characterized by uplift of pre-Tertiary basement rocks in western and northern Hungary during the Quaternary (Figs. I, 2).

3. Data set and methodology The area of this study includes the eastern, middle and southwestern part of Hungary. A number of data sets were used in this sequence stratigraphic work. 6000 km multifold seismic sections were correlated with more than 100 hydrocarbon exploration wells. Third- and fourth-order depositional sequences were identified on these sections. In this study, magnetostratigraphic ages were available from four continuously cored wells (Figs. 2 and 12). Within these wells, many narrow intervals of reversed and normal polarity (“extra” reversals), excursions, transitions and intervals of mixed polarity are present in the sections (Elston et al., 1985, 1990; Lantos et al., 1994). These zones were correlated with the geomagnetic po-

85

larity time scale (Berggren et al., 1985) by means of lithostratigraphic. biostratigraphic and seismic stratigraphic time markers (Fig. 3) and K/Ar data (Lantos et al., 1994). Volcanic horizons are known within the Pannonian Basin: ignimbrites and pyroclastics of rhyelite composition in the eastern part (I 1-12 Ma). submarine basalts in the middle and southern parts (7-10 Ma), Na-alkaline basalt tuffs and lavas in Transdanubia (2.8-5.4 Ma) (Poka, 1988). The sequence stratigraphic model relied on the methodology and terminology of Vail et al. (19771, Vail (19871, Van Wagoner et al. (1987). Posamentier and Vail (1988) and Vail et al. (lY91). Well-log sequence stratigr~~phy and detailed systems tracts interpretation are based on the techniques described by Vail and Wornardt (IYYO), Van Wagoner et al. (1990). Mitchum and Van Wagoner (1990) and Posamentier et al. (1092). The most detailed correlation concentrated on the eastern part of the country: thus, the seismic and well-log examples presented in this papei were chosen from the eastern region.

4. Sequences

and systems tracts

A fully developed theoretical depositional sequence is described below, including the most important lithologic, sedimentologic, well-log and seismic characteristics of systems tracts. Not all the systems tracts are developed in each sequence. All the interpreted Neogcnc sequences in the Pannonian Basin are Type-l sequences. related to erosional truncation (Figs. 7-10). The seismic facies are summarized in Fig. 4.

The LST consist of three successive depositional units-basin floor fan, slope fan, prograding complex-which are not coeval (Vail, lY87). The &sin floor fan (bff) was deposited during early relative water-level fall. On well-logs the bff has blocky, sharp contacts with the other units (Fig. 5). On seismic profiles the bff is most commonly characterized by one relatively high-amplitude and continuous reflection (Figs. 6, 13a, 13b

BESSARABIAN __~__--___e__-------~

Fig. 3, Neogene biostratigraphy and tentative correlation of the Central Paratethys (Pannonian Basinj concept @U&l et al., 1984%

Fig. 3. Characteristic

WEDGE

SHEET

WEDGE

WEDGE MOUND

SHEET

wismk

MODERATE

HIGH

HIGH

MODERATE

LOW

MODERATE HIGH

MODERATE

FAIR /LOW

HIGH

CONTINUITY

units recognized in this study.

FAIR MODERATE

HIGH

HIGH

MODERATE HIGH

MODERATE

HIGH

facies of depositional

SHINGLED CLINOFORM

PARALLEL

PARALLEL

CLINOFORM

CHAOTIC

SUBPARALLEL

LOW MODERATE

SUBPARALLEL DIVERGENT

WEDGE

HIGHSTAND SYSTEMS TRACTS

TURBIDITES

DEEP-WATER

wEDGE

PROGRADING

SLUMPS

DISTAL TURBIDITES

LEVEE

HIGH

PARALLEL HUMMOCKY

LENS SHEET

SHEET

PROGRADING COMPLEX

FAN

SLOPE

CHANNEL

HIGH

PARALLEL MOUNDED

SHEET MOUNDED

AMPLITUDE

‘soT;yL

zEMRNAL

TRANSGRESSIVE SYSTEMS TRACTS

r 2

z

8

:

