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Genesis of Late Cretaceous toe-of-slope breccias in the Bakony Mts, Hungary
ELSEVIER
Sedimentary Geology 128 (1999) 51–66
Genesis of Late Cretaceous toe-of-slope breccias in the Bakony Mts, Hungary J. Haas * Geological Research Group of the Hungarian Academy of Sciences, Eo¨tvo¨s Lora´nd University 1088, Budapest Mu´zeum krt. 4=A, Budapest, Hungary Received 15 December 1997; accepted 2 May 1999
Abstract Toe-of-slope megabreccia deposits of an Upper Cretaceous rudist platform were studied in the Bakony Mountains. During the Turonian–early Senonian tectogenesis an articulated basin came into being in the area of the Bakony; depressions and highs were formed roughly parallel with the structural strike of the mountains. Inundation of the highs led to the evolution of carbonate platforms. The investigated platform shows an asymmetric architecture. In contrast to its gentle northern slope a steep erosional slope bounded the platform to the south with large, low-angle aprons, the site of deposition of megabreccias. The breccia accumulation occurred within a single 3rd-order cycle commencing in the early highstand and reaching its climax in the late highstand. 1999 Elsevier Science B.V. All rights reserved. Keywords: Bakony Mountains; Upper Cretaceous; carbonate platform; slope; megabreccia; sea-level changes
1. Introduction Carbonate megabreccias accumulating on or at the toe of slopes of carbonate platforms may provide valuable information on the features of the eroding platforms, conditions prevailing on the slope and also in the site of deposition itself. However, determination of their genesis is not easy because various endogenic and exogenic processes may be responsible for the breccia formation. One of the most debated questions is the relationship between the megabreccia generation and relative sea-level changes, hence the sequence stratigraphic importance of the megabreccia formations.
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In the Bakony Mts of Hungary, due to Pre-Gosau tectogenesis and subsequent relative sea-level rise, elongated, asymmetric isolated carbonate platforms came into being. Along the toe of their steeper slope, megabreccia bodies of more than 100 m in thickness were formed. Based on detailed studies, mainly of cores, the aim of the present paper is to analyze those factors which may have played an important role in megabreccia formation and lithoclast deposition, with special regard to the relationship to possible sea-level changes.
2. Geologic setting and stratigraphy Senonian formations of hundreds of meters in thickness occur in the western part of the Transdanu-
0037-0738/99/$ – see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 7 - 0 7 3 8 ( 9 9 ) 0 0 0 6 1 - 5
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J. Haas / Sedimentary Geology 128 (1999) 51–66
TR. Gyõr
10
0
20 km
K
isa
lfö
ld PSB
?
od Ug gh H MSB
N. ony k Ba
TRASDANUBIAN RANGE UNIT
Budapest
e
ba Rá
Lin
ny ko a B
? ? Zalaegerszeg
?
Zala Basin
Veszprém
S. ?
Keszthely
e Lak
extension of Senonian platform carbonates
on
t Bala
Balaton Line
hemipelagic carbonates PSB
Pápa Sub-basin
MSB
Magyarpolány Sub-basin
Fig. 1. Extension of the Senonian and distribution of the Campanian formations in the Transdanubian Range Unit.
bian Range Unit, i.e. in the Bakony Mountains and in the basement of the Kisalfo¨ld (Small Plain) and the North Zala Basin in west Hungary (Fig. 1). In the early part of the Alpine evolutionary stage (from the late Paleozoic to the Late Cretaceous) the Transdanubian Range Unit was located between the Upper Austro–Alpine and the South Alpine realms (Haas et al., 1990). Multi-stage collision of the Adriatic microplate and other lithosphere fragments in the foreland of the European plate led to tectonic deformations in the Transdanubian Range during the Late Cretaceous. Late Cenomanian–Turonian collision (Pre-Gosau phase) resulted in uplift and intense erosion. This was followed by the formation of a large collapse basin in the early Senonian. The evolution of this basin came to an end in the Paleocene, due to the next collision event (Laramian phase). Between the two collision phases a typical tectonically controlled 2nd-order transgression–regression depositional cycle was formed in the basin (Haas, 1983). As a combined effect of tectonics and subaerial erosion, an articulated basin came into being by the Santonian with elongated highs and depressions be-
tween them, roughly parallel to the structural strike of the Transdanubian Range. The study area is located on the western side of the Northern Bakony (Fig. 2), representing the central part of the Senonian basin. The investigated breccia bodies were explored in the Magyarpola´ny Sub-basin, south of the Ugod High where a rudist platform had been developed in the Campanian. The depression north to the Ugod High is called the Pa´pa Sub-basin (see Fig. 1). The major lithostratigraphic units of the Senonian cycle and their stratigraphic position are shown in Fig. 3. Due to significant topographic differences which strongly influenced the paleogeographic setting in the early stage of basin evolution, the stratigraphic successions of the depressions and the highs were completely different (Haas, 1983). In the depressions the karstified Mesozoic bedrocks and locally bauxites upon them are covered by fluvial–lacustrine deposits (Csehba´nya Formation) or a limnic to parallic coal-bearing formation (Ajka Coal Formation). They are overlain by marls, rich in brackish water fossils (lower member of the Ja´ko´ Formation), which progress into marls and calcareous marls of neritic marine facies (upper member of the Ja´ko´ Forma-
J. Haas / Sedimentary Geology 128 (1999) 51–66 0
53
10 km
PÁPA Pa-2.
Ugod
Te Tfõ-4. vel Hi li Tapolcafõ Tfõ-4
Vinár-1.
Cel-1.
Cseh-13
Bakonyjákó Dabrony-1. Bj-26. Magyarpolány Mp-38. Dv-3.
Polány Fm . Ajka
Devecser Ukk-.1.
Ugod-Polány transitional unit. Ugod Fm.
Ukk-3.
GY-7.
Jákó Fm.
Gy-5.
Ajka Fm.
Sp-2. Csebánya Fm.
Sümeg
extension of the Ugod Fm below the Polány Fm. outcrops of megabreccias subsurface data on the megabreccia assumed extension of the megabreccia bodies
Fig. 2. Outcrops and subcrops of the Senonian formations and locality of the most important borehole sections in the Bakony area.
tion). They in turn are overlain by calcareous marls with Inoceramus and globotruncanids (lower member of the Pola´ny Marl Formation). The studied thick megabreccia bodies (Ja´ko´hegy Breccia Member — Nagy, 1957) are locally intercalated into this member. The upper member of the Pola´ny Marl Formation consists of silty marl with thin sandstone interlayers. The succession on the highs is simpler. The bedrocks or bauxites are overlain by rudist limestones (Ugod Limestone) which are followed by the Pola´ny Formation through a transitional interval. The chronostratigraphy of the Senonian sequences is based mainly on foraminifera, nannoplankton and sporomorphs (Go´cza´n, 1964; Siegl-Farkas, 1983;
Fe´legyha´zi, 1983; Bodrogi and Fogarasi, 1995; SieglFarkas and Wagreich, 1996; Bodrogi et al., 1998). According to the palynologic data terrestrial deposition began in the depressions during the Santonian. In our study area (Magyarpola´ny Sub-basin), the first nannofossil-bearing marine layers with Calculites obscurus appear in the lower member of the Ja´ko´ Formation. The first appearance of Bronsonia parca constricta indicates the Santonian=Campanian boundary in the lower part of the Pola´ny Formation within palynozone D. Ceratolithoides aculeus appears at the top of the Ja´ko´hegy Breccia. The upper part of the Pola´ny Marl can be characterized by Qadrum gothicum; thus it is also Campanian (A. Fogarasi, pers. commun.).
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J. Haas / Sedimentary Geology 128 (1999) 51–66
L IT H O ST R AT I G R A P HY STAGES
platform
basin My
78 Ganna Mb. 80
CAMPANIAN POLÁNY FM. JB
82
UGOD FM. 84 JÁKÓ FM.
SANTONIAN AJKA FM.
86
88
CONIACIAN ?
CSEHBÁNYA FM.
bauxite
bauxite PRE-SENONIAN FORMATIONS
Fig. 3. Lithostratigraphic chart of the Senonian in the Transdanubian Range.
In the Pa´pa Sub-basin palynological data are available (F. Go´cza´n, in Haas, 1981). These data constrain synchronous marine flooding in the two sub-basins, in the palyno-zone D that is in the late Santonian near the Santonian=Campanian boundary. The inundation of the Ugod High may have begun later, in palyno-zone E (F. Go´cza´n, in Haas, 1981), which corresponds to the lower member of the Pola´ny Formation in the depressions.
