succession of west central Jordan - stratigraphy and crises
MFT 15 MFT 11
6.3 Cyclicities
vertical burrows fenestral fabrics
hardground and/or ferrigenous crusts
bioclasts
B Strongbioturbation oncoids
ooids oyster
‘patch reef’
intraformational reworking dolomite bed+
iron impregnation thin lamination algal lamination large benthic foraminifers
5 m rudist
‘patch reef’
gastropods
B
B B
LOWER CENOMANIAN Naur Limestone (Member b)Formation / Member Cyclicities of different scale Systems Tracts
Substage
MA 1 16 km MA 2
HSTTSTTSTLST
36°00'
Amman
35°30' 36°30'
Dead Sea
30°30' 32°00'
31°30'
31°00'
0 25 50 km
MA2 MD6
AY1 MA1
B B
TURONIAN Wadi as Sir LimestoneFormation / Member Cyclicities of different scale Systems Tracts
Stage MD6 AY166 km
HSTTST
Lowstand Systems Tract
Legend
Transgressive Systems Tract
TST
Highstand Systems Tract
HST LST
Cenomanian Cycles
Turonian Cycles
Hardgrounds with vertical borings and/or iron crusts (reddish, brownish colours) are predominantly determined as cycle boundaries, but transgressive surfaces are also used to separate shallow cycles (‘HST’) from deeper water deposits (‘TST’).
Therefore, the onset of a transgression is mostly reflected by a sharp boundary between fossiliferous shallow subtidal or intertidal limestones and marly limestones with abundant planktic foraminifers, bioclasts or reworked clasts (e.g. MFTs 8, 10, 11; Fig. 5). On the other hand, fossiliferous limestones, yielding e.g. diverse larger benthic foraminifers, gastropods or rudists imply fully marine, shallow subtidal to shallow intertidal facies (e.g. MFTs 4, 6, 9; Fig. 5) that may reflect HST conditions.
Overlying limestones or dolostones that contain a non-diverse fauna or fenestrate fabrics point to shallow intertidal or supratidal environments (e.g. MFTs 1, 2, 5; Fig.
5) and reflect a relative sea-level lowstand (LST). Between neighbouring sections, major cycle boundaries can be correlated (Fig. 7).
6.3.2. Cyclicities of Turonian platform carbonates
The entire cliff-building unit of the middle/upper Turonian Wadi As Sir Formation exhibits cyclic bedding that differs from Cenomanian platform cycles (mentioned above). Turonian cycles can not be subdivided in dm-scale but exhibit a thickness of a few metres to about 20m. Moreover, different components and sedimentary patterns characterise the Turonian cycles, exemplarily illustrated by two sections from the central and the southern study area (Fig. 7). Bioturbated, bioclastic limestones, with reworked clasts and rudist fragments, and locally oolithic limestones indicate transgressive phases (e.g. MFTs 9-13; Fig. 5), while predominantly thin and/or wavy laminated dolomitic limestones and dolostones characterise highstand deposits on Turonian platforms (MFTs 2, 3, 5; Fig. 5). Thick dolostone successions that include algal laminations reflect deposition in peritidal environments and may be assigned to a relative sea-level lowstand. Cycle boundaries are less obvious than in Cenomanian successions. However, irregular and occasionally bored surfaces or iron crust are also observed within Turonian cycles of different order and are defined as cycle boundaries (Fig. 7). Two main differences are obvious, when comparing the Cenomanian and Turonian platform cycles within the study area: rudist patch reefs prevail in Cenomanian TST’s, while oolithic shoals locally predominate Turonian TST’s. Furthermore, fossiliferous and bioturbated limestones occur in the upper part
of Cenomanian HST’s, but peritidal thin laminated limestones and dolostones predominate among Turonian highstand deposits.
