Eustatic implications of late Miocene depositional sequences in the Melilla Basin, northeastern Morocco

Sedimentary

Geology ELSEVIER

Sedimentary

Geology

107 (1997) 147- 165

Eustatic implications of late Miocene depositional sequences in the Melilla Basin, northeastern Morocco Kevin J. Cunningham

a.*, Richard H. Benson b, Kruna Rakic-El Bied ‘, Larry W. McKenna a

aDepartment

of Geology, University of Kansas, 120 Lindley Hall, Lawrence, Kansas 66045, USA ‘Smithsonian Institution, U.S. National Museum, Washington, D.C. 20560, USA ’Smithsonian Institution, 12 Avenue de France, Rabat, Morocco Received

18 August

1995; accepted

28 May 1996

Abstract The age (-5.78 Ma or lower chron C3r) of the major drawdown of the Paleo-Mediterranean Sea during the Messinian Salinity Crisis has been established by combining results from stratigraphy, paleontology, magnetostratigraphy, and argon dating for a late Miocene sedimentary succession in the Melilla Basin, NE Morocco. This event is inferred from a marine-to-continental series of carbonate and siliciclastic rocks that record the end of Messinian marine deposition in the Melilla Basin and presumably marks the final isolation of the Paleo-Mediterranean Sea. The evidence from the Melilla Basin is approximately coeval with an increase in benthic foraminiferal S’*O values from a deep-marine section in the Bou Regreg valley, NW Morocco (Hodell et al., 1994). This increase suggests that a glacio-eustatic lowering of sea level, at least, contributed to the final closure of the Mediterranean during the Messinian Salinity Crisis. The marine-to-continental succession onlaps a carbonate complex that contains evidence for multiple relative sea-level changes leading up to the main drawdown. From bottom to top, the carbonate complex is composed of: (1) an onlapping ramp; (2) a prograding bioclastic platform; (3) a prograding and, locally, downstepping Porites-reef complex; and (4) a topography-draping sequence composed of grainstones, Porites reefs, and stromatolites (terminal carbonate complex of Esteban, 1979). The transgressive ramp correlates to relatively low values of benthic foraminiferal 6180 values from a Tortonian-tolower Messinian section at Bou Regreg (Hodell et al., 1994). This correlation indicates, at least in part, a link between rising sea level and a reduction in global ice volume during deposition of the ramp. A major fall in relative sea level (-60 m) occurred near the demise of the reef complex during chron C3n. In at 5.95 f 0.10 Ma. This signals the initiation of drawdown and changing environmental conditions in the Melilla Basin (a marginal basin), and perhaps the entire Paleo-Mediterranean Sea. A megabreccia interpreted as forming by solution collapse of evaporites on the basin margin of the reef complex occurs at the base of the terminal carbonate complex. Updip, a major subaerial unconformity separates the reef complex and terminal carbonate complex. Evaporite deposition likely occurred during this exposure event and has been dated at 5.82 f 0.02 Ma near the base of chron C3r. We contend that these evaporites, restricted to the shallow Melilla Basin, are related to the continuation of the initial stage of the major drawdown of the Paleo-Mediterranean Sea. Keywords:

Messinian

*Corresponding Geophysics,

Salinity Crisis; Morocco;

Melilla Basin; late Miocene;

eustacy; depositional

author. Current address: Rosenstiel School of Marine and Atmospheric Science,

University

of Miami, 4600 Rickenbacker

0037-0738/97/$17.00 Copyright PZZ SOO37-0738(96)00037-l

Causeway,

sequences

Division of Marine Miami, Florida 33149, USA. Fax: +l 305 361-4094.

0 1997 Elsevier Science B.V. All rights reserved.

Geology

and

148

K.J. Cunningham

et nl./Srdimenta~

1. Introduction It has long been debated whether glacio-eustatic lowering of sea level (Ruggieri, 1967; Hsti et al., 1973a,b; Cita and Ryan, 1979; Mtiller and Hsti, 1987; Hodell et al., 1994) or tectonic induced narrowing of the Atlantic-Mediterranean seaways (Ruggieri, 1967; Hsi.i et al., 1973b; Weijermars, 1988; Benson and Rakic-El Bied, 1991a), or even a combination of the two mechanisms (Kastens, 1992) caused the isolation and consequent desiccation of the Paleo-Mediterranean Sea during the Messinian (the ‘Messinian Salinity Crisis’). The prevailing idea is that an increase in global ice volume reduced sea level and contributed to the formation of a sill between the Atlantic and Paleo-Mediterranean Sea. Without a source of recharge of oceanic waters from the Atlantic, excess evaporation produced desiccation of the restricted basin. A test of this hypothesis would be to document evidence that demonstrates a corresponding age for the evaporites produced by evaporation and a eustatic lowering. The purpose of this study was to identify relative sea-level changes during the time leading up to and during the Messinian Salinity Crisis in a late Miocene succession of mainly carbonate deposits at the proximal end (Melilla Basin) of the last connection (Rifian Corridor) of the Paleo-Mediterranean Sea with the Atlantic (Fig. 1). An attempt was then made to correlate these changes to: (1) Jr*0 variations (Hodell et al., 1994), a proxy for global sea-level changes, from a deep-marine section (Bou Regreg valley) located at the Atlantic threshold of the Rifian Corridor (Fig. 1); and (2) the Sorbas Basin, which has a similar history of basin fill and situated on the western margin of the Mediterranean Basin (Fig. 1). The deposits from the Melilla Basin, which include coral reefs, in NE Morocco are punctuated by ash falls from nearby acid volcanoes. The Melilla Basin deposits left one of the few well-dated records (Cunningham et al., 1994; Cunningham, 1995) of relative sea-level changes (recorded on a relative sea-level curve) just before the main Crisis ‘drawdown’. sequence stratigraphy, biostratigraphy, Using magnetostratigraphy, and 40Ar/39Ar dating the relative sea-level history of the Melilla Basin has been precisely dated (Cunningham et al., 1994; Cunning-