3. Evolution of the basin Based on the evaluation of outcrop and core data, the Senonian evolution of the study area is summarized below. By the early Senonian, a WSW–ENE-trending, elongated, asymmetric high had come into being, leading to separation of a southern and a northern sub-basin. The Ugod High may have been bounded by steep multiple faults to the south. In contrast, a gentle slope came into existence on the northern side of the Ugod High. The elevation of the top of the high above the bottom of the depression may have been about 120–150 m. Filling up of the basins began in the Santonian. In the initial evolutionary stage, a fluvial environment came into being in the northern sub-basin (Csehba´nya Formation) and flu-
vial, lacustrine and paludal deposition occurred in the southern one (Csehba´nya and Ajka Formations). This was followed by an abrupt facies change in the second evolutionary stage, at the end of the Santonian, which led to inundation and the establishment of shallow neritic brackish-water conditions in both sub-basins (lower member of the Ja´ko´ Marl). At this stage the difference in altitude of the top between the high and the bottom of the shallow sea may have been about 20–50 m. Subsequently the relative sea-level rise continued. This is clearly reflected in the fundamental change in the biota upsection in the Ja´ko´ Formation. It is highly probable that the rising sea-level reached the top of the Ugod High at the very end of the Santonian, when the transitional layers between the Ja´ko´ and the Pola´ny Formations were deposited in the basin. The definite trend of upward-increasing carbonate content within the Ja´ko´ Marl may indicate the establishment of the subtidal carbonate factory on the surrounding platforms. Following the inundation, the tectonically preformed, steep southern slope was transformed into a submarine erosional slope, whereas on the more gently dipping northern side an accretional slope came into being. At the toe of the steep slope megabreccias were formed, while on the low-angle slope bioclasts (mainly of platform origin) were accumulated.
J. Haas / Sedimentary Geology 128 (1999) 51–66
The subsequent transgression (i.e. the initial stage of the next 3rd-order cycle) led to a significant reduction in the extension of the platform. It is reflected in retrogradation and significant back-stepping of the facies zones on the northern slope, and in the cessation of the lithoclast and rudite to arenite-size bioclast accumulation (at least in the inner part) of the southern sub-basin. The next sea-level rise and the coeval increase of fine terrigenous influx may have caused the final drowning of the Ugod Platform and also of the other rudist platforms in the Senonian basin of the Transdanubian Range, in the middle part of the Campanian.
4. Basic features of the megabreccia deposits 4.1. General characteristics Along the southern slope of the Ugod High, in a belt at least 20 km long and at least 5 km wide,
55
megabreccia bodies were encountered in the lower member of the Pola´ny Marl Formation. Outcrops of megabreccias are known on Tevel Hill and on Ja´ko´ Hill west of Bakonyja´ko´ (see Fig. 2), and they were also encountered in some boreholes in the neighbourhood of Magyarpola´ny and further southwest in the core of well Devecser Dv-3 (see Figs. 3 and 4). The first breccia horizons appear about 20 m above the lower boundary of the formation. These are mud-supported breccias with scattered lithoclasts or plasticlasts. The frequency of the megabreccia interlayers increases upward leading to a predominance of lithoclasts upsection. This part of the Pola´ny Formation was defined as the Ja´ko´hegy Breccia Member (Go´cza´n et al., 1979). The thickness of the Ja´ko´hegy Breccia may exceed 100 m. The megabreccias are generally mud-supported in the lower part of the member and grain-supported in the upper part. The size of lithoclasts varies between mm to m scales, and they are unsorted as a rule. Bioclastic calcarenite–calcirudite intercalations also occur.
Fig. 4. Lithoclasts in muddy matrix in core Dv-3 (629.5 m).
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J. Haas / Sedimentary Geology 128 (1999) 51–66
4.2. Petrographic features of the studied section Core Mp-38 (for location see Fig. 2) is a key section of the megabreccia-bearing Pola´ny Formation. In the lower member of the Pola´ny Marl the first lithoclastic interlayer appears at 406 m. Upsection the frequency of the lithoclastic layers and proportion and size of the clasts increase. The lower boundary of the Ja´ko´hegy Member was defined at 352 m (Fig. 5). The upper boundary of the member is sharp. Above 270 m, the lithoclasts abruptly disappear. In the lithoclastic layers, the proportion of the clasts is 50–90% (Figs. 5 and 6). Mud-supported texture characterizes the lithoclastic interlayers of the Pola´ny Marl and the lower and uppermost part of the Ja´ko´hegy Breccia, whereas grain-supported texture prevails in the middle part of the Ja´ko´hegy Member. Microstylolitic grain contacts are common in the latter interval. The size of the lithoclasts is between 0.1 and 10 cm. The maximum grain size was found also in the middle part of the Ja´ko´hegy Member. The roundness of the clasts is highly variable. Angular grains are predominant, but well-rounded ones occur as well. The petrographic data of the lithoclastic intervals are summarized in Fig. 5a. A detailed microscopic study was carried out on the lithoclastic layers in order to investigate the microfacies characteristics of the lithoclasts and their matrix. The diagenetic features of the lithoclasts were also studied. The results are shown in Fig. 6. Based on this analysis microfacies types were defined. Most of the types were recognised in both the matrix and the lithoclasts, but certain types appeared only in the matrix or in the lithoclasts. Major characteristics of the defined microfacies types are summarized in Table 1. Based on the analysis of 118 thin sections, the relations of the microfacies of the lithoclasts and the matrix are shown in Table 2. According to these data, microfacies A is the most frequent in the lithoclasts (31%), while types B, C and E are present in about
equal proportions (14–17%). In the matrix, microfacies type E is definitely predominant (64%), the frequency of type D is significant (22%), while types B, C and F are poorly represented (4–6%) (Fig. 7) In many cases, signs of cementation and other early diagenetic features could be recognized in the lithoclasts, mainly in type A, but occasionally also in types B and C. The interparticle pores are filled as a rule with equant sparry calcite; drusy mosaic and syntaxial overgrowths are common. The biomolds are generally filled by coarse sparite. In two B-type lithoclasts, vug pores were observed. In two A-type lithoclasts, isopachous fibrous rims were visible around the grains; in another sample, fibrous sparite lined the biomoldic pores. 4.3. Geometry of the megabreccia deposits The core data in the environs of Magyarpola´ny also provided some information on the geometry of the megbreccia deposits. In these cores features and thickness of the Pola´ny Formation and within it the megabreccias are shown in Fig. 8. In the 3.5-km-long cross-section connecting the cores Mp-42, Mp-44 and Mp-41, significant southeastward thinning of the Ja´ko´hegy Breccia is clearly visible. These trends suggest the southward or southeastward dipping of the slope. The difference in thickness of the member is much less along the profile between cores Mp-42 and Mp-38 (also in a north–south direction). This can be explained either by a change in the orientation of the paleoslope or by subsequent tectonic displacements.
5. Discussion 5.1. Mechanisms of megabreccia formation Carbonate megabreccias form on or at the toe of metastable submarine slopes. The formation and deposition are triggered and controlled by various en-
Fig. 5 (see following two pages). Lithology and facies characteristics of the lithoclastic interval of core Mp-38. (a) Quantity, size, roundness and microfacies-types of the lithoclasts. (b) Petrography, microfacies-types and facies interpretation of the matrix. Abbreviations: M D mudstone, W D wackestone, P D packstone, G D grainstone; Glob. D Globotruncana, Het. D Heterohelix, pl.For. D other planktic forams, Ca.sph. D calcisphaerulids, b.For. D benthic forams, Ech. D echiniderms, Ostr. D ostracods, Moll. D molluscs.
J. Haas / Sedimentary Geology 128 (1999) 51–66
L
(a)
I
T
m 260
0
50
H
O
C
grain-size logmm
quantity % 100
0
1
L
A
57
S
roundness 2
10 10 10
0 1 2 3 4
T
S
microfacies type A
B
C
D
E
270
280
290
330
340
BRECCIA
320
J Á K Ó H E GY
310
MEMBER
300
350
360
370
380
390
400
410 1
limestone
2
argillaceous limestone
Fig. 5.
3
calcareous marl
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J. Haas / Sedimentary Geology 128 (1999) 51–66
M A T R I X
Moll.
Rudists
b.For .
Ostr.
Ca. sph.
pl.For .
Glob.
270
280
MEMBER
290
BRECCIA
300
310
J Á K Ó H E GY
320
330
340
350
360
370
380
390
400
410 4
lithoclast
5
plastoclast
6
microfacies type
fossilis
Het.
rudite
P G
silt
M W
m 260
grain-size arenite
texture
Ech.