ti

3

z

r 0

FAN

BASIN FLOOR

DEPOSITIONAL UNITS

WIDESPREAD

WIDESPREAD

WIDESPREAD

WIDESPREAD

WIDESPREAD

DOWNDIP

DIPPING FROM THE CHANNEL

NAROOW LINEAR

WIDESPREAD FAN

LATERAL DISTRIBUTION

DOWNLAP

CONCORDAN

CONCORDAN

ONLAP LANDWARD

EROSIVE

CONCORDANT DOWNLAP

CONCORDANT DOWNLAP

EROSIVE CONCORDAN

CONCORDAN ON SEQUENCI

LOWER BOUNDARY

3

XX

CT. Vakarcs et al. / Tectonophysics 240 (1994) 81-106

.

i

5

rc7/5 #7/4 #7/4 #713 #f/2

Xc614

#613

Fig. 5.

G. Vakarcs et al. / Tectonophysics 240 (1994) 81-106

XY

LEGEAID SEOUENCE

--___... --

BOUNDARY

TOP OFTRANSGRESSIVE TOP OF LOWSTAND

SYSTEMS

TOP OF BASIN FLOOR TOP OF SLOPE

#8

#8/l

6.3

SYSTEMSTRACT

HIGHSTAND

B

TRANSGRESSIVE .

TRACT

FAN

FAN

NUMBER

OF THIRD-ORDER

NUMBER

OF FOUMKORDER

AGE OF THIRDORDER IN MILLION

m

.

LOWSTANO

SYSTEMS

SYSTEMS

E

BASIN FLOOR

f.ggg

SLOPE FAN

l3!@

LOWSTAND

SEQUENCE SEOUENCE

SEQUENCE

TRACTS

SYSTEMSTRACT TRACT

FAN

PROGRADING

COMPLEX

BOUNDARY BOUNDARY

BOUNDARY

YEARS (HAQ ET AL.. 1987)

Fig. 5. Well-log example showing continuous deposition from alluvial, later shallow-water (#l. 17.5) to deep-water t#2, 16.5) (middle part of fourth-order sequence #h/5) and later to neritic environments (middle part of fourth-order sequence #h/S--top of the well). Location of well-log is shown in Fig. 2. See seismic interpretation in Fig. 6. Based on its petrologic character the rhyolite tuff within the upper part of sequence #l (17.5) suggests a strong relationship with the Tokaj Mountain (north Hungary, see location in Fig. 1) Tar Rhyolite Tuff Formation. The radi~}metric age of this formatjon is 16.4 (i-b.8 Mat. Note that in the deep-basin facies (sequences #2 (16.5)-middle part of fourth-order sequence #h/S). the sequence boundary is indicated by the sandstone bodies of the LST overlying several tens of meters of TST and HST clays.

and 14). The slope j&r (sfl was deposited during late reIative water-level fail. The slope fan consists of channel levee lobes (l), distal turbidites (2) and chaotic slumps (3). On well-logs they are characterized by rounded and crescent (I), blocky and rounded (2) or mainly shale-rich (3) well-log patterns (Fig. 5). On seismic profiles the most common reflection patterns are parallel, high-amplitude and subparallel, low- to moderate-amplitude f 11, parallel, high-amplitude (2) and chaotic (3) (Figs. 10 and 14). The lowstand progra&g complex (Ipc) was deposited during early relative water-level rise. From the base to the top, on well-logs, the lower part of the prograding complex (PC> is characterized by several, irregular blocky sandstone bodies (deep-water turbidites), thick shale beds (delta slope faciesl and inverted “Christmas tree”-shaped sandstone bodies (delta front) becoming thicker and coarser upward (Fig. 5). On seismic profiles it has a strata1 pattern of moderate to high reflection continuity and amplitude and prograding (sigmoid and oblique) clinoform bedding (Figs. 10 and 15).