Geology 107 (1997) 147-165

ham, 1995). It is now possible to compare the local evidence to other areas of the Mediterranean Basin, such as the Sorbas Basin. This is the first time that well-dated evidence of relative sea-level changes, based on local outcrop studies (Melilla Basin) from within the Paleo-Mediterranean Sea, have been correlated with S’*O variations (Hodell et al., 1994) from outside the Paleo-Mediterranean Sea (Bou Regreg). Results suggest global ice-volume changes influenced specific local events in the Melilla Basin during the Messinian Salinity Crisis. The strength of these data is that they are from the straits (Rifian Corridor), which formed the connection between global and regional changes (Fig. 1). 2. Methods 2. I. Field methods Thirty-six detailed measured sections, mapping of strata1 geometries, facies associations, depositionalsequence boundaries on topographic maps and photo mosaics, and petrography formed the foundation for the stratigraphic results presented herein (Cunningham, 1995). A relative sea-level curve was developed by estimating accommodation space using physical measurements of strata1 and surface geometries, facies relationships, and, where possible, quantitative methods described by Goldstein and Franseen (1995). 2.2. Analytical

methods

2.2.1. Magnetostratigrphy The reader is referred to Cunningham et al. (1994) and Cunningham (1995) for a detailed description of the magnetostratigraphic methods used in this study. 2.2.2. 40Ar/‘YAr dating Nine volcanic rock samples were dated for this study. The Ar-isotopic measurements were conducted at the New Mexico Geochronological Research Laboratory. Sanidines were separated from crushed samples using standard heavy liquid, magnetic, and hand-picking techniques. Approximately 30 crystals were wrapped in aluminum foil and sealed in evacuated quartz tubes along with interlaboratory standard Fish Canyon Tuff sanidine (27.84

K.J. Cunningham

et al./Sedimentary

Geology 107 (1997) 147-165

qdorhotsene \

0

Undifferentiated

Neogene

a Fringing-beefs

fl*

-wQ’p /; ++!!S Y

Platfoim Ra

Ap

Tertiary Volcanic Rot :ks Clastic Wedge Metamorphic Rocks Fig. 1. Location and generalized geologic map of an Upper Miocene carbonate complex in the Melilla Basin, NE Morocco. The carbonate complex developed on the south flank of a paleohigh composed of Paleozoic(?) metamorphic rocks, a Tertiary volcanic massif, and Serravallian(?)-to-Tortonian sedimentary rocks (the elastic wedge). A carbonate ramp underlies a carbonate platform and fringing reefs (Fig. 2). Arrows show the direction of progradation of the fringing reefs. Solid squares designate localities where volcanic rock samples were collected for argon-age analyses. The border separating Morocco and Spain (Melilla) is shown as a bold dashed line. Location of Section FA-1 and volcanic rock sample #FA-1 (Fig. 2) is shown by a solid-black square. The map of the Melilla Basin has been modified after Guillemin et al. (1983). The inset map, modified after Benson et al. (1991). shows the late Miocene paleogeography of the western Mediterranean and location of the Melilla and Sorbas Basins, and Bou Regreg valley.

Ma). The standard was used to monitor the neutron dose received during the 24-h irradiation in the L-67 position of the University of Michigan Ford reactor. Following irradiation, the samples were unpacked and individual crystals were placed in holes drilled in a copper disk. In general ten sanidine crystals

(-2-5 mg) for each rock sample and flux monitors were fused within an ultra-high vacuum argon extraction system with a 1OW Synrad CO2 continuous laser. Evolved gases were purified for two minutes using a GP-50 SAES getter operated at f450”C and a SAES ST-172 getter operated at room temperature.

K.J. Cunningham et al. /Sedimentary

150 Table 1 Isochron ages and uncertainties

for volcanic

Sample number

n

V4a V4 v3 v2 V2 VI VI Izarorene Sammar

FA-1 B-l IF-l-42.9 4- 19-4 93-8 93-2 93- 1 IR-1 .O SM-84

13 11 12 7 8 10 7 10 12

used for calculating

isochron

107 (1997)

147-165

ash samples from the Melilla Basin

Volcanic ash

Parameters

Geology

Isochron

Uncertainty (1 sigma)

40136

age

Uncertainty (I sigma)

5.79 5.82 5.95 6.68 6.12 7.0 7.0 6.86 6.9

f f It zt * zt + It i

280 328 291 223 380 423 244 261

i 15

ages are explained

0.02 0.02 0.10 0.02 0.02 0.14 0.14 0.02 0.2

261

i f i f f f f f

18 10 57 35 40 40 17 5

in Section 2

Argon-isotopic compositions were determined with a Map Analyzer Products 215-50 mass spectrometer. Blanks were measured numerous times throughout the course of the analyses and were very reproducible. Typical blanks (including machine backgrounds) were 15, 0.3, 0.05, 0.7, and 0.4 x 10-l’ moles as masses 40, 39, 38, 37, and 36, respectively. J-factors were determined to 50.25% by fusing five separate two-crystal aliquots from seven vertical positions over the 4.5 cm irradiation geometry. The flux gradient over the sample interval was less than 0.5%. Correction for interfering nuclear reactions were determined using KzSO4 glass and CaF2. These values are: (40Ar/39Ar)k = 0.0225 f 0.0007, (36Ar/37Ar)ca = 0.00027 + 0.00002, and (39Ar/37Ar)c, = 0.00070 f 0.00002. Reported ages and precision of 40Ar/39Ar-dated volcanic rocks differ slightly in this paper relative to those reported in Cunningham et al. (1994). Ages reported in this paper (Table 1) are isochron ages, whereas those reported in Cunningham et al. (1994) were reported as integrated ages. We changed from integrated to isochron ages because isochron ages are more accurate. Since individual crystals were fused in a single heating step, isochron ages are reported. Integrated ages are more typically reported when a ‘step-wise’ heating of crystals is employed. Isochron ages are produced by plotting results from individual rock samples (e.g. 10 independent fusions) on a 36Ar/40Ar vs 39Ar/40Ar plot. The x-intercept of the ‘best fit’ line through the resulting data array is proportional to the age of the sample (Phillips and Onstott, 1986; Dalrymple et al., 1988). All errors are