Afra Springs
AF 1
LOWER CENOMANIANSubstage Formation / Member LOWER CENOMANIANNaur Limestone (Member b)Formation / Member Substage
Silla SI1+2
cycle number cycle number
1m 1m
Naur Limestone (Member b)
1 1
2 2
3 3
4 5 4 6 5
6 7
7 8
8 9
9 10
10 11
11
12
12
13
13
14
14 16
16
17
17
18
18
19
19
20
20
21
21
22
22
23
23
24
24
25
25
26
26
27
27
28
28
29
29
30
30
31
31
32 33 34 35 36 37 38 39 40 41 42
36°00' 35°30'
Legend
Amman
36°30' Dead Sea
30°30' 32°00'
31°30'
31°00'
0 25 50 km SI1+2 AF1
AF1 16 km SI1+2
Major surface / ferrigenous crust Correlation of main positive ‘deviation peaks’
Intraformational reworking Hardground / vertical boring
a
a b
c d e g
f h
Fig. 8: Two detailed lower Cenomanian (Naur Formation, Member b; compare Fig. 2) sub-sections of the southern study area (AF 1, SI1+2; compare map), with cm to m-scaled shallowing-up cycles are illustrated. The cumulative deviation from the mean cycle thickness of both sections has been plotted against the cycle number (curves in grey box), following Sadler et al. (1993). These curves are interpreted as accommodation plots (see text).
Comparisons of both curves resulted in seven correlatable positive peaks (dashed lines, ‘a’ –
‘g’), while the uppermost peak (‘h’) is only observed in the plot of section AF1. Within sections AF1 the peaks mainly coincide with reworked beds, while in section SI1+2 the largest positive deviations mainly correlate with surfaces that are marked by dolomitised beds with iron impregnation / red colours, and vertical borings. For explanation of signatures see legend.
6.3.3. Accommodation plots
To verify lateral correlations and to provide sequence stratigraphic interpretations, the accommodation rates of the described cyclicities have been statistically investigated. Cyclically bedded units of the Cenomanian to Turonian succession were measured in detail in several sections of the study area. Following the scheme of Sadler et al. (1993) and Strasser et al. (1999; ‘elementary sequence’), the cycles were counted and the deviation of the mean cycle thickness was plotted. Cyclicities
within the lower cliffs of the Naur Limestone Formation (member ‘b’; Figs. 2, 8) are exemplarily illustrated and described based on four sections of the central and southern study area (WK1, TB1, AF1, SI1+2; Figs. 1, 9).
The cyclically bedded successions of the same interval of the two sections AF1 and SI1+2 (Fig. 8) contain different lithologies and sedimentary patterns. To provide a reproducible definition of cycles, the facies succession (deep to shallow subtidal or subtidal to peritidal; compare e.g. Osleger, 1991; Strasser et al., 1999) from the deepest to the shallowest part has been determined.
In section AF1, reworked layers (packstones with bioclasts, intraclasts or subrounded pebbles) indicate a transgressive phase, while massive, strongly bioturbated limestones mark a relative sea-level highstand. The shallowest part of a cycle is predominantly marked by platy, thinly laminated limestones and dolostones. Cycle boundaries are either defined between these ‘deepest’ and ‘shallowest’ parts or they coincide with ferrigenous crusts on top of the shallowing-up successions (Fig. 8).
In contrast, the cyclic bedding of member b in section SI1+2 (Fig. 8) is reflected by an upward thinning (compare Goldhammer et al., 1993), from about 30 cm thick beds at the base to 5 cm thick layers on top. Massive limestones or dolostones represent
‘deep’ cycle parts, while thin lamination and reworking patterns mark the shallower units. Cycles in section SI1+2 are mostly topped by hardgrounds, iron-crusts, and vertical borings, respectively. These surfaces reflect the cycle boundaries.
Cycles of both sections were counted and the deviation from the mean cycle thickness was plotted against the cycle number (Fig. 8). A positive divergence from the mean cycle thickness reflects higher accommodation rates, whereas negative deviations exhibit lower accommodation rates (e.g. Goldhammer et al., 1993;
Strasser, 1999).
A comparison of both curves exhibits similar patterns, such as several positive peaks of accommodation rate and rhythmic divergences from the mean cycle thickness (Fig. 8). Seven main peaks have been correlated between both plots (‘a-g’, Fig. 8).
Moreover, enveloping curves have been graphically created (touching all ‘positive peaks’ on the left side of the curve; Fig. 9) and applied for comparison of two other sections (TB1, WK1; Figs. 1, 9).
In general, the absolute cycle number decreases toward the south from 80 cycles in section WK1 to 31 cycles in section SI1. Nevertheless, seven curve maxima,
mentioned above, can be correlated between all four plots, and an additional peak is related to the topmost cycles of the sections WK1, TB1 and AF1 (peak ‘h’, Fig. 9).