4.2. Transgressirle systems tracts (TST) The transgressive systems tract was formed during the maximum rate of water-level rise. In neritic environments, the TST is characterized by upward-fining depositional units. On well-log the TST shows a “Christmas tree”-shaped pattern, in which upward-thinning and upward-fining inverted “Christmas tree”-shaped sandstone bodies are repeated (Fig. 51. On seismic profiles. if this systems tract is thin, it is characterized by one continuous high-amplitude reflection. If it is thicker, an apparent truncation pattern is typical (Figs. 9, 10 and 14). In deeper-water environments condensed sections are formed. On weh-log curves this unit is represented by the highest shale content (Fig. 5); on seismic sections it is characterized by one widespread high-amplitude reflection (Figs. 7 and 141. 4.3. Hightand systems tracts (HST) The highstand systems tract was deposited during the late relative water-level rise. On well-

40

G. Vakarrs et al. / Tectonophysics

240 (1994) 81-106

2.5

3

3.5

2.5

3.5

-

-- ------#8 8.3

THIRL)-ORDERBWNQARY TOP OF TF?mE SY’STWS TRACT TOP OF LOWSTAM SYST’WS TRACT FOW?7H=O~+?~.W~ARY TOP OF TRB SYTRACT TOPOF LCWS’WWS~S’FWBTRACT FOURTltORIXR SHWENCE:SGWMRY BiuMBmoF SEwmcE BQUMMRY AGE OF THR&ORW!I SEQW#& @OWQARY IN MUUQN YEARS WkQ ET AL.. l!I87I

Fig. 6. Uninterpreted (a) and interpreted (b) seismic example of deep-water sequences. Location of profile is shown in Fig. 2. See well-log interpretation in Fig. 5. In the deep basins sequence boundaries are overlain by turbidites pinching out towards the basin margins at the boundary of the preceding sequence. The clays of the TST and HST cover LST units. a situation that probably occurs in the basin margins as well.

91

1

LEGEND - - - -#8

THIRD-ORDER SEQUENCE BOUNDARY TOP OF TRANSGRESSIVE SYSTEMS TRACT TOP OF LOWSTAND SYSTEMS TRACT NUMBER OF SEQUENCE BOUNDARY

6.3 e

AGE OF THIRD-ORDER SEQUENCE BOUNDARY IN MILLION YEARS (HAQ ET AL., 1987) REFLECTION TERMINATION

Fig. 7. Uninterpreted (a) and interpreted (b) seismic sections. Location of profile is shown in Fig. 2. This seismic line shows the initial progradation of depositional sequence #5 (12.5). Geometry of the clinoform reflections indicates that the initial deltas prograded into a shallow water (100-200 m). On the northeastern end of this section a huge angular un~~~nformity can be predicted. Based on the identified truncations (see arrows), from sequence #5 (12.5) minimum 1000 m thick sediments were eroded due to the Quaternary uplift. A volcanic intrusion can be identified on the southwestern end of the section. near I s. Based on K/Ar data the age of this vulcanite is 11.1 + 0.7 Ma (from Well-7, SzCky-Fux and Kozak. 1985; see well location in Fig. 2). The grey bold line represents the seismically interpreted age of this intrusion. Above this line the younger strata have the same thickness and a normal deposition. Below this line the older strata were deformed due to the intrusion. Therefore, the age of the volcanic intrusion has to be younger than the age of sequence #S (123, but older than sequence #b (10.5). This date approximately confirms the correlating age of the Haq et al. (19871 curve (12.5 and 10.5 Ma).

Fig. 8. ~nj~te~~eted (a) and interpreted (b) seismic profile showing sequence boundary #6 IlO.5). Location of pro% is shown in Fig. 2. Strong erosional truncation characterizes the sequence boundary. Progradation of the Iowstand prograding wedge. from the northeast continued in gradually deepening water (200-400) m.