(b)
rudite-size bioclast
Fig. 5 (continued).
7
bioturbation
D
E
F
J. Haas / Sedimentary Geology 128 (1999) 51–66
59
Fig. 6. Lithoclastic rocks in core Mp-38: (1) densely packed lithocalcirudite with microstylolitic grain contacts (308.3 m); (2, 3) poorly sorted lithoclast in muddy matrix (2: 291.0 m; 3: 329.3 m).
dogenic and exogenic processes (Spence and Tucker, 1997). Mechanisms of breccia formation and processes leading to accumulation of the breccia rockbodies can be determined by analysis of their sedimentological features. Results of this analysis in the studied case are summarized below.
For extension and geometry of the megabreccias only sporadic data are available except for the Magyarpola´ny area. According to these data the breccias occur in a length of at least 20 km along the southern foreland of the Ugod High. However it is not established whether the breccias form a continuous
Table 1 Main characteristics of the defined microfacies types
A B C D E F
Texture
Grain-size
Typical fossils
Facies
packstone–grainstone wackestone–packstone wackestone–packstone (grainstone) wackestone–packstone packstone–wackestone mudstone–wackestone
arenite arenite arenite (silt)
rudists, echinoderms, benthonic forams, Pienina algae mollusc shell fragments, echinoderms (benthonic forams) echinoderm fragments, benthonic forams
rudist platform slope terrace (proximal) slope terrace (medium)
silt silt
echinoderm fragments (calcisphaerulids) calcisphaerulids, molluscsechinoderms planktonic forams (globotruncanids)
slope terrace (distal) toe-of-slope pelagic basin
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J. Haas / Sedimentary Geology 128 (1999) 51–66
Table 2 Relationship between the microfacies types of the lithoclasts and the host rocks (matrix) of the lithoclasts %
mf F
-
-
-
-
-
-
21.2 E
-
2
-
2
19
2
17.0 D
-
1
1
4
12
2
14.4 C
-
1
2
9
4
1
16.1 B
-
-
-
5
14
-
31.3 A
-
1
2
6
26
2
A
B
C
D
E
F
0
4.2
4.2
22.0 63.6 6.0
0
clasts matrix
mf %
slope apron or separated fans. Based on data in the Magyarpola´ny area, a large, gently sloping apron(s) can be assumed which came into being along the toe of the slope. An interesting regularity was observed in the sedimentological features of the breccias along the sections. In the lower part of the breccia-bearing interval, mud-supported breccias were found with a typical grain size of 0.2–5 cm. They can be interpreted as debris flow deposits (Crevello and Schlager, 1980; Reading, 1986). This progresses into coarse, unsorted, grain-supported breccias upsection, with cmto m-size clasts which can be interpreted as rockfall deposits (Reading, 1986). In the topmost part of the breccia interval, the grain size shows a decreasing trend and the mud-supported character of the megabreccia beds returns. Rockfall deposits may have been deposited at the base of the steep slopes which were surrounded by debris flow (debrite) deposits outward. Based on microfacies studies of the matrix and the lithoclasts of the breccia layers further information
was obtained for sites of formation and deposition of the clasts. A summarizing depositional model is presented in Fig. 9. According to the statistical analysis of the investigated lithoclasts, they were derived partly from the margin of the rudist platform (microfacies A) and partly from the upper and lower parts or terraces of the slope (microfacies B, C, D, E). These types of lithoclasts of different origin were deposited generally together both in the rockfall and debris flow deposits. It suggests that back-stepping erosion may have caused collapses of the platform margin and the slope, leading to slope failures and triggering clastification as well as mixing of clasts of various origin and gravity transport. Frequently present meteoric cement in the lithoclasts of microfacies A suggests temporary subaerial exposure of the isolated platform. However, it is plausible that the reworked slope (slope terrace) deposits should have also been in a consolidated or semi-consolidated state in the time of their redeposition. According to microfacies analysis of the matrix of the lithoclasts and the muddy interlayers, the site of rockfall deposition may have been in the belt of deposition of macrofacies D, whereas the debrites may have been preferentially deposited in the zone of microfacies E, i.e. in periplatform depositional environments where predominantly fine-grained platform or upper slope derived bioclastic (microfacies D) or predominantly pelagic (microfacies E) carbonate deposition took place in the tranquil intervals between the collapse episodes. However, microfacies studies of the lithoclasts indicate that even these distal slope zones may have occasionally collapsed. 5.2. Depositional environments Based on the study of the surface exposures and cores in the study area and taking into consideration the sedimentological observations discussed above, a general depositional model can be established for the time when the transgression reached the top level of the asymmetric paleohigh (Ugod High) between
Fig. 7. Microfacies types of lithoclasts in core Mp-38: (1) microfacies type A, calcarenite grainstone (320.0 m); (2) microfacies type D, calcisiltite packstone (293.0 m); (3) lithoclast of type C in E-type matrix (356.8 m); (4) lithoclast of type F in C-type matrix (330.0 m).
J. Haas / Sedimentary Geology 128 (1999) 51–66
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J. Haas / Sedimentary Geology 128 (1999) 51–66
Fig. 8. Cross-section in the neighbourhood of Magyarpola´ny showing stratigraphic setting and trend in the thickness of lithoclastic formations. Location of the section is shown on the inset map.
Fig. 9. General facies model for the southern foreslope of the Ugod Platform.
J. Haas / Sedimentary Geology 128 (1999) 51–66
the southern and northern sub-basins (see Fig. 2). Inundation of this high led to the formation of a 5– 8 km wide isolated carbonate platform with a steep southern slope and a much gentler northern one. On the platform rudist limestones were formed, on the northern slope redeposited bioclasts of platform origin and at the foot of the southern slope lithoclastic sediments were deposited. In addition to the topographic asymmetry of the platform, the differences in features of the foreslope deposits also indicate differences in the early diagenesis of the deposited sediments. Characteristics of the low-angle accretional slope on the northern margin of the isolated platform and facies zones of the Ugod Platform are not discussed in the present paper (for these data see: Go´cza´n et al., 1979; Haas and Pa´lfalvy, 1989; Haas, 1999). For an environmental reconstruction of the southern part of the platform (Fig. 9) only the lithoclasts of the megabreccias provided data. In the marginal zone, a medium- to high-energy carbonate sand shoal environment may have existed (microfacies A), similar to that on the northern margin. Meteoric cement in the lithoclasts indicates the early lithification of the carbonate sand, most probably during the shortterm subaerial exposure intervals. 5.3. Steep erosional slope with depositional terraces (microfacies B–D) An abundance of lithoclasts in the southern foreland of the Ugod Platform indicates a steep erosional slope bounding the platform to the south. However, the fact that in addition to the platform facies (A) various microfacies types of the foreslope were also found in the lithoclasts, suggests that as a result of the pre- and syn-depositional tectonic activity, terraces could have formed on the slope where accumulation of sediments may have occurred; consolidated or semi-consolidated sediments of these terraces may also have been subject to clastification and reworking. The echinoderm-mollusc calcarenite (microfacies B) and the echinoderm calcarenite-calcisilt (C) microfacies were probably deposited under shallow marine conditions but below the euphotic zone. One part of the bioclasts was transported from the platform but the other part (e.g. the majority of the
63
crinoids) may have lived on the slope terrace. The relatively elevated position of this environment is also indicated by the fact that this facies is fairly common in the lithoclasts. At the toe of the slope a large amount of lithoclasts of varied origin were deposited, mainly by rockfall. Periplatform taluses were formed. The intraparticle pores of the grain-supported breccia were filled mainly by echinoderm calcisilt mud (microfacies D) which may have also been deposited on the top of the taluses during quiet periods. This interpretation is also supported by the observation that this microfacies type is common both in the lithoclasts and in the matrix of the lithoclasts. Debrites were deposited in the more distal part of the toe-of-slope environment. Slump structures are also common. Mollusc-echinoderm silt with a large amount of calcisphaerulids (microfacies E) is the characteristic matrix. However, this microfacies-type also appears in the lithoclasts. This moderately deep, pelagic environment may have been very similar to that of the distal zone of the northern slope. 5.4. Hemipelagic basin (microfacies F) This was a relatively deep (shallow bathyal) hemipelagic basin. The pelagic nature of the depositional environment is indicated by the almost exclusively planktonic biota. Its distant location from the coeval platforms is also supported by the fact that bioclasts and lithoclasts of platform origin did not reach this environment as a rule. However, the high carbonate content of the deposited mud may have originated at least partly from the surrounding platforms. 5.5. Megabreccias and sea-level cycles According to the comprehensive review of Spence and Tucker (1997) “both rises and falls of relative sea-level can have either casual or direct link to the initiation of gravitational instability” and consequently megabreccia formation. The available data indicated that sea-level falls and lowstands were more favourable for the megabreccia deposition and only a few examples of megabreccias deposited during rising or highstand of relative sea-level were reported. However, in many cases the relationship