WK1 TB1 AF1 SI1
cycle number
cumulative deviation from mean cycle thickness (m)
Formation / MemberNaur Limestone (Member b) Lower CenomanianSubstage 26km
17km 19km
36°00'
Amman
35°30' 36°30'
Dead Sea
30°30' 32°00'
31°30'
31°00'
0 25 50 km
SI1+2 AF1
WK1 TB1 a
b c e f g
d h
a b c e f g
d h
Legend
a
f cumulative deviation from
mean cycle thickness enveloping curve (touching all
‘positive peaks’)
enveloping curve (touching the eight correlated ‘positive peaks’; a-h) correlation of eight ‘positive peaks’
correlation of enveloping curve maxima flexure points (FPs) of enveloping curve Correlation of FPs / Sequence boundary CeJo1
WK1 TB1 AF1 SI1
Naur Limestone (Member b) Lower Cenomanian
? CeJo1 cyclic
set
A
B
CeJo1
a b c e f g
d h
Fig. 9A: The accommodation plots of sections AF1 and SI1+2 (see Fig. 8) are compared with two plots of a section of the southern (TB1) and the central study area (WK1, compare map on the right). Eight peaks of positive deviation from the mean cycle thickness (compare Sadler et al., 1993; see Fig. 8) are correlated (thin dashed lines), while the two main peaks are accentuated (peaks ‘c’ and ‘f’). A higher-ranking trend in accommodation fluctuation is indicated for each section (dotted curve), created by graphic correlation of an enveloping curve on the left (‘positive’) side of the accommodation plot.
9B: A second enveloping curve is created (thick dashed line; graphic correlation of the eight main positive peaks ‘a’ to ‘h’) and these higher-ranking accommodation trends are compared for the four sections of Figure 9A. Two cyclic sets (Ι+ΙΙ) can be subdivided, resulting from the second enveloping curve and each containing a rising accommodation trends, a maximum, and decreasing accommodation. Furthermore, each cyclic set includes four of the defined
‘positive accommodation peaks’, a flexure point of the second enveloping curve (behind the two accommodation maxima), and a coinciding sequence boundary (CeJo1, CeJo1Ι). For explanations of signatures see legend.
After connecting these eight maxima by another enveloping curve (Fig. 9B), two main accommodation maxima and one minimum are visible, corresponding with two major
‘cyclic sets’ (Fig. 9). ‘Cyclic set Ι’ exhibits about double thickness compared to ‘cyclic set ΙΙ’ (Fig. 9B). Moreover, both sets include a curve maximum, as well as four main accommodation peaks (peak a-d, set Ι; peak e-h, set ΙΙ; Fig. 9B). The enveloping curves contain two flexure points (behind positive maxima; Fig. 9B), the upper one coinciding with sequence boundary (SB) CeJo1, separating the ‘pre-CeJo1 HST’
from the overlying transgressive deposits (Figs. 2, 9B).
6.3.4. Discussion
Lateral correlation of accommodation plots exhibits coincidences between major trends but also differences concerning the higher-frequent accommodation changes.
Considering, the discussion about use and interpretation of ‘Fischer plots’ (e.g. by Sadler et al., 1993; Boss and Rasmussen, 1995; Grötsch, 1996) the factors controlling platform sedimentation, possible mistakes and restrictions for the applicability of accommodation plots can be estimated.
After Sadler et al. (1993), about 50 cycles are required for an accommodation plot with a representative mean cycle thickness. Lower cycle numbers may produce a statistical mistake that cannot be excluded for the studied accommodation plots based on cycle numbers between 31 and 80.
Furthermore, the problem of ‘missing beats’ (e.g. Balog et al., 1997) implies that not all relative sea-level fluctuations are recorded on the shallow platform and that condensation and reworking resulted in incomplete plots and modified accommodation rates. The southward decrease of cycle numbers, mentioned above, is probably related to an amalgamation of cycles towards the continent.
The completeness of cycles and a primary accommodation rate are, among others, the prerequisites for discussing ‘Fischer Plots’ as accommodations plots. Other basic assumptions, like neglecting of compaction and subsidence (e.g. Fischer, 1964;
Goldhammer et al., 1993) are probably not fulfilled in this study.