G. Vakarcs rl al. / Tectomphysics

240 (1994) NJ-106

93

5. Age of sequences

logs, the HST consists of gradually thickening and coarsening upward sandstone bodies (Fig. 5). On seismic section parallel to slightly divergent, moderate-amplitude reflections are typical (Figs. 9, 10 and 14).

Twelve sequence boundaries labeled #l through #12 were identified on seismic sections and well-logs and were correlated with age data.

2.5

b.

SW

2

2.5

LEGEND -

I

- - ---

THiRD-ORDER SEQUENCE BOUNDARY TOP OF TRANSGRESSIVE SYSTEMS TRACTS TOP OF LOWSTAND SYSTEMS TRACTS FOURTH-ORDER SEQUENCE BOUNDARY

Fig. 0. Uninterpreted

(a) and interpreted

sequence #9 (5.5). Location one high-amplitude highstand relatively

deposits deeper

time perrod.

As a result of the dramatic

occurred

totally

water

AGE OF THIRD-ORDER SEQUENCE BOUNDARY IN MILLION YEARS (HAQ ET AL., 1987) REFLECTION TERMINATION NUMBER OF SEQUENCE BOUNDARY

c-#B

showing the final progradational

of profile is shown in Fig. 2. In this example

reflection.

(6.31, delta progradation

(b) seismic profile

6.3

infilled

in shallower

water

the shelf region

change in the sedimentary

(200-400

environment

m) again. For the first time during

and the final deltas

offers a good possibility to seismically

third-order

identify

sequence

the transgresaive systems tract is characterized

prograded

fourth-order

directly

after sequence

boundary

the delta sedimentation,

into the basin.

sequences

boundary. by just only

formed

during

#X the

Pr[~grad~lti(~n into a the late highstand

I'

2

0

0

1

1

a

2

G. Vakarcs et al. / Tectonophysics 240 (1994) 81-106

Unfortunately biostratigraphic data were not available to determine ages of these sequences because the Pannonian Basin became gradually isoiated from the world sea at the Sarmatian/ Pannonian boundary. From the Pannonian the isolation was complete (Steininger et al., 1988). The identified sequence boundaries were correlated with four magnetostratigraphically calibrated key-wells (Figs. 2, 10, 11 and 121, with three wells in which K/Ar dates were available (Figs. 2 and 7) and with one well in which a regionally known tuff horizon was identified (Fig. 5). In the case of four sequence boundaries 1#7, 8.2; #8, 6.3; #IO, 4.2; and #ll, 3.8) excellent correlation was found between the magnetostratigraphic age of the sequences and the ages of the boundaries shown in the Haq et al. (1987) curve (Figs. 10, 11 and 12). Correlation of sequences # 1 (17.5) (Fig. 5), #5 (12.5) (Fig. 7), #6 (IO.51 (normal deposition above a 12.1 k 0.4 Ma volcanic layer in Well-7 (SzCky-Fux and Kozak, 1985) and volcanic layer (9.61 Ma) above the sequence boundary in Well-6 (Pogacsas et al., 1987)) and #I2 (3.0) (Fig. 12) are only approximate. Unfortunately, a few data were available for sequences #2 (16.51, #3 (X5.5), #4 (13.8) and #9 (5.51, therefore we adopted the age of the Haq et al. (1987) curve. These results suggest that the water-level fluctuation in the Pannonian Lake presumably reflects the eustatic change of the world sea level during this period of time. Presently we do not have enough chronological data to further refine the Haq et al. curve (19871, therefore we accept