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J. Haas / Sedimentary Geology 128 (1999) 51–66
between the megabreccia deposition and the sealevel setting was not determined or satisfactorily proven. Since a solution of this problem would be of crucial importance from the point of view of the sequence analysis, we also attempted to evaluate our observations from this aspect. Two approaches are given below. Based on the evaluation of the facies changes on the gentle northern slope and in the basin at the toe of the steeper southern slope the recognition of the 3rd-order cycles will be discussed first. Then the matrix of the lithoclasts will be evaluated since studies of modern carbonates suggest that maximum shedding of loose silt to arenite-size carbonate particles occurred during highstand of relative sealevel (Droxler and Schlager, 1985; Reijmer et al., 1988; Schlager et al., 1994). Cyclicity was recognised in the studied successions of both the northern and southern slopes. It was very obvious and easily recognisable in the cored sections at Tapolcaf˝o belonging to the northern platform margin and slope (Go´cza´n et al., 1979; Haas and Pa´lfalvy, 1989). Based on the facies analysis, two 3rd-order relative sea-level cycles, superimposed by 4th- and probably 5th-order ones, could be recognised (Fig. 10). Sea-level rise at the beginning of the first 3rd-order cycle led to the inundation of the Ugod High, colonization of the platform biota and initiation of the carbonate factory on the top of the paleohigh. The early evolutionary stage of the platform was followed by a significant platform progradation during the highstand stage of the cycle. The low angle of the northern slope may have favoured rapid progradation. The first cycle may have ended with a sea-level drop which probably resulted in the subaerial exposure of the top of the platform and consequently reduced shedding and interruption of the slope accretion. The next sea-level rise led to the back-stepping of the platform. It resulted in the appearance of the deeper slope facies above the platform and upper slope facies. It was followed by a highstand progradation, superimposed however by 4th- and 5th-order oscillations (retrogradations and progradations) prior to the final drowning. In the Magyarpola´ny Mp-38 core, recognition of cyclicity was more difficult. However, based on the microfacies analysis the cyclic alternation of the depositional facies could be detected (Figs. 5 and 10).
Within the lower member of the Pola´ny Formation two 3rd-order cycles could also be interpreted. The first cycle initiated with a sea-level rise, the same one which resulted in the establishment of the carbonate factory on the Ugod High. This event is probably reflected in the significant increase of the carbonate content in the deepening-upward basinal succession in the transitional interval between the Ja´ko´ and Pola´ny formations, where a neritic pycnodont-bearing facies progresses into a deep pelagic Inoceramus–Globotruncana facies (microfacies F). The first distal debrite interlayers appear in the E microfacies (Fig. 5), most probably subsequent to the maximum flooding. Massive appearance of the typical debrites can be correlated with the predominance of the calcisphaere microfacies (E). It is followed by a segment showing prevalence of the D microfacies in the matrix and rockfall deposits. They may have formed in the highstand stage of the first cycle. Positive correlation between the amount of lithoclasts and the amount of sand-size bioclasts of platform origin appear to support this interpretation (see Fig. 5), since shedding of the platform bioclasts was probably more intense in the highstand periods when the platform was covered by a shallow sea (Droxler and Schlager, 1985). The first debris flow deposits may have formed during the early highstand. This was followed by basinward extension of the toe-of-slope megabreccia aprons during the late highstand. Traces of high-frequency cyclicity were also detectable in this interval. The intercalations of the pelagic F facies, poor in particles of platform origin, may indicate the short-term lowstand and transgressive intervals, whereas layers rich in platformderived bioclasts and also in lithoclasts may correspond to the highstand stages. The second 3rd-order cycle is made up of pelagic carbonates; rudite to arenite-sized grains are missing (Fig. 10). This means that due to retrogradation of the platform only the silt or lutite-sized carbonate mud may have reached the internal part of the basin south to the Ugod High. However, the high carbonate content of the basin sediments indicates survival of the platforms in the immediate neighbourhood. The end of the second cycle is indicated by a significant change in lithology in the form of a remarkable increase in clay and siliciclastic silt content.
J. Haas / Sedimentary Geology 128 (1999) 51–66
65
Fig. 10. Comparison of depositional sequences formed on the northern and southern slopes of the Ugod Platform.
It is worth mentioning that the inferred sea-level cycles discussed above are amazingly similar to those recognized in the Sinai and other correspond-
ing successions in the Middle East and North Africa (Lu¨ning et al., 1998).
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J. Haas / Sedimentary Geology 128 (1999) 51–66
6. Conclusions
References
In the Turonian–early Senonian, a post-orogenic collapse basin dissected by tectonically controlled highs came into being in the western side of the Bakony area. Relative sea-level rise in the Santonian led to step-by-step inundation of the articulated basin giving rise to the formation of isolated platforms on the highs. In the foreland of the steep slope of one of the platforms, megabreccias have been accumulated in remarkable thicknesses. Based on microfacies characteristics of the lithoclasts it can be assumed that they derived from the platform margin, the upper and the lower slope. The clasts were mixed in the course of their redeposition. This means that breccias were formed by intense back-stepping erosion which led to collapse of large parts of the rim of the platform and the foreslope. Lithoclasts formed rockfall talus at the toe of the slope which were surrounded by large, low-angle aprons, site of deposition of periplatform hemipelagic sediments during the tranquil intervals between the collapse episodes. Based on cycle-analysis of continuous core sections it was concluded that the breccia deposition took place within a single 3rd-order depositional cycle. The first debris flow deposits were formed after maximum flooding. Increasing frequency of debris flow deposits and the subsequent appearance of the rockfall breccias indicate the initiation of the slope progradation during the early highstand and continuation thereof during the late highstand. The breccia formation may have also be influenced by the high-frequency cyclicity causing intense meteoric cementation in the platform margin and the assumed synsedimentary tectonic activity contributing to the instability of the slope.
Bodrogi, I., Fogarasi, A., 1995. The Santonian=Campanian boundary in the ‘Senonian’ of the Bakony Mts. (Transdanubian Range). 2nd Int. Symp. Cretaceous Stage Boundaries, Brussels, 8–16 September 1995, 23. Bodrogi, I., Fogarasi, A., Yazikova, E.A., Sztano´, O., Ba´ldi-Beke, M. 1998. Upper Cretaceous of the Bakony Mts. (Hungary), sedimentology, biostratigraphy, correlation. Zbl. Geol. Pala¨ont. I, 11=12, 1179–1194. Crevello, P.D., Schlager, W., 1980. Carbonate debris sheets and turbidites, Exuma Sound, Bahamas. J. Sediment. Petrol. 50 (4), 1121– 1148. Droxler, A.W., Schlager, W., 1985. Glacial versus interglacial sedimentation rates and turbidite frequency in the Bahamas. Geology 13, 799–802. Fe´legyha´zy, L., 1983. Research into the nannoplankton stratigraphy of the Upper Cretaceous in the southern Bakony Mountains. Annu. Rep. Hung. Geol. Inst. 1983, 143–155. Go´cza´n, F., 1964. Stratigraphic palynology of the Hungarian Upper Cretaceous. Acta Geol. 8, 229–264. ´ ., Mo´ra-Czabalay, L., Rimano´czy, A ´ ., Go´cza´n, F., Siegl-Farkas, A Haas, J., 1979. The Ugod Limestone Formation (Senonian Rudist Limestone) in the Bakony Mountains. Ann. Hung. Geol. Inst. 61, 149. Haas, J., 1981. Marine Upper Cretaceous Formations in the Bakony Mts. PhD. dissertation, Budapest, 158 pp. Haas, J., 1983. Senonian cycle in the Transdanubian Central Range. Acta Geol. Hung. 26, 21–40. Haas, J., 1999. Late Cretaceous isolated platform evolution in the Bakony Mountains (Hungary). Geol. Carpathica (in press). Haas, J., Pa´lfalvi, S., 1989. Ugod Limestone (Upper Cretaceous) fa´ FI Annu. Rep. cies key-sections in the Bakony Mountains. MA 1987, 35–57. Haas, J., Csa´sza´r, G., Kova´cs, S., Vo¨ro¨s, A., 1990. Evolution of the western part of the Tethys as reflected by the geological formations of Hungary. Acta Geod. Geophys. Mont. Hung. 25, 325–344. Lu¨ning, S., Marzouk, A.M., Morsi, A.M., Kuss, J., 1998. Sequence stratigraphy of the Upper Cretaceous of central-east Sinai, Egypt. Cretaceous Res. 19 (2), 153–196. Nagy, E., 1957. Authigenetic brecciation in the Upper Cretaceous strata around Pa´pa, North Western Hungary. Fo¨ldt. Ko¨zl. 87, 346– 347. Reading, H.G., 1986. Sedimentary Environments and Facies. Blackwell, Oxford. Reijmer, J.J.G., Schlager, W., Droxler, A.W., 1988. Site 632. Pliocene–Pleistocene sedimentation in the Bahamian basin. Proc. ODP, Sci. Results 101, 213–220. ´ ., 1983. Palynology of the Senonian Formations at Siegl-Farkas, A ˜ sle´nytani Vita´k (Discussiones Paleontologicae) Magyarpola´ny. O 29, 59–69. ´ ., Wagreich, M., 1996. Age and palaeoenvironment Siegl-Farkas, A of the spherulite-bearing Pola´ny Marl Formation (Late Cretaceous, Hungary) on the basis of palynological and nannonplankton investigations. Acta Biol. Szeged. 35, 23–36. Schlager, W., Reijmer, J.J.G., Droxler, A., 1994. Highstand shedding of carbonate platforms. J. Sediment. Res. B 64, 270–281. Spence, G.H., Tucker, M.E., 1997. Genesis of limestone megabreccias and their significance in carbonate sequence stratigraphic models: a review. Sediment. Geol. 112, 163–193.