reported at the 1 sigma confidence level, calculated from the uncertainty in the x-intercept (Table 1). Detailed results of single crystal laser fusion of sanidine crystals for 40Ar/39Ar dating are shown in Table 2. 3. Geologic setting The Melilla Basin is a post-erogenic basin in which marine deposition began possibly as early as Serravallian or, at latest, during the Tortonian. Guillemin and Houzay (1982) recognized four distinct tectonic phases in the Melilla Basin. The first is an episode of extension that included deposition of conglomerates, marls, siltstones, and sandstones, referred to as the ‘elastic wedge’ (Cunningham et al., 1994) and rhyolitic volcanism at Trois Fourches at the north end of the basin (Figs. 1 and 2). Although Guillemin and Houzay (1982) assign a Tortonian age to this event, their discussion implies the possibility of a Serravallian age. The second phase is a Tortonian compressive stage that steeply tilted beds of the elastic wedge during uplift of the metamorphic core of the Tarjat anticline (Fig. 1) and probably caused left-lateral movement along the Dchar-Rana fault (Fig. 1). The third phase is a ‘Messinian’ extensional episode which is coincident with deposition of a thick succession of shallow-marine carbonates, marls, and siliciclastics, with acidic volcanism at Trois Fourches and alkaline volcanism at a stratovolcano called Gourougou (Fig. l), and with southwestward termination of the Dchar-Rana fault by a north-trending normal fault. This normal fault par-

K.J. Cunningham

Table 2 Results of single-crystal Melilla Basin Sample

36Ar/39Ar x 10-s

Power

ov (FA-1) V4a volcanic 1640-O 1 1640-02 1640-03 1640-04 1640-05 1640-06 1640-07 1640-08 1640-09 1640-10 1640-l 1 1640-12 1640-13 (B-l) V4 volcanic 1.7 283-OlA 0.3 283-02A 1.7 283-02B 1.7 283-03A 1.7 283-04A 1.7 283-05A 1.7 283-06A 1.7 283-07A 1.7 283-08A 1.7 283-09A 1.7 283-10A (IF-l-42.9) 896-01 896-02 896-03 896-04 896-05 896-06 896-07 896-08 896-09 896-10 896-11 896-12

37Ar/39Ar x 10-a

heating)

for 40Ar/39Ar

40Ar/39Ar

Geology 107 (1997) 147-165

dating of sanidine

39Ar/40Ar

36Ar/40Ar x 10-4

ash (Figs. 1 and 2). Samples 1 to 5 are single sanidine crystals; 6.6 0.2427 1.67 4.1200 24 2.3 0.2304 1.55 4.3400 98 0.1 0.2299 2.00 4.3500 6 0.002 0.2439 1.54 4.1000 1 0.2 0.2463 1.62 4.0600 8 0.003 0.2463 1.39 4.0600 1 0.1 0.2463 1.60 4.0600 5 0.2 0.2457 1.58 4.0700 8 1.7 0.2347 1.56 4.2600 71 0.2445 0.004 1.61 4.0900 2 0.2373 0.1 1.90 4.2100 4 0.2415 0.1 1.53 4.1400 3 0.2457 0.004 1.63 4.0700 1

ash (Figs. 1 and 2). Single-crystal sanidine; 1.63 0.8934 21 1.68 0.9102 32 1.62 0.8332 2 1.57 0.8286 2 1.62 0.8320 2 1.60 0.8311 3 1.64 0.9466 7 1.64 0.8425 3 1.62 0.8389 2 1.59 0.8355 3 1.62 0.8307 2

V3 volcanic

(4-19-4) V2 volcanic 1.7 1.7 1.7 1.7 1.7 1.7 1.7 1.7 1.7 1.7

285-OlA 285-02A 285-03A 295-04A 295-05A 285-06A 285-07A 285-08A 285-09A 285-10A

laser fusion (single-step

et al./Sedimentary

ash (Figs. 1 and 2). Single-crystal 1.55 5.2000 286 1.66 4.9000 190 1.71 4.6200 70 1.65 4.7900 150 1.76 4.4600 21 1.28 6.5500 664 1.75 4.4000 -91 1.67 4.5300 -87 1.43 4.4700 4 1.46 4.4500 13 1.49 5.3800 250 1.75 5.4600 382

J = 0.004017 1.1193 1.0987 1.2002 1.2069 1.2019 1.2032 1.1812 1.1869 1.1920 1.1969 1.2038

2.4 3.5 0.2 0.2 0.2 0.4 0.8 0.4 0.2 0.4 0.2

sanidine; J = 0.0007658 0.1923 5.5 0.2041 3.9 1.5 0.2165 0.2092 3.1 0.2242 0.5 0.1527 10.1 0.2273 -2.1 0.2208 -1.9 0.2237 0.1 0.2247 0.3 0.1959 4.6 0.1832 7.0

ash (Figs. 1 and 2). Single-crystal sanidine; J = 0.004024 1.0442 0.6 1.78 0.9577 6 1.0562 0.3 1.I2 0.9468 3 1.0452 0.5 1.76 0.9568 5 1.0551 0.3 1.67 0.9478 3 1.0535 0.1 1.77 0.9492 1 1.0410 0.1 1.74 0.9606 1 1.0525 0.3 1.76 0.9501 3 1.0516 0.3 1.73 0.9509 3 1.0409 0.3 1.79 0.9607 3 1.0461 0.2 1.71 0.9559 2

crystals

KJCa

151

from volcanic

40Ar* /39Ark

ash samples

40Ar”

Age (Ma)

f S.D.

97.9 92.9 99.2 99.5 99.0 99.5 99.2 99.0 94.7 99.4 99.3 99.3 99.5

5.79 5.78 6.19 5.86 5.79 5.79 5.78 5.77 5.78 5.84 5.99 5.90 5.80

0.02 0.04 0.04 0.04 0.03 0.02 0.02 0.02 0.02 0.01 0.02 0.01 0.02

(%) _I= 0.000706 30.6 4.0335 32.9 4.0319 25.5 4.3152 33.1 4.0795 31.5 4.0194 36.7 4.0397 31.9 4.0275 32.3 4.0293 32.7 4.0342 31.7 4.0655 26.9 4.1805 33.4 4.1110 31.3 4.0497

from the

31.3 30.4 31.5 32.5 31.5 31.9 31.1 31.1 31.5 32.1 31.5

0.9104 0.7958 0.8047 0.8004 0.8044 0.8003 0.8050 0.8120 0.8105 0.8065 0.8042

90.7 87.4 96.6 96.6 96.7 96.3 95.1 96.4 96.6 96.5 96.8

5.86 5.76 5.82 5.79 5.82 5.79 5.92 5.98 5.86 5.84 5.82

0.02 0.01 0.02 0.02 0.01 0.02 0.02 0.02 0.02 0.02 0.03

32.9 30.7 29.8 30.9 29.0 39.9 29.2 30.6 35.7 35.0 34.3 29.2

4.3368 4.3169 4.3936 4.3163 4.3797 4.5654 4.6464 4.7656 4.4387 4.3922 4.6214 4.3134