‘)s

the ages of this curve in the discussion that follows. 6. Discussion Based on the interpretation of seismic and well-log profiles from the Neogene sedimentary fill of the Pannonian Basin, several depositional sequences can be identified. The major unconformities between these sequences correspond to third-order sequence boundaries (Tari et al., 1992b). The overall progradation of delta systems into the deep isolated basin displays a geometry similar to a shelf margin setting (Vakarcs et al., 1992) as described by Vail (1987). The lacustrine depositional sequences of the Pannonian Basin show the same strata1 patterns as the marine ones. All the third-order sequences consist of a lowstand (LST), transgressive (TST) and highstand systems tracts (HST). The individual third-order depositional sequences are characterized by slightly different proportions between the systems tracts. However, in the case of any given sequence, the particular geometry can be traced throughout the study area. The common elements of the third-order sequences arc the anomalously thick lowstand prograding complex and relatively thin transgressive and highstand deposits. This feature is believed to result from a very large shelf region (100-200 km wide) compared to the width of the lowstand prograding complex (20-50 km) (Vakarcs et al., 1992). The building blocks of the identified thirdorder sequences and systems tracts are higher-

Fig. 10. (a) Uninterpreted seismic section. See (b) for interpretation and discussion. Location of profile is shown in Fig. 2. (b) Interpreted dip-oriented seismic section, showing the typical pattern of third- and fourth-order sequences. Location of profile is shown in Fig. 2. In water depths of 600-1000 m, the thick lowstand systems tract of sequence #8 contains several fourth-order sequences. These higher-order sequences show the same characteristics as the third-order sequences. The fourth-order lowstand systems tracts are well developed and consist of basin floor fan, slope fan and prograding complex. The basin floor fan, however, sometimes is missing. The typical late lowstand aggradation and back-stepping series of TST also can be seen in this section. The third-order transgressive systems tracts are characterized by two backstepped reflection. The shallow-water progradation of the third-order highstand systems tracts is clearly identified by the slightly deepening shingled reflections. One of the most important magnetostratigraphic key wells (Well-4) is located at the northeastern end of the section. In this case, the correlation between the magnetostratigraphic ages of sequences #7 and #8 and the ages of the Haq et al. (1987) curve (8.2, 6.3) is excellent. (See Fig. 11 for well-log and magnetostratigraphic interpretation and correlation.)

-.

.*

.

.

”

m in Fig. 2. See seismic section (Fig. 10) and text for explanation. Note, that in this case excellent correlation was achieve4 between the ~a~n~tostrati~a~~i~ age of the sequences and those bomdary ages shown in the Haq et al. (1987) curve. __I_...

.

.

8

7

6

-5

4

2

,”

c3

-

3 0

4A

4

2A

_

_

detailed

-.

.=

_

I

/

,-

.-

--

._-

chart between

-.

*

legend in Fig. 5.

Fig. 12. Correlation

-

8

-

7

6 -

5 -

3 -

-

2

a

_ _,,

,

jlj

the magnltostratigraphic

.-

4.2

3.8

3.I

Q.3

0.7

E. i 5

-

-\

^%a

,_

.a

._

I_ I.

--

---

,,,

_

,//

HIRD-ORDEI -

-

-

-

-1

--

-_\_

et at.. 19941 and the identified

4.9

“&a

-R4

-6.7

IE 500

-6.5

-6.0 ,-

A

k iti

z

1 WELL-1 -

data (Haq rt al.. 1987: I.antos

_-*_....”

---.--

--I--

-.-a.-_

--.--

.$

it a/-

,I2 3.0

WELL-3

.YR

sequence

,lllL

1

and systems tracts boundaries.

9.9

8.2

,7.9

,7*4

6.7

6.3

A

G

z

z-

WELL-2 -

See

G. Vakarcs et al. / Tectorrophysics 240 (1994) 81-106

a.

b.

0

1

2

3

4

_. .._

CT

= .._

WI

us 6.3 -._. -

FWJLT REFLECTION

Fig. 13. (a) Uninterpreted seismic section. See (b) for interpretation. Location of profile is shown in Fig. 2. (b) Neogene sequence stratigraphic model of the Pannonian Basin. Location of profile is shown in Fig, 2. See detailed legend of the profile in (c); see text for explanation. (c) Detailed legend for (a) and (b), and Figs. 14 and 15.