Acknowledgements Investigations providing the basis of the present paper were carried out by the author at the Hungarian Geological Institute. The work was also supported by the Hungarian Academy of Sciences. The author is indebted to Attila Fogarasi (Budapest) for the discussions on the biostratigraphy.
Sedimentary Geology 128 (1999) 51–66
Genesis of Late Cretaceous toe-of-slope breccias in the Bakony Mts, Hungary J. Haas * Geological Research Group of the Hungarian Academy of Sciences, Eo¨tvo¨s Lora´nd University 1088, Budapest Mu´zeum krt. 4=A, Budapest, Hungary Received 15 December 1997; accepted 2 May 1999
Abstract Toe-of-slope megabreccia deposits of an Upper Cretaceous rudist platform were studied in the Bakony Mountains. During the Turonian–early Senonian tectogenesis an articulated basin came into being in the area of the Bakony; depressions and highs were formed roughly parallel with the structural strike of the mountains. Inundation of the highs led to the evolution of carbonate platforms. The investigated platform shows an asymmetric architecture. In contrast to its gentle northern slope a steep erosional slope bounded the platform to the south with large, low-angle aprons, the site of deposition of megabreccias. The breccia accumulation occurred within a single 3rd-order cycle commencing in the early highstand and reaching its climax in the late highstand. 1999 Elsevier Science B.V. All rights reserved. Keywords: Bakony Mountains; Upper Cretaceous; carbonate platform; slope; megabreccia; sea-level changes
1. Introduction Carbonate megabreccias accumulating on or at the toe of slopes of carbonate platforms may provide valuable information on the features of the eroding platforms, conditions prevailing on the slope and also in the site of deposition itself. However, determination of their genesis is not easy because various endogenic and exogenic processes may be responsible for the breccia formation. One of the most debated questions is the relationship between the megabreccia generation and relative sea-level changes, hence the sequence stratigraphic importance of the megabreccia formations.
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C36 1 266 4947; E-mail: [email protected]
In the Bakony Mts of Hungary, due to Pre-Gosau tectogenesis and subsequent relative sea-level rise, elongated, asymmetric isolated carbonate platforms came into being. Along the toe of their steeper slope, megabreccia bodies of more than 100 m in thickness were formed. Based on detailed studies, mainly of cores, the aim of the present paper is to analyze those factors which may have played an important role in megabreccia formation and lithoclast deposition, with special regard to the relationship to possible sea-level changes.
2. Geologic setting and stratigraphy Senonian formations of hundreds of meters in thickness occur in the western part of the Transdanu-
0037-0738/99/$ – see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 7 - 0 7 3 8 ( 9 9 ) 0 0 0 6 1 - 5
52
J. Haas / Sedimentary Geology 128 (1999) 51–66
TR. Gyõr
10
0
20 km
K
isa
lfö
ld PSB
?
od Ug gh H MSB
N. ony k Ba
TRASDANUBIAN RANGE UNIT
Budapest
e
ba Rá
Lin
ny ko a B
? ? Zalaegerszeg
?
Zala Basin
Veszprém
S. ?
Keszthely
e Lak
extension of Senonian platform carbonates
on
t Bala
Balaton Line
hemipelagic carbonates PSB
Pápa Sub-basin
MSB
Magyarpolány Sub-basin
Fig. 1. Extension of the Senonian and distribution of the Campanian formations in the Transdanubian Range Unit.
bian Range Unit, i.e. in the Bakony Mountains and in the basement of the Kisalfo¨ld (Small Plain) and the North Zala Basin in west Hungary (Fig. 1). In the early part of the Alpine evolutionary stage (from the late Paleozoic to the Late Cretaceous) the Transdanubian Range Unit was located between the Upper Austro–Alpine and the South Alpine realms (Haas et al., 1990). Multi-stage collision of the Adriatic microplate and other lithosphere fragments in the foreland of the European plate led to tectonic deformations in the Transdanubian Range during the Late Cretaceous. Late Cenomanian–Turonian collision (Pre-Gosau phase) resulted in uplift and intense erosion. This was followed by the formation of a large collapse basin in the early Senonian. The evolution of this basin came to an end in the Paleocene, due to the next collision event (Laramian phase). Between the two collision phases a typical tectonically controlled 2nd-order transgression–regression depositional cycle was formed in the basin (Haas, 1983). As a combined effect of tectonics and subaerial erosion, an articulated basin came into being by the Santonian with elongated highs and depressions be-
tween them, roughly parallel to the structural strike of the Transdanubian Range. The study area is located on the western side of the Northern Bakony (Fig. 2), representing the central part of the Senonian basin. The investigated breccia bodies were explored in the Magyarpola´ny Sub-basin, south of the Ugod High where a rudist platform had been developed in the Campanian. The depression north to the Ugod High is called the Pa´pa Sub-basin (see Fig. 1). The major lithostratigraphic units of the Senonian cycle and their stratigraphic position are shown in Fig. 3. Due to significant topographic differences which strongly influenced the paleogeographic setting in the early stage of basin evolution, the stratigraphic successions of the depressions and the highs were completely different (Haas, 1983). In the depressions the karstified Mesozoic bedrocks and locally bauxites upon them are covered by fluvial–lacustrine deposits (Csehba´nya Formation) or a limnic to parallic coal-bearing formation (Ajka Coal Formation). They are overlain by marls, rich in brackish water fossils (lower member of the Ja´ko´ Formation), which progress into marls and calcareous marls of neritic marine facies (upper member of the Ja´ko´ Forma-
J. Haas / Sedimentary Geology 128 (1999) 51–66 0
53
10 km
PÁPA Pa-2.
Ugod
Te Tfõ-4. vel Hi li Tapolcafõ Tfõ-4
Vinár-1.
Cel-1.
Cseh-13
Bakonyjákó Dabrony-1. Bj-26. Magyarpolány Mp-38. Dv-3.
Polány Fm . Ajka
Devecser Ukk-.1.
Ugod-Polány transitional unit. Ugod Fm.
Ukk-3.
GY-7.
Jákó Fm.
Gy-5.
Ajka Fm.
Sp-2. Csebánya Fm.
Sümeg
extension of the Ugod Fm below the Polány Fm. outcrops of megabreccias subsurface data on the megabreccia assumed extension of the megabreccia bodies
Fig. 2. Outcrops and subcrops of the Senonian formations and locality of the most important borehole sections in the Bakony area.
tion). They in turn are overlain by calcareous marls with Inoceramus and globotruncanids (lower member of the Pola´ny Marl Formation). The studied thick megabreccia bodies (Ja´ko´hegy Breccia Member — Nagy, 1957) are locally intercalated into this member. The upper member of the Pola´ny Marl Formation consists of silty marl with thin sandstone interlayers. The succession on the highs is simpler. The bedrocks or bauxites are overlain by rudist limestones (Ugod Limestone) which are followed by the Pola´ny Formation through a transitional interval. The chronostratigraphy of the Senonian sequences is based mainly on foraminifera, nannoplankton and sporomorphs (Go´cza´n, 1964; Siegl-Farkas, 1983;
Fe´legyha´zi, 1983; Bodrogi and Fogarasi, 1995; SieglFarkas and Wagreich, 1996; Bodrogi et al., 1998). According to the palynologic data terrestrial deposition began in the depressions during the Santonian. In our study area (Magyarpola´ny Sub-basin), the first nannofossil-bearing marine layers with Calculites obscurus appear in the lower member of the Ja´ko´ Formation. The first appearance of Bronsonia parca constricta indicates the Santonian=Campanian boundary in the lower part of the Pola´ny Formation within palynozone D. Ceratolithoides aculeus appears at the top of the Ja´ko´hegy Breccia. The upper part of the Pola´ny Marl can be characterized by Qadrum gothicum; thus it is also Campanian (A. Fogarasi, pers. commun.).
54
J. Haas / Sedimentary Geology 128 (1999) 51–66
L IT H O ST R AT I G R A P HY STAGES
platform
basin My
78 Ganna Mb. 80
CAMPANIAN POLÁNY FM. JB
82
UGOD FM. 84 JÁKÓ FM.
SANTONIAN AJKA FM.
86
88
CONIACIAN ?
CSEHBÁNYA FM.
bauxite
bauxite PRE-SENONIAN FORMATIONS
Fig. 3. Lithostratigraphic chart of the Senonian in the Transdanubian Range.
In the Pa´pa Sub-basin palynological data are available (F. Go´cza´n, in Haas, 1981). These data constrain synchronous marine flooding in the two sub-basins, in the palyno-zone D that is in the late Santonian near the Santonian=Campanian boundary. The inundation of the Ugod High may have begun later, in palyno-zone E (F. Go´cza´n, in Haas, 1981), which corresponds to the lower member of the Pola´ny Formation in the depressions.
3. Evolution of the basin Based on the evaluation of outcrop and core data, the Senonian evolution of the study area is summarized below. By the early Senonian, a WSW–ENE-trending, elongated, asymmetric high had come into being, leading to separation of a southern and a northern sub-basin. The Ugod High may have been bounded by steep multiple faults to the south. In contrast, a gentle slope came into existence on the northern side of the Ugod High. The elevation of the top of the high above the bottom of the depression may have been about 120–150 m. Filling up of the basins began in the Santonian. In the initial evolutionary stage, a fluvial environment came into being in the northern sub-basin (Csehba´nya Formation) and flu-
vial, lacustrine and paludal deposition occurred in the southern one (Csehba´nya and Ajka Formations). This was followed by an abrupt facies change in the second evolutionary stage, at the end of the Santonian, which led to inundation and the establishment of shallow neritic brackish-water conditions in both sub-basins (lower member of the Ja´ko´ Marl). At this stage the difference in altitude of the top between the high and the bottom of the shallow sea may have been about 20–50 m. Subsequently the relative sea-level rise continued. This is clearly reflected in the fundamental change in the biota upsection in the Ja´ko´ Formation. It is highly probable that the rising sea-level reached the top of the Ugod High at the very end of the Santonian, when the transitional layers between the Ja´ko´ and the Pola´ny Formations were deposited in the basin. The definite trend of upward-increasing carbonate content within the Ja´ko´ Marl may indicate the establishment of the subtidal carbonate factory on the surrounding platforms. Following the inundation, the tectonically preformed, steep southern slope was transformed into a submarine erosional slope, whereas on the more gently dipping northern side an accretional slope came into being. At the toe of the steep slope megabreccias were formed, while on the low-angle slope bioclasts (mainly of platform origin) were accumulated.
J. Haas / Sedimentary Geology 128 (1999) 51–66
The subsequent transgression (i.e. the initial stage of the next 3rd-order cycle) led to a significant reduction in the extension of the platform. It is reflected in retrogradation and significant back-stepping of the facies zones on the northern slope, and in the cessation of the lithoclast and rudite to arenite-size bioclast accumulation (at least in the inner part) of the southern sub-basin. The next sea-level rise and the coeval increase of fine terrigenous influx may have caused the final drowning of the Ugod Platform and also of the other rudist platforms in the Senonian basin of the Transdanubian Range, in the middle part of the Campanian.
4. Basic features of the megabreccia deposits 4.1. General characteristics Along the southern slope of the Ugod High, in a belt at least 20 km long and at least 5 km wide,
55
megabreccia bodies were encountered in the lower member of the Pola´ny Marl Formation. Outcrops of megabreccias are known on Tevel Hill and on Ja´ko´ Hill west of Bakonyja´ko´ (see Fig. 2), and they were also encountered in some boreholes in the neighbourhood of Magyarpola´ny and further southwest in the core of well Devecser Dv-3 (see Figs. 3 and 4). The first breccia horizons appear about 20 m above the lower boundary of the formation. These are mud-supported breccias with scattered lithoclasts or plasticlasts. The frequency of the megabreccia interlayers increases upward leading to a predominance of lithoclasts upsection. This part of the Pola´ny Formation was defined as the Ja´ko´hegy Breccia Member (Go´cza´n et al., 1979). The thickness of the Ja´ko´hegy Breccia may exceed 100 m. The megabreccias are generally mud-supported in the lower part of the member and grain-supported in the upper part. The size of lithoclasts varies between mm to m scales, and they are unsorted as a rule. Bioclastic calcarenite–calcirudite intercalations also occur.
Fig. 4. Lithoclasts in muddy matrix in core Dv-3 (629.5 m).
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J. Haas / Sedimentary Geology 128 (1999) 51–66
4.2. Petrographic features of the studied section Core Mp-38 (for location see Fig. 2) is a key section of the megabreccia-bearing Pola´ny Formation. In the lower member of the Pola´ny Marl the first lithoclastic interlayer appears at 406 m. Upsection the frequency of the lithoclastic layers and proportion and size of the clasts increase. The lower boundary of the Ja´ko´hegy Member was defined at 352 m (Fig. 5). The upper boundary of the member is sharp. Above 270 m, the lithoclasts abruptly disappear. In the lithoclastic layers, the proportion of the clasts is 50–90% (Figs. 5 and 6). Mud-supported texture characterizes the lithoclastic interlayers of the Pola´ny Marl and the lower and uppermost part of the Ja´ko´hegy Breccia, whereas grain-supported texture prevails in the middle part of the Ja´ko´hegy Member. Microstylolitic grain contacts are common in the latter interval. The size of the lithoclasts is between 0.1 and 10 cm. The maximum grain size was found also in the middle part of the Ja´ko´hegy Member. The roundness of the clasts is highly variable. Angular grains are predominant, but well-rounded ones occur as well. The petrographic data of the lithoclastic intervals are summarized in Fig. 5a. A detailed microscopic study was carried out on the lithoclastic layers in order to investigate the microfacies characteristics of the lithoclasts and their matrix. The diagenetic features of the lithoclasts were also studied. The results are shown in Fig. 6. Based on this analysis microfacies types were defined. Most of the types were recognised in both the matrix and the lithoclasts, but certain types appeared only in the matrix or in the lithoclasts. Major characteristics of the defined microfacies types are summarized in Table 1. Based on the analysis of 118 thin sections, the relations of the microfacies of the lithoclasts and the matrix are shown in Table 2. According to these data, microfacies A is the most frequent in the lithoclasts (31%), while types B, C and E are present in about
equal proportions (14–17%). In the matrix, microfacies type E is definitely predominant (64%), the frequency of type D is significant (22%), while types B, C and F are poorly represented (4–6%) (Fig. 7) In many cases, signs of cementation and other early diagenetic features could be recognized in the lithoclasts, mainly in type A, but occasionally also in types B and C. The interparticle pores are filled as a rule with equant sparry calcite; drusy mosaic and syntaxial overgrowths are common. The biomolds are generally filled by coarse sparite. In two B-type lithoclasts, vug pores were observed. In two A-type lithoclasts, isopachous fibrous rims were visible around the grains; in another sample, fibrous sparite lined the biomoldic pores. 4.3. Geometry of the megabreccia deposits The core data in the environs of Magyarpola´ny also provided some information on the geometry of the megbreccia deposits. In these cores features and thickness of the Pola´ny Formation and within it the megabreccias are shown in Fig. 8. In the 3.5-km-long cross-section connecting the cores Mp-42, Mp-44 and Mp-41, significant southeastward thinning of the Ja´ko´hegy Breccia is clearly visible. These trends suggest the southward or southeastward dipping of the slope. The difference in thickness of the member is much less along the profile between cores Mp-42 and Mp-38 (also in a north–south direction). This can be explained either by a change in the orientation of the paleoslope or by subsequent tectonic displacements.
5. Discussion 5.1. Mechanisms of megabreccia formation Carbonate megabreccias form on or at the toe of metastable submarine slopes. The formation and deposition are triggered and controlled by various en-
Fig. 5 (see following two pages). Lithology and facies characteristics of the lithoclastic interval of core Mp-38. (a) Quantity, size, roundness and microfacies-types of the lithoclasts. (b) Petrography, microfacies-types and facies interpretation of the matrix. Abbreviations: M D mudstone, W D wackestone, P D packstone, G D grainstone; Glob. D Globotruncana, Het. D Heterohelix, pl.For. D other planktic forams, Ca.sph. D calcisphaerulids, b.For. D benthic forams, Ech. D echiniderms, Ostr. D ostracods, Moll. D molluscs.
J. Haas / Sedimentary Geology 128 (1999) 51–66
L
(a)
I
T
m 260
0
50
H
O
C
grain-size logmm
quantity % 100
0
1
L
A
57
S
roundness 2
10 10 10
0 1 2 3 4
T
S
microfacies type A
B
C
D
E
270
280
290
330
340
BRECCIA
320
J Á K Ó H E GY
310
MEMBER
300
350
360
370
380
390
400
410 1
limestone
2
argillaceous limestone
Fig. 5.
3
calcareous marl
58
J. Haas / Sedimentary Geology 128 (1999) 51–66
M A T R I X
Moll.
Rudists
b.For .
Ostr.
Ca. sph.
pl.For .
Glob.
270
280
MEMBER
290
BRECCIA
300
310
J Á K Ó H E GY
320
330
340
350
360
370
380
390
400
410 4
lithoclast
5
plastoclast
6
microfacies type
fossilis
Het.
rudite
P G
silt
M W
m 260
grain-size arenite
texture
Ech.
(b)
rudite-size bioclast
Fig. 5 (continued).
7
bioturbation
D
E
F
J. Haas / Sedimentary Geology 128 (1999) 51–66
59
Fig. 6. Lithoclastic rocks in core Mp-38: (1) densely packed lithocalcirudite with microstylolitic grain contacts (308.3 m); (2, 3) poorly sorted lithoclast in muddy matrix (2: 291.0 m; 3: 329.3 m).
dogenic and exogenic processes (Spence and Tucker, 1997). Mechanisms of breccia formation and processes leading to accumulation of the breccia rockbodies can be determined by analysis of their sedimentological features. Results of this analysis in the studied case are summarized below.
For extension and geometry of the megabreccias only sporadic data are available except for the Magyarpola´ny area. According to these data the breccias occur in a length of at least 20 km along the southern foreland of the Ugod High. However it is not established whether the breccias form a continuous
Table 1 Main characteristics of the defined microfacies types
A B C D E F
Texture
Grain-size
Typical fossils
Facies
packstone–grainstone wackestone–packstone wackestone–packstone (grainstone) wackestone–packstone packstone–wackestone mudstone–wackestone
arenite arenite arenite (silt)
rudists, echinoderms, benthonic forams, Pienina algae mollusc shell fragments, echinoderms (benthonic forams) echinoderm fragments, benthonic forams
rudist platform slope terrace (proximal) slope terrace (medium)
silt silt
echinoderm fragments (calcisphaerulids) calcisphaerulids, molluscsechinoderms planktonic forams (globotruncanids)
slope terrace (distal) toe-of-slope pelagic basin
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J. Haas / Sedimentary Geology 128 (1999) 51–66
Table 2 Relationship between the microfacies types of the lithoclasts and the host rocks (matrix) of the lithoclasts %
mf F
-
-
-
-
-
-
21.2 E
-
2
-
2
19
2
17.0 D
-
1
1
4
12
2
14.4 C
-
1
2
9
4
1
16.1 B
-
-
-
5
14
-
31.3 A
-
1
2
6
26
2
A
B
C
D
E
F
0
4.2
4.2
22.0 63.6 6.0
0
clasts matrix
mf %
slope apron or separated fans. Based on data in the Magyarpola´ny area, a large, gently sloping apron(s) can be assumed which came into being along the toe of the slope. An interesting regularity was observed in the sedimentological features of the breccias along the sections. In the lower part of the breccia-bearing interval, mud-supported breccias were found with a typical grain size of 0.2–5 cm. They can be interpreted as debris flow deposits (Crevello and Schlager, 1980; Reading, 1986). This progresses into coarse, unsorted, grain-supported breccias upsection, with cmto m-size clasts which can be interpreted as rockfall deposits (Reading, 1986). In the topmost part of the breccia interval, the grain size shows a decreasing trend and the mud-supported character of the megabreccia beds returns. Rockfall deposits may have been deposited at the base of the steep slopes which were surrounded by debris flow (debrite) deposits outward. Based on microfacies studies of the matrix and the lithoclasts of the breccia layers further information
was obtained for sites of formation and deposition of the clasts. A summarizing depositional model is presented in Fig. 9. According to the statistical analysis of the investigated lithoclasts, they were derived partly from the margin of the rudist platform (microfacies A) and partly from the upper and lower parts or terraces of the slope (microfacies B, C, D, E). These types of lithoclasts of different origin were deposited generally together both in the rockfall and debris flow deposits. It suggests that back-stepping erosion may have caused collapses of the platform margin and the slope, leading to slope failures and triggering clastification as well as mixing of clasts of various origin and gravity transport. Frequently present meteoric cement in the lithoclasts of microfacies A suggests temporary subaerial exposure of the isolated platform. However, it is plausible that the reworked slope (slope terrace) deposits should have also been in a consolidated or semi-consolidated state in the time of their redeposition. According to microfacies analysis of the matrix of the lithoclasts and the muddy interlayers, the site of rockfall deposition may have been in the belt of deposition of macrofacies D, whereas the debrites may have been preferentially deposited in the zone of microfacies E, i.e. in periplatform depositional environments where predominantly fine-grained platform or upper slope derived bioclastic (microfacies D) or predominantly pelagic (microfacies E) carbonate deposition took place in the tranquil intervals between the collapse episodes. However, microfacies studies of the lithoclasts indicate that even these distal slope zones may have occasionally collapsed. 5.2. Depositional environments Based on the study of the surface exposures and cores in the study area and taking into consideration the sedimentological observations discussed above, a general depositional model can be established for the time when the transgression reached the top level of the asymmetric paleohigh (Ugod High) between
Fig. 7. Microfacies types of lithoclasts in core Mp-38: (1) microfacies type A, calcarenite grainstone (320.0 m); (2) microfacies type D, calcisiltite packstone (293.0 m); (3) lithoclast of type C in E-type matrix (356.8 m); (4) lithoclast of type F in C-type matrix (330.0 m).
J. Haas / Sedimentary Geology 128 (1999) 51–66
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J. Haas / Sedimentary Geology 128 (1999) 51–66
Fig. 8. Cross-section in the neighbourhood of Magyarpola´ny showing stratigraphic setting and trend in the thickness of lithoclastic formations. Location of the section is shown on the inset map.
Fig. 9. General facies model for the southern foreslope of the Ugod Platform.
J. Haas / Sedimentary Geology 128 (1999) 51–66
the southern and northern sub-basins (see Fig. 2). Inundation of this high led to the formation of a 5– 8 km wide isolated carbonate platform with a steep southern slope and a much gentler northern one. On the platform rudist limestones were formed, on the northern slope redeposited bioclasts of platform origin and at the foot of the southern slope lithoclastic sediments were deposited. In addition to the topographic asymmetry of the platform, the differences in features of the foreslope deposits also indicate differences in the early diagenesis of the deposited sediments. Characteristics of the low-angle accretional slope on the northern margin of the isolated platform and facies zones of the Ugod Platform are not discussed in the present paper (for these data see: Go´cza´n et al., 1979; Haas and Pa´lfalvy, 1989; Haas, 1999). For an environmental reconstruction of the southern part of the platform (Fig. 9) only the lithoclasts of the megabreccias provided data. In the marginal zone, a medium- to high-energy carbonate sand shoal environment may have existed (microfacies A), similar to that on the northern margin. Meteoric cement in the lithoclasts indicates the early lithification of the carbonate sand, most probably during the shortterm subaerial exposure intervals. 5.3. Steep erosional slope with depositional terraces (microfacies B–D) An abundance of lithoclasts in the southern foreland of the Ugod Platform indicates a steep erosional slope bounding the platform to the south. However, the fact that in addition to the platform facies (A) various microfacies types of the foreslope were also found in the lithoclasts, suggests that as a result of the pre- and syn-depositional tectonic activity, terraces could have formed on the slope where accumulation of sediments may have occurred; consolidated or semi-consolidated sediments of these terraces may also have been subject to clastification and reworking. The echinoderm-mollusc calcarenite (microfacies B) and the echinoderm calcarenite-calcisilt (C) microfacies were probably deposited under shallow marine conditions but below the euphotic zone. One part of the bioclasts was transported from the platform but the other part (e.g. the majority of the
63
crinoids) may have lived on the slope terrace. The relatively elevated position of this environment is also indicated by the fact that this facies is fairly common in the lithoclasts. At the toe of the slope a large amount of lithoclasts of varied origin were deposited, mainly by rockfall. Periplatform taluses were formed. The intraparticle pores of the grain-supported breccia were filled mainly by echinoderm calcisilt mud (microfacies D) which may have also been deposited on the top of the taluses during quiet periods. This interpretation is also supported by the observation that this microfacies type is common both in the lithoclasts and in the matrix of the lithoclasts. Debrites were deposited in the more distal part of the toe-of-slope environment. Slump structures are also common. Mollusc-echinoderm silt with a large amount of calcisphaerulids (microfacies E) is the characteristic matrix. However, this microfacies-type also appears in the lithoclasts. This moderately deep, pelagic environment may have been very similar to that of the distal zone of the northern slope. 5.4. Hemipelagic basin (microfacies F) This was a relatively deep (shallow bathyal) hemipelagic basin. The pelagic nature of the depositional environment is indicated by the almost exclusively planktonic biota. Its distant location from the coeval platforms is also supported by the fact that bioclasts and lithoclasts of platform origin did not reach this environment as a rule. However, the high carbonate content of the deposited mud may have originated at least partly from the surrounding platforms. 5.5. Megabreccias and sea-level cycles According to the comprehensive review of Spence and Tucker (1997) “both rises and falls of relative sea-level can have either casual or direct link to the initiation of gravitational instability” and consequently megabreccia formation. The available data indicated that sea-level falls and lowstands were more favourable for the megabreccia deposition and only a few examples of megabreccias deposited during rising or highstand of relative sea-level were reported. However, in many cases the relationship
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J. Haas / Sedimentary Geology 128 (1999) 51–66
between the megabreccia deposition and the sealevel setting was not determined or satisfactorily proven. Since a solution of this problem would be of crucial importance from the point of view of the sequence analysis, we also attempted to evaluate our observations from this aspect. Two approaches are given below. Based on the evaluation of the facies changes on the gentle northern slope and in the basin at the toe of the steeper southern slope the recognition of the 3rd-order cycles will be discussed first. Then the matrix of the lithoclasts will be evaluated since studies of modern carbonates suggest that maximum shedding of loose silt to arenite-size carbonate particles occurred during highstand of relative sealevel (Droxler and Schlager, 1985; Reijmer et al., 1988; Schlager et al., 1994). Cyclicity was recognised in the studied successions of both the northern and southern slopes. It was very obvious and easily recognisable in the cored sections at Tapolcaf˝o belonging to the northern platform margin and slope (Go´cza´n et al., 1979; Haas and Pa´lfalvy, 1989). Based on the facies analysis, two 3rd-order relative sea-level cycles, superimposed by 4th- and probably 5th-order ones, could be recognised (Fig. 10). Sea-level rise at the beginning of the first 3rd-order cycle led to the inundation of the Ugod High, colonization of the platform biota and initiation of the carbonate factory on the top of the paleohigh. The early evolutionary stage of the platform was followed by a significant platform progradation during the highstand stage of the cycle. The low angle of the northern slope may have favoured rapid progradation. The first cycle may have ended with a sea-level drop which probably resulted in the subaerial exposure of the top of the platform and consequently reduced shedding and interruption of the slope accretion. The next sea-level rise led to the back-stepping of the platform. It resulted in the appearance of the deeper slope facies above the platform and upper slope facies. It was followed by a highstand progradation, superimposed however by 4th- and 5th-order oscillations (retrogradations and progradations) prior to the final drowning. In the Magyarpola´ny Mp-38 core, recognition of cyclicity was more difficult. However, based on the microfacies analysis the cyclic alternation of the depositional facies could be detected (Figs. 5 and 10).
Within the lower member of the Pola´ny Formation two 3rd-order cycles could also be interpreted. The first cycle initiated with a sea-level rise, the same one which resulted in the establishment of the carbonate factory on the Ugod High. This event is probably reflected in the significant increase of the carbonate content in the deepening-upward basinal succession in the transitional interval between the Ja´ko´ and Pola´ny formations, where a neritic pycnodont-bearing facies progresses into a deep pelagic Inoceramus–Globotruncana facies (microfacies F). The first distal debrite interlayers appear in the E microfacies (Fig. 5), most probably subsequent to the maximum flooding. Massive appearance of the typical debrites can be correlated with the predominance of the calcisphaere microfacies (E). It is followed by a segment showing prevalence of the D microfacies in the matrix and rockfall deposits. They may have formed in the highstand stage of the first cycle. Positive correlation between the amount of lithoclasts and the amount of sand-size bioclasts of platform origin appear to support this interpretation (see Fig. 5), since shedding of the platform bioclasts was probably more intense in the highstand periods when the platform was covered by a shallow sea (Droxler and Schlager, 1985). The first debris flow deposits may have formed during the early highstand. This was followed by basinward extension of the toe-of-slope megabreccia aprons during the late highstand. Traces of high-frequency cyclicity were also detectable in this interval. The intercalations of the pelagic F facies, poor in particles of platform origin, may indicate the short-term lowstand and transgressive intervals, whereas layers rich in platformderived bioclasts and also in lithoclasts may correspond to the highstand stages. The second 3rd-order cycle is made up of pelagic carbonates; rudite to arenite-sized grains are missing (Fig. 10). This means that due to retrogradation of the platform only the silt or lutite-sized carbonate mud may have reached the internal part of the basin south to the Ugod High. However, the high carbonate content of the basin sediments indicates survival of the platforms in the immediate neighbourhood. The end of the second cycle is indicated by a significant change in lithology in the form of a remarkable increase in clay and siliciclastic silt content.
J. Haas / Sedimentary Geology 128 (1999) 51–66
65
Fig. 10. Comparison of depositional sequences formed on the northern and southern slopes of the Ugod Platform.
It is worth mentioning that the inferred sea-level cycles discussed above are amazingly similar to those recognized in the Sinai and other correspond-
ing successions in the Middle East and North Africa (Lu¨ning et al., 1998).
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6. Conclusions
References
In the Turonian–early Senonian, a post-orogenic collapse basin dissected by tectonically controlled highs came into being in the western side of the Bakony area. Relative sea-level rise in the Santonian led to step-by-step inundation of the articulated basin giving rise to the formation of isolated platforms on the highs. In the foreland of the steep slope of one of the platforms, megabreccias have been accumulated in remarkable thicknesses. Based on microfacies characteristics of the lithoclasts it can be assumed that they derived from the platform margin, the upper and the lower slope. The clasts were mixed in the course of their redeposition. This means that breccias were formed by intense back-stepping erosion which led to collapse of large parts of the rim of the platform and the foreslope. Lithoclasts formed rockfall talus at the toe of the slope which were surrounded by large, low-angle aprons, site of deposition of periplatform hemipelagic sediments during the tranquil intervals between the collapse episodes. Based on cycle-analysis of continuous core sections it was concluded that the breccia deposition took place within a single 3rd-order depositional cycle. The first debris flow deposits were formed after maximum flooding. Increasing frequency of debris flow deposits and the subsequent appearance of the rockfall breccias indicate the initiation of the slope progradation during the early highstand and continuation thereof during the late highstand. The breccia formation may have also be influenced by the high-frequency cyclicity causing intense meteoric cementation in the platform margin and the assumed synsedimentary tectonic activity contributing to the instability of the slope.
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Acknowledgements Investigations providing the basis of the present paper were carried out by the author at the Hungarian Geological Institute. The work was also supported by the Hungarian Academy of Sciences. The author is indebted to Attila Fogarasi (Budapest) for the discussions on the biostratigraphy.