The question if ‘Fischer Plots’ are equivalent to accommodation plots and if they can be interpreted as sea-level curves, is discussed by e.g. Boss and Rasmussen (1995).
However, facies changes from deeper (e.g. subtidal) environments to shallow (e.g.
supratidal) facies belts reflect relative fluctuations of the water depth. Therefore, the single units of each cycle are equated with systems tracts (mentioned above) and if
we assume that the position of SB CeJo1 coincides with the flexure point within
‘cyclic set ΙΙ ’ (mentioned above), the flexure points within cyclic set Ι may point to an additional SB within the lower Cenomanian Naur Formation (CeJo1 Ι, Fig. 9B).
Considering the mentioned possible sources of error, the following controlling factors for sedimentation and accommodation on the shallow shelf are discussed.
Autocyclic control
Peritidal shallowing-up cycles imply after Ginsburg, (1971), Goldhammer et al.
(1993), Grötsch (1996) and Strasser et al. (1999) a successively filling of the available accommodation space within shallow subtidal areas and therefore autocyclic mechanisms. Carbonate production prevails in this case during a relative sea-level rise. This shallowing is related to gradual progradation of tidal flat facies belts. Coevally, the carbonate production decreases, while subsidence persists.
Thus, after a period of non-deposition, a deepening and an increasing productivity result and a new peritidal cycle begins (Ginsburg, 1971; Strasser, 1991; Pratt et al., 1992). These mechanisms effect vertical facies changes and induce lateral shifts of facies belts. Moreover, cycles that contain a peritidal unit are most probably affected by erosion and/or reworking, as well as by diagenetic alteration and compaction (e.g.
Grötsch, 1996).
Allocyclic control
Subtidal cycles that do not contain a peritidal unit and which are separated by a rapid deepening on top of each cycle, indicate after Osleger (1991) and Goldhammer et al.
(1993) a lack of shoreline progradation and an allocyclic control of shelf deposition.
Allocyclic factors may be tectonical movements (subsidence) or eustasy (e.g.
Goldhammer et al., 1993; Strasser et al., 1999; Grötsch, 1996). Moreover, previous authors often discussed higher-frequent cycles as a result of Milankovitch-driven mechanisms (‘orbitally forced’; e.g. Strasser, 1994; Balog et al., 1997; Gale et al., 2002). Eustatic oscillations, triggered by orbital mechanisms could produce cycles with a duration of about 400 ky, 100 ky, 41 ky or 23 ky to 19 ky (e.g. Fischer, 1991).
Peritidal cycles seem to predominate the investigated cyclic successions and imply an autocyclic control. Nevertheless, occurrences of subtidal cycles, and the uniform subdivision into cyclic sets over tens of kilometres on the platform reflect predominating allocyclic control. Increasing and decreasing subsidence have to be considered as possible controlling factors, but the bundling of accommodation changes imply eustasy as prevailing allocyclic control.
We calculated the average duration of the single analysed cycles to test a possible Milankovitch-control. In sections of the central and southern study area a lower Cenomanian age is attributed to the members ‘a’ and ‘b’ of the Naur Formation (Powell, 1989; Schulze et al., 2003). If we follow the scheme of Schulze et al. (2003), sequence boundary (SB) CeJo1 on top of member b (Naur Formation, Figs. 2, 9B) is comparable with SB Ce 2 of Haq et al. (1987). Compared to the standard chronostratigraphy of Gradstein et al. (1995) the age of Ce 2 is 97.4 Ma. Moreover, this time scale defines an age of 98.9 Ma for the base of the lower Cenomanian.
Therefore, we can calculate a time span of 1.5 Ma for the members ‘a’ and ‘b’ of the Naur Formation and an average of 750.000 y for each member. Considering that each cycle of member b represents the identical time, the resulting average duration of a cycle varies from 9.400 y to 24.200 y per cycle (variations are related to the different cycle numbers of each section).
We assumed an amalgamation of cycles from north to south on the platform as an explanation for the decrease of cycle numbers within the single sections (Fig. 9). It follows that the calculation of about 9.400 y / cycle for the section with the highest cycle number (section WK1, Fig. 9) is probably more precise than the longer time intervals for cycles of sections further south. Therefore, the measured cycles of member b (Naur Formation) do not reflect Milankovitch-forcing because shortest
‘orbital cycles’ span about 19.000 to 23.000 y (precession).