99

I

1 b

*

I&E

NNEI *

Fig. 14. (a) Uninterpreted seismic section. See (b) for jnter~retatjon. Location of profile is shown in Figs. 5 and 6. (bl Neogene sequence stratigraphic model of the Pannonian Basin. L.ocation of profile is shown in Fig. 2. See detailed legend of the profile in Fig. 13~: see text for explanation.

Fig. 1.5. (a) Unisterpreted seismic section. See (b) for interpretation. Location of profile is shown in Fig. 2. (b) Neogene sequence stratigraphic model of the Pannonian Basin. Location of profile is shown in Fig. 2. See detailed legend of the profile in Fig. 13~; see text for explanation.

G. Vakarcs et al. / Tectonophysics

I b. 1 -I s

Fig. 13

BEKES

Fig. 14

I

Fig. 15

I

DERECSKE

BASIN

101

240 (1994) 81-106

I

TROUGH N

cl

1 2 3 4 5

#8

6

LEGEND

NUMBER OF SEOUENCE SOUNOARY AGE OF THIRD-ORDER SEQUENCE BOUNDARY 6.3 IN MILLION YEARS (HA0 ET AL., 1987,

-7

THIRD-ORDER SEO”ENCE SO”NOARY FOURTH-ORDER SEQUENCE BOUNDARY --EXTRAPOLATED SEWENCESOUNOARY FAULT + REFLECTION TERMINATION

i

BEKES

BASIN

DERECSKE

TROUGH

1

N

0 1

UPLIFT

Fig. 16. Sequence stratigraphic model of the Pannonian Basin. Location of profile is shown in Fig. 2a. Uninterpreted seismic section. includes Figs. 13, 14 and 15b. Depth converted line-drawing showing only the interpreted third- and fourth-order sequence boundar-ies. The enhanced subsidence of deep basin areas (e.g. Cloetingh et al., 1985, 1989b. 1992) may represent the effect of the intraplate stress coupled with uplift in northern Hungary. During deposition of sequences #h (10.5) and #8 (6.3). the deep (5.5 km) Derecske Trough was filled. The southern part of the section (BCkCs Basin, 6.5 km deep) remained unfilled during this period. Late Pliocene to Recent late-stage compression caused the uplift of the Derecske Trough (see eroGon) and subsidence of the Bekcs Basin. This process is still going on.

I(12

G Vakarcs et al. / Tecmnophysics 240 (1994) 81-106

order depositional sequences and/or parasequences (Tari et al., 1992b, 1993). Resolution seems to be on the order of 0.1-0.5 Ma. These higher-order sequences are interpreted as fourthorder sequences and believed to be associated with Milankovitch-type climatic fluctuations in the drainage areas of rivers flowing into the Pannonian Lake. The fourth-order depositional sequences mostly tend to be well developed in the lowstand prograding wedge, but rarely they also occur within the final stage of the highstand systems tracts Wakarcs et al., 1993) (Figs. IO, 1% 15). The fourth-order parasequences tend to be mainly developed in the transgress& systems tracts and in the early highstand systems tracts. Systematic sequence stratigraphic analysis of the eastern, middle and southwestern parts of Hungary allows identification of twelve thirdorder and nineteen fourth-order sequence boundaries in the Neogene sedimentary fill of the Pannonian Basin (Figs. 13a, 13b, 14, 1.5)Wakarcs et al., 1992; Vakarcs, 1993). The number of identified third-order depositional sequences corresponds to that of the published global eustatic curve for this period of time. Because the Pannonian Basin was gradually isofated from the world sea, reliable biostratigraphic age data were not available. Therefore, the ages of these scquences were determinated using magnetostratigraphic and radiometric data (Figs. 1O- 121. In the case of four sequence boundaries, the correlation is excellent, but for another four sequence boundaries it is only approximate. These results suggest that the water level in the isolated lacustrine Pannonian Basin fluctuated in the same phase as the global sea level (PogLcsPs et al., 1988, 3992b. 1994; Vakarcs and Vamai, 1991; Vakarcs et al., 1992; Vakarcs, 1993). Seismic stratigraphic analyses indicate that deltas prograded into shallow water during the first stage of delta progradation (from the upper part of sequence #4 (13.8) to the lower part of sequence #7 (8.2)). Following the total isolation from the global sea (Pannonian/ Sarmatian boundary, Steininger et al., 19881, the sedimentary basin gradually deepened from shallow water (200 m) to deeper water (800-1000 m). The isolation resulted in increased water levels, since the