83.4 88.1 95.1 90.3 98.2 69.7 105.6 105.2 99.3 98.7 85.9 79.0

5.91 5.98 5.98 5.97 5.96 6.21 6.33 6.48 6.04 5.98 6.30 5.87

0.05 0.07 0.04 0.08 0.08 0.14 0.15 0.15 0.11 0.09 0.12 0.07

28.7 29.7 29.0 30.6 28.8 29.3 29.0 29.5 28.5 29.8

0.9175 0.9166 0.9216 0.9189 0.9241 0.9361 0.9197 0.9212 0.9301 0.9297

95.8 96.9 96.3 96.9 97.4 97.4 96.8 96.9 96.8 97.2

6.65 6.64 6.69 6.66 6.70 6.78 6.66 6.68 6.74 6.74

0.02 0.01 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02

152

K.J. Cunningham

et aL/Sedimentary

Geology

107 (1997)

147-165

Table 2 (continued)

Sample

Power

36Ar/39Ar x 10-s

n-v (93-S) V2 volcanic

287-OlA 287-02A 287-03A 287-04A 287-05A 287-06A 287-07A 287-08A 287-09A 287-10A

1.7 1.7 1.7

1.7 1.7 1.7 1.7 1.7

1.7 1.7

ash (Figs. 6 10 3 6 14 6 5 3 3 170

37Ar/3qAr x lo-’

“0Ar/39Ar

39Ar/4QAr

36Ar/40Ar x 10-1

WCs

“OAr /39Ark

hoAr‘

Age

=k S.D.

(7@‘0) (Ma)

1 and 2). Single-crystal 1.79 1.68 I .74 1.71 1.93 1.77 1.79 1.84 1.72 3.85

sanidine; J = 0.004025 0.6 0.9656 1.0356 1.1 0.9416 I .0620 0.3 0.9558 1.0462 0.6 0.9699 1.0310 I .4 1.0004 0.9996 0.3 0.958 1 1.0437 I .0392 0.S 0.9632 0.9636 1.0378 0.3 0.9607 1.0409 0.3 1.6355 0.6114 10.4

(93-2) Vl volcanic 1.7 288.OIA 2.0 288-02A 2.0 288-03A 2.0 288-04A 2.0 288.05A 2.0 288-06A 2.0 288-07A 2.0 288-08A 2.0 288-09A 2.0 288-1OA

ash (Figs. 1 and 2). Single-crystal sanidine: 0.9906 3 0.87 0.9637 2 0.87 0.988 1 5 0.88 0.9852 2 0.88 0.9984 12 0.93 0.9804 2 0.84 0.9740 2 0.88 0.9765 4 0.85 0.9744 3 0.86 0.9960 5 0.85

(93-l) VI volcanic 1.7 284-OlA 1.7 284-02A 1.7 284-03A 1.7 284-04A 1.7 284-05A 1.7 284-06A 1.7 284-07A 1.7 284-08A 1.7 284-09A 1.7 284-10A

ash (Figs. 1 and 2). Single-crystal sanidine; 0.9803 7 0.93 0.9845 2 0.95 0.9694 3 0.88 0.9953 13 0.89 0.9760 3 0.88 0.9694 1 0.87 0.9785 2 0.86 0.9798 1 0.86 0.9674 2 0.88 0.9655 2 0.90

28.5 30.4 29.3 29.8 27.9 29.8 28.5 27.7 29.7 13.3

0.9268 0.9200 0.9268 0.9322 0.939 I 0.9287 0.9288 0.9349 0.9298 1.1125

96.0 94.7 97.0 96. I 93.9 96.9 96.4 97.0 96.8 68.0

6.72 6.67 6.72 6.76 6.81 6.73 6.73 6.78 6.74 8.06

0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.78

1.0241 1.0263 I .0040

0.3 0.2 0.5 0.2 1.2 0.2 0.2 0.4 0.3 0.5

58.7 58.7 58.0 58.0 54.9 60.8 58.0 60.0 59.3 60.0

0.9587 0.9360 0.9513 0.9562 0.9405 0.9513 0.9449 0.9442 0.9444 0.960 1

96.8 97.1 96.3 97. I 94.2 97.0 97.0 96.7 96.9 96.4

6.95 6.78 6.89 6.93 6.81 6.89 6.85 6.84 6.84 6.96

0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02

J = 0.004021 1.0201 1.0157 1.0316 1.0047 1.0246 1.0316 I .0220 I .0206 1.0337 1.0357

0.7 0.2 0.3 1.3 0.3 0.1 0.2 0.1 0.2 0.2

54.9 60.0 58.0 57.3 58.0 58.7 59.3 59.3 58.0 56.7

0.9372 0.957 I 0.9386 0.9339 0.9450 0.944 I 0.9497 0.9556 0.9385 0.9389

95.6 94.2 96.8 93.8 96.8 97.4 97.1 97.5 97.0 97.2

6.79 6.93 6.80 6.76 6.84 6.84 6.88 6.92 6.80 6.80

0.02 0.02 0.02 0.04 0.02 0.02 0.02 0.02 0.01 0.02

.I = 0.004024

I .0095 1.0377 1.0120 1.0150

1.0016 1.0200

1.0267

(IR-1 .O) lzarorene 2.6 724-01 2.6 724-02 2.6 724-03 2.6 724-04 2.6 724-0.5 2.6 724-06 2.6 724-07 2.6 724-08 2.6 724-09 2.6 724-10

volcanic ash (Fig. 5). Single-crystal sanidine; J = 0.0009112 4.2438 0.2356 0.2 10 0.90 4.2478 0.2354 0.6 24 1.13 4.3054 0.2323 0.9 35 0.98 4.3076 0.2321 0.9 36 0.89 4.2241 0.2367 0.3 12 0.94 4.43 18 0.2256 1.9 83 1.18 4.4768 0.2234 2.3 103 0.95 4.3941 0.2276 1.5 65 0.92 4.2789 0.2337 0.6 26 0.98 4.3621 0.2292 1.5 65 1.03

56.7 45.2 52.1 57.3 54.3 43.3 53.7 55.5 52. I 49.6

4.1924 4.1552 4.1822 4.1612 4.1679 4.1655 4.1500 4.1816 4.1818 4.1400

98.8 97.8 97.1 97.1 98.7 94.0 92.7 95.2 97.7 95.1

6.88 6.82 6.86 6.86 6.84 6.84 6.81 6.86 6.86 6.79

0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02

(SM-84) Sammar 2.6 725-01 2.6 725-02 2.6 725-03 2.6 725-04 2.6 725-05 2.6 725-06

volcanic ash (Fig. 5). Single-crystal sanidine; J = 0.0009120 4.3477 0.2300 12.4 540 0.82 4.4393 0.2253 21.9 970 0.95 4.493 1 0.2226 23.8 1070 0.83 4.4770 0.2234 20.1 900 0.83 4.3947 0.2275 14.8 650 0.86 4.2933 0.2329 5.6 240 0.82

62.2 53.7 61.5 61.5 59.3 62.2

4.1682 4.1325 4.1544 4.1883 4.1819 4.2025

95.9 93.1 92.5 93.5 96.2 97.9

6.85 6.79 6.82 6.88 6.87 6.90

0.01 0.01 0.02 0.02 0.02 0.02

K.J. Cunningham

et al./Sedimentary

Geology

107 (1997) 147-165

153

Table 2 (continued)

Sample

725-07 725-08 725-09 725-10 725-l 1 725-12 725-13

36Ar/39Ar x 10-S

37Ar/39Ar x lo-’

40Ar/39Ar

0V 2.6 2.6 2.6 2.6 2.6 2.6 2.6

5420 330 310 220 150 340 710

0.95 0.81 0.86 0.85 0.84 0.83 0.93

5.7776 4.2976 4.2852 4.2268 4.2580 4.2756 4.3563

Power

40Ar* is radiogenic

argon, 39Ark is K-derived

39Ar/40Ar

0.1731 0.2327 0.2334 0.2366 0.2349 0.2339 0.2296

36Ar/40Ar x 10-4 93.8 7.7 7.2 5.2 3.5 8.0 16.3

WCs

60.0 63.0 59.3 60.0 60.8 61.5 54.9

40Ar’ /39Ark

4.1555 4.1792 4.1712 4.1409 4.1909 4.1530 4.1260

40Ar’

Age

WI

(Ma)

71.9 97.2 97.3 98.0 98.4 97.1 94.7

6.82 6.86 6.85 6.80 6.88 6.82 6.78

f S.D.

0.03 0.02 0.02 0.03 0.03 0.03 0.04

39Ar.

allels the upper reach of Irhzer Amekrfine (Fig. 1; Guillemin and Houzay, 1982). Results from biostratigraphy, magnetostratigraphy, and 40Ar/39Ar dating of this study combined with dating reported by Guillemin and Houzay (1982) indicate that this extensional, basin-filling phase began during the Tortonian. The fourth phase consists of renewed compression, probably in the late Pliocene. This event produced reverse faulting that offset basaltic lava flows at the Miocene-Pliocene boundary near the village of Farkhsna (Fig. 1; Guillemin and Houzay, 1982). Building on the stratigraphic framework presented earlier for the Melilla Basin by Choubert et al. (1966), Guillemin and Houzay (1982), and Saint Martin et al. (1991), detailed results of the senior author’s field studies (Cunningham, 1995) show the basic stratigraphic pattern of a Neogene carbonate complex (Figs. l-3). It is composed, from bottom to top, of: (1) an onlapping bryozoan red-algal limestone ramp; (2) a prograding molluscan bioclastic limestone platform; (3) a dolomitized prograding and, locally, downstepping fringing-reef complex dominated by a Porites-coral framework; and (4) a dolomitized topography-draping sequence composed of grainstones, Porites reefs, and stromatolites (terminal carbonate complex or TCC of Esteban, 1979). Overlying and onlapping a subaerial exposure surface at the top of the TCC is a marine-tocontinental succession of carbonates and siliciclastics referred (Cunningham et al., 1994) to as the mixed carbonate-siliciclastic complex (MCSC). The MCSC records the end of late Miocene marine deposition within the Melilla Basin and presumably correlates to final isolation of the Paleo-Mediterranean Sea during the Messinian Salinity Crisis.

4. Correlations based on magnetostratigraphy and biostratigraphy 4.1. The Tortonian-Messinian

boundary

The Tortonian-Messinian boundary is defined in the Mediterranean at the first occurrence datum (FOD) of the planktonic foraminifer Globorotalia conomiozea in section Falconara on Sicily (d’onofrio et al., 1975; Colalongo et al., 1979). Unfortunately linkage of Falconara to the geomagnetic polarity time scale (GPTS) has never been accomplished because of remagnetization of the section (Langereis and Dekkers, 1992). Integrating radiometric ages from volcanogenic layers and a FOD of G. conomiozea from sections in the Northern Apennines of Italy, Vai et al. (1993) have reported an age of 7.26 f 0.10 Ma for the Tortonian-Messinian boundary. Krijgsman et al. (1994) have suggested ages for the Tortonian-Messinian boundary on Crete of either 6.92 Ma using the GPTS of Cande and Kent (1992) or 7.10 Ma utilizing a linear interpolation between two new calibration points; an astronomical age from Schackleton et al. (1995) and a radiometric age from Baksi (1993). In an Atlantic section in the Bou Regreg valley (Fig. l), NW Morocco, Benson et al. (1991) and Benson and Rakic-El Bied (1995) identified the FOD G. conomiozea near the change between chrons C3Bn and C3Ar (Fig. 4). Dating by Arias et al. (1976) in the Melilla Basin at the Izarorene Section (Fig. 5) has existed as a standard for the Tortonian-Messinian boundary (6.5 Ma) in western Morocco. Our biostratigraphic results of a composite section which includes the Izarorene Section (Fig. 5) shows that

1.54

K.J. Cunningham

UIl lzE3 I

et aL/Sedimentaq

Geology 107 (1997) 147-165

Basalts

Lacustrine

.&5

.-_;

carbonates

Conglomerates

Shallow-marine peloidal grainstones stromatolites

i iIII i Ezzz ....:.... c_-l

&

Patch reefs

1.

Lime mudstones

&

,“b Dl megabreccias Shallow

dolomitized

TCC

Sandstones

:.:., :::.:.. :: m .‘_

::::::::::::‘:.,::‘ii:i: .I\

./..

:::I:l:j:~.::I:I:::::.::::

Bioclastic grainstones & packstones/Ha//meda-rich

+v Lrzl

Dolomitized

fringing

reefs

--A-

C-

i-i --

Marls & diatomites

-I ISEI lulmmnl I%@ _Y

Marls

Mollusc-rich packstones

Volcanic

lime & grainstones

ashes

Bryozoan &red algal lime packstones & grainstones

Marls,

El

siltstones.

Metamorphic Erosional Subaerial

TORT. SERR.?

sandstones

rocks

unconformities unconformities

40

40Ar/39Ar dates w

5.79

+0.02

Ma (#FA-1

@

5.82

kO.02

Ma [#B-l)

@

5.95

iO.10

Ma (#IF-l

K/

40

Ar dates

)

-42.9)

Fig. 2. Interpretative stratigraphic column for the Melilla Basin. Stratigraphic ages are based on K/Ar radiometric ages from Hemandez and Bellon (1985) for V5 and integrated magnetostratigraphy, biostratigraphy, and isochron ages from 40Ar/39Ar analyses of singlecrystal, laser-fused sanidines for volcanic ashes Vl, V2, V3, V4, and V4a (Table 1). Abbreviations RI and R2 represent depositional sequences within the ramp, and BPI, BP2, and BP3 within the bioclastic platform, and KC represents the terminal carbonate complex. Stratigraphic location of measured Section FA-I (Fig. 1) is shown.

Fig. 3. Composite cross-section youngest reefs of stratigraphic plane of B-B’ has been rotated basement. The mega-breccia at

0

200 meters

t

51

at base of TCC

2km

m

Reef Margin & Shallow-Marine TCC

] Reef Complex Exposures

I

Alt Amar

f

-.

%

9

I

c

”

I

I

in the Basin Margin or

Measured Sections

Margin or Shallow-Marine

TCC

Grainstones & Packstones in the Reel

Wackestones

& Bioclastic Packstones

Halimeda

@ @

Red-Algal Ramp Marls & Diatomites

Bryozoan

Molluscan Bioclastic Platform

Peloidal grainstones & stramatolites

$@s

f

la/

Basement

of the carbonate complex and overlying mixed carbonate-siliciclastic complex (MCSC). The cross-section was constructed by projecting the section A-A’ at Ibebenhadoussene and section B-B’ at Oued Mezilri along the depositional strike of the carbonate complex. The vertical approximately 130” clockwise into the vertical plane of A-A’. The Serravallian(?)-to-Tortonian elastic wedge (Fig. 2) is included in the the base of the terminal carbonate complex (KC) is interpreted as formed by dissolution of evaporites. For further explanation see Fig. 2.

Basin Margin

K.J. Cunningham et al./Sedimentary

156

Geology 107 (1997) 147-165

Melilla Basin

*

Ain E I Beida Section 5.25

-@

5.82

k0.02

Ma

tO.lOMa

-05.95

LO

G. conomiozea

~0

G. mediterranea

LO

G. menardii

6.19 LO

_3

6.26

FO G. conomiozea G. mediterranea

6.55

G. menardii

G. conomiozea

&

6.91

FO ----_,G. conomiozea

2 :

7.06

Vl = 7.0 +O.l4Ma & 7.0 kO.14 Ma

6 25 m

Z 2

a

= major

subaerial

+ *

= precession

*

from

Hernandez

cycles

unconformity of Benson

& Bellon

et al. (19950)

(1985]

Fig. 4. Correlation of polarity-reversal stratigraphies and biostratigraphies of the Melilla Basin and the Ain el Beida Section at Bou Regreg (Fig. 1). The Ain el Bied section is modified after Benson et al. (1995a). Detailed stratigraphic positions for volcanic ashes Vl-V5 are shown in Fig. 2. See Cunningham (1995) for sections used in construction of the Melilla Basin magnetostratigraphy. In polarity columns: black = normal, white = reversal, and angled slashes = no samples. Abbreviations are as follows: FO = first occurrence, LO = last occurrence, and MCSC = mixed carbonate siliciclastic complex. The chronostratigraphy is calibrated to a composite of Baksi’s (1995) geomagnetic extrapolations (the termination of chron C3r and from chron C3An.lr and older) and Benson et al. (1995a.b) astronomic stratigraphy of the Ain el Beida Section (the onset and termination of chron C3An.l; 6.19 and 5.94 Ma, respectively). The Messinian-Pliocene boundary is after Hilgen (1991) and Tortonian-Messinian boundary is after Benson et al. (1991) and Benson and Rakic-El Bied (1995). See Cunningham et al. (1994) for discussion of the ambiguous normal polarity zone (gray subchron) in the terminal carbonate complex (TCQ.

K.J. Cunningham et al./Sedimentary

Geology IO7 (1997) 147-165

IR-1

1.57

Height(m)

Lithologic Legend

IR-1

measured section

?? = village

-I

SA-1

FAD G. conomiozea (Arias

Height 100

et al., 197~

(m) to.4

* +

-60

(SM-84) 6.9 kO.2 Ma

+

1H

Ma (biotlte)

(This study)

Foraminiferal Assemblages Assemblage +

Assemblage G. G. G. G. G. G.

+ FO G. conomiorea

mediterranea saheliana exerta saphoae conoidea conomiozea

1

2:

G. ac. acostaensis G. ac. humerosa G. galanae Globigerina bulloides Globigerinoides spp. Orbulina universa Hastigerina spp.

Assemblage

3:

Orbulina suturalis Globigerina spp. Orbulma universa

(This study) 0

+

Fig. 5. Stratigraphic cross-section from the SW portion of the Melilla Basin. Section SA-1 is at the village of Sammar and Section IR-1 is about 1.5 km northeast of the village of Izarorene. The isochron ages from 40Ar/3gAr analyses of single-crystal, laser-fused sanidines (samples IR-1.0 and SM-84) for a volcanic ash that can be correlated between the two sections are shown. Five K/Ar dates from Arias et al. (1976) for Section IR-1 are also shown. Alternating foraminiferal assemblages described by Rakic-El Bied and Benson (1995) are shown for Section SA-1. Foraminiferal assemblages 2 and 3 are not differentiated on Section SA- 1. The relative stratigraphic positions of the first occurrence (FO) for G. conomiozea defined of this study and first appearance datum (FAD) of Arias et ai. (1976) are shown.

158

K.J. Cunningham

et al./Sedimentary

the Tortonian-Messinian boundary of Arias et al. (1976) should be abandoned. Beneath the TortonianMessinian boundary proposed by Arias et al. (1976) Rakic-El Bied and Benson (1995) report that a G. conomiozea-G. mediterranea foraminiferal assemblage alternates with foraminiferal assemblages in which the G. conomiozea-G. mediterranea assemblage is not present. G. mediterranea is a planktonic foraminiferal counterpart to G. conomiozea and is similarly considered a Messinian biomarker (Sierro, 1985). This alternation of foraminiferal assemblages occurs in marls, about 80 m in thickness, of Section SA-1 at the village of Sammar (Fig. 5). Field mapping indicates that faulting did not produce this oscillation in assemblages. A volcanic ash which occurs near the top of these alternations yielded 40Ar/39Ar ages of 6.86 f 0.02 Ma and 6.9 f 0.2 Ma (Fig. 5 and Table 1). The biostratigraphic succession of Section SA-1 (Fig. 5) demonstrates that the presence of G. conomiozea and G. mediterranea are not always constant in vertical stratigraphic sections and could lead biostratigraphers to misjudge the FOD of these biomarkers in short sections. 4.2. Western Rifian Corridor and Mediterranean correlations The magnetic-reversal stratigraphy for the Melilla Basin has been correlated to two areas, containing marine sections (Bou Regreg and Sorbas Basin; Fig. l), which have been dated by magnetostratigraphy and biostratigraphy. One of the sections (Bou Regreg) was also dated by stable-isotope stratigraphy (Hodell et al., 1994) and astronomic tuning of sedimentary cycles (Benson et al., 1995a,b; Benson and Rakic-El Bied, 1995). The sections are: (1) a composite section of Tortonian-to-Pliocene marine sediments in the Bou Regreg valley, NW Morocco (Figs. 1 and 4; Hodell et al., 1989, 1994); and (2) a Messinian-to-Pliocene section in the Sorbas Basin, located in the Betic Cordillera, SE Spain (Figs. 1 and 6; Gamier et al., 1994). The Bou Regreg section was exterior to the Paleo-Mediterranean Sea, located at the western threshold of the Rifian Corridor (Fig. 1). It is important because, relative to the Melilla Basin, it is the closest deep-marine section with a continuous succession of Tortonian-to-Pliocene marine sediments uninterrupted by Mediterranean desicca-

Geology

107 (1997)

147-165

tion events and has all of the stratigraphic criteria necessary to serve as a reference for this time interval (Hodell et al., 1989, 1994; Benson et al., 1995a; Benson and Rakic-El Bied, 1995). The Sorbas Basin is marginal to the Mediterranean Basin (Fig. 1) and is rimmed by a shallow-marine carbonate complex (Riding et al., 1991; Braga and Martin, 1992; Martin and Braga, 1994) similar to the carbonate complex of the Melilla Basin, allowing tentative correlations between the shallow-marine successions of the two basins. Correlation of the magnetic-reversal stratigraphy of the Melilla Basin to the two areas was aided by foraminiferal biostratigraphy and ““Ar/39Ar ages of volcanics interbedded within the carbonate complex of the Melilla Basin (Cunningham et al., 1994). 4.2.1. Bou Regreg The range of G. conomiozea in the Melilla Basin was observed from within chron C3ar to within chron C3an.lr, resulting in the correlation of the magnetic reversal stratigraphies between the Melilla Basin and the Ain el Beida Section (Bou Regreg) shown in Fig. 4. G. conomiozea has also been reported as young as chron C3an.lr from the Vera Basin (Benson and Rakic-El Bied, 1991b) and Sorbas Basin (Gamier et al., 1994) SE Spain. The new ‘“Ar/“‘Ar dates from the Melilla Basin corroborate the correlation in Fig. 4 and reinforce new astronomic dates for Bou Regreg (Benson et al., 1995a,b). The bryozoan red-algal ramp, an onlapping, transgressive deposit, correlates to relatively low values of S’*O measured from benthic foraminifers from the blue marls of the Bou Regreg valley (Fig. 7; Hodell et al., 1994). This correlation indicates, at least in part, a link between rising sea level and reduction in global ice volume during deposition of the ramp. 4.2.2. Sorbas Basin Braga and Martin (1992) and Martin and Braga (1994) report that fringing reefs and bioherms (Cantera Member) rimming the Sorbas Basin predate the evaporites (Yesares Member) shown in Fig. 8; and stratigraphic interpretations by Martin and Braga (1994) imply that the reefs and bioherms are laterally equivalent to basinal marls, calcisiltites, silts, and diatomites (Fig. 8). Extrapolation of the magnetostratigraphy and lithostratigraphy of Gamier et

K.J. Cunningham

et al./Sedimentary

Sorbas Basin -?

159

Geology 107 (1997) 147-165

Melilla Basin

* @

5.80 5.87

k0.29 Ma 8( +0.29Ma -

+@

5.79

20.02

t@

5.82

+@I

5.95

kO.02

Ma

Ma

&O. 10 Ma

LO G. conomiozea

2 L

LO G. mediterranea LO G. menardii

a cr)

3Bn FO G. conomiozea

a=L

&

G. medterranea

FO G. menardii

+@

*@

v2 =

6.68f0.02

Ma

& 6.72 kO.02

Ma

Vl = 7.0 kO.14 Ma & 7.0 f0.14Ma w

= major *

subaerial

from Hernandez

unconformity

& Bellon

[l 985)

Fig. 6. Correlation of polarity reversal stratigraphies and biostratigraphies of the Melilla Basin, Morocco, and the Sorbas Basin, Spain (Fig. 1). Stippled lines connect reversal boundaries. Solid lines connect interpreted lithologic correlations. Range in occurrence of Globororalia conomiozea is indicated for each basin. Ranges in occurrence of G. mediterranea and G. menardii are indicated for the Melilla Basin only. Other abbreviations are as follows: FO = first occurrence, LO = last occurrence, L and S = limestone and sandstone, D and M = diatomite and marl, 2. = limestone, RL = red loams, C = conglomerate, XC = terminal carbonate complex, and MCSC = mixed carbonate siliciclastic complex. In polarity columns: black = normal polarity, white = reversed polarity, and angled slashes = no samples. Sorbas Basin data modified after Gamier et al. (1994). See Cunningham et al. (1994) for a discussion of the ambiguous normal polarity zone (gray subchron) in the TCC.

al. (1994) to the lithostratigraphic correlations of Braga and Martin (1992) and Martin and Braga (1994) suggest that the fringing reefs and bioherms of the Sorbas Basin (Fig. 8) are equivalent to the same interval of polarity reversals as that exhibited by the interbedded diatomites and marls shown

in Fig. 6. Comparison of the lithostratigraphy, biostratigraphy, and magnetostratigraphy of the Melilla Basin with the inferred linkage in the Sorbas Basin between Gautier et al.‘s (1994) basinal magnetobiostratigraphy and lithostratigraphy and Martin and Braga’s (1994) lithostratigraphy suggests that the

160

K.J. Cunningham

et al./Sedimentaly

Melilla Basin, Morocco

Bou Regreg, Morocco 0.2

I

-)c

0.6

I

I

1.0

I

1.4

Geology 107 (1997) 147-165

1.8

Relative Sea-Leve I Curves

I I I I I 400

300

200

100

0

-8.0 Hodell et al. (1994)

? o=argon

age\ ‘7

Fig. 7. Correlation section of a aI80 record for Bou Regreg (Fig. 1; after Hodell et al., 1994) interpretative relative sea-level curves for the Melilla Basin, major lithostratigraphic units of the Melilla Basin, and a portion of a geomagnetic polarity time scale (GP TS) of Baksi (1995). with ages modified at the onset and termination of chron C3anln by astronomic tuning of cycles at Ain el Beida (Fig, 4) by Benson et al. (1995a,b). The placement of the Miocene-Pliocene boundary is after Hilgen (1991) and the Tortonian-Messinian boundary is after Benson et al. (1991) and Benson and Rakic-El Bied (1995). TCC = terminal carbonate complex, MCSC = mixed carbonate-siliciclastic complex, and GPTS = geomagnetic polarity time scale. Vertical error bars for argon-dated volcanic ashes are shown. See Fig. 3 for stratigraphic positions of the volcanic ashes designated by circled V’s For further explanation see Fig. 2.

fringing-reefs of the Melilla Basin are of the same or similar age as the fringing reefs and bioherms of the Sorbas Basin (cf. Figs. 6 and 8). A subaerial erosion surface that separates the reefs and TCC at the margin of the Sorbas Basin (Martin and Braga, 1994) is interpreted to be correlative to the basinal evaporites shown in Fig. 8 (Riding et al., 1991). Within the Melilla Basin a subaerial exposure surface, located near the base of chron C3r, separates the fringing-reefs and the TCC (Fig. 6). A megabreccia overlies the downdip equivalent of the surface on

the basin margin (Fig. 3) and is inferred to represent collapse resulting from dissolution of evaporites. Perhaps the evaporites of the Sorbas Basin, which occur just above the onset of chron C3r (Fig. 6), are represented by the subaerial exposure surface separating the fringing-reefs and TCC and by the megabreccia of the Melilla Basin. A volcanic ash (V4) which overlies the subaerial exposure surface separating the reef complex and TCC (Figs. 2 and 6) has an 40Ar/39Ar age of 5.82 * 0.02 Ma (Table 1). Gautier et al. (1994) concluded that the Messinian

K.J. Cunningham et al./Sedimentary

Salinity Crisis began at 5.7 Ma, near the base of chron C3r, using the GPTS of Baksi (1993). They correlate the Messinian evaporites of the Sorbas Basin with the classic sections of Sicily (Gautier et al., 1994~. 1107). We question whether Gautier et al.‘s (1994) correlation of the evaporites in the shallow marginal Sorbas Basin and the deepbasinal evaporites of Sicily is appropriate. Any regional drawdown would have first affected shallow marginal basins of the Paleo-Mediterranean Sea. We consider the inferred evaporites of the Melilla Basin related to the initial stages of the Messinian Salinity Crisis. The main drawdown of the Messinian Salinity Crisis occurred slightly later than 5.82 Ma and left no record of evaporites in the Melilla Basin. 4.2.3. Melilla Basin In the Melilla Basin at Section FA-1 (Figs. 1 and 2) a volcanic ash (V4a) occurs below the base of shallow-marine, cross-bedded, peloidal and coated-grain grainstones that contain a few miliolids. Above the grainstones are marine sandstones which grade upwards into continental conglomerates overlain by a distinct paleosol. Overlying the paleosol are charophyte-bearing lacustrine carbonates (Guillemin and Houzay, 1982). The V4a ash (Fig. 2) is located approximately 20 m below the last Messinian marine deposit within the Melilla Basin. The 40Ar/39Ar age of the ash (5.79 & 0.02 Ma) and the depositional rates for modern oolites (550-2000 mm/kyr; Schlanger, 1981) and for small deltaic siliciclastics (our approximate average of 5000 mrn/kyr using data from Enos, 1991) suggest that final Messinian marine deposition within the Melilla Basin was at approximately 5.78 Ma. This change to continental deposition is inferred to be coincident with final closure of the Rifian Corridor during the latest Messinian. Increasing global ice volumes may have reduced sea level at this time and, at least, played a role in this closure. This conclusion is supported by an approximately coeval relative increase in benthic foraminiferal Al80 values (Hodell et al., 1994) recorded at Bou Regreg (Fig. 7). 5. Conclusions (1) A Tortonian-to-lower Messinian carbonate ramp that forms the base of a carbonate complex

Geology 107 (1997) 147-165

161

in the Melilla Basin correlates to relatively low values of benthic foraminiferal S’80 values (Hodell et al., 1994) from deep-marine marls in the Bou Regreg valley, NW Morocco. This correlation indicates, at least in part, a link between rising sea level and a reduction in global ice volume during deposition of the ramp. (2) A major fall in relative sea level (-60 m) occurred near the demise of a Messinian reef complex in the Melilla Basin during chron C3n.ln at 5.95 f 0.10 Ma. This signals the initiation of drawdown and changing environmental conditions in the Melilla Basin, and perhaps throughout the PaleoMediterranean