lake was characterized by a positive hydrographic budget. At sequence boundary #8 (6.3), the water level decreased dramatically (approximately 21X) m water-level fall, Vakarcs et al., in prep.). The Mediterranean Sea experienced a large sea-level drop during the Messinian (Hsii et al., 1977) and a major part of the water mass of the Pannonian Lake might have been drained also (Csatci, 1993). Presently, one of the most important unsolved questions is the cause of the observed water-level fails. Two alternative models were proposed (Tari et al., 1992a). One possibility is that the thirdorder sequences reflect changes in intra-plate stress (e.g., Cloetingh et al.. I985 1989a. 1991). which were indeed very pronounced during the Neogene (e.g., Diivinyi and Hot-v&h, 1990). Alternatively, the third-order water-level variations affected the Pannonian Lake through a major river system as base levet changes (e.g., Posamentier and Vail, 1988). Tectonic activity was minimal during the postrift phase of the back-arc Pannonian Basin (HorvQth et al., 1986; Horvlth, 1990) and the rate of the thermal subsidence was relatively low (Horvrith et al., 1988). However, ongoing Neogene compression between the European and African plates resulted in a generaf compression in the Pannonian Basin (e.g., Csontos et al., 1991). Differential subsidence and uplift (Cloetingh ct al., 1985, 1989b, 19911, frequently seen on seismic sections in the Pannonian Basin, may represent the effect of changing intraplate stress. Finally we reiterate the point made by Tari et al. (1992a). The origin of the third-order depositional sequences in the Pannonian Basin remains an open problem. In order to have better and perhaps more discriminative constraints on the above-mentioned alternative explanations, further detailed sequence stratigraphic studies should be carried out in the entire intraCarpathian region.

7. Conclusions (1) The methodology of sequence stratigaphy, using reflection seismic and well-log data, in conjunction with magnetostratigraphy, sedimentology

G. Vakarcs et al. / Tectonophysics 240 (1994) 81-106

and radiometric age dating, can be applied to the lacustrine sedimentary strata of the Pannonian Basin. (2) Within the Pannonian Basin, Type-l depositional sequences and systems tracts can be identified. These lacustrine depositiona sequences show the same strata1 patterns as the marine ones. (3) The major unconformities between these depositional sequences correspond to third-order sequence boundaries. AlI the third-order depositional sequences consist of lowstand (LST), transgressive (TST) and highstand systems tracts (HST). The common elements of the third-order sequences are the anomalously thick lowstand prograding complex and the relatively thin transgressive and highstand deposits. (4) Twelve third-order sequence boundaries were identified in the Neogene sedimentary fill of the Pannonian Basin. The number and the age of these sequences correspond well to that of the published global eustatic curve for this period of time. (5) The building blocks of the identified third-order depositional sequences and systems tracts are fourth-order sequences and/or parasequences. The fourth-order sequences tend to be weI1 developed mainiy in the lowstand prograding wedge, but occasionally they also occur within the final stage of the highstand systems tracts. The fourth-order parasequences tend to be developed mostly in the transgressive systems tracts and in the early highstand systems tracts. (6) The interpreted sequence stratigraphic method is a valuable tool to recognize the effects of late-stage compression, such as differential subsidence and uplift in certain parts of the basins.

Acknowledgements The authors thank the leaders of MOL Hungarian Oil and Gas Company. Special thanks are extended to Istvan Szaloky, Bela Bard&z and Kgroly Molnar for their permission to publish the seismic data and interpretations of this paper. Thanks also to Ferenc Horvgth for his editorial

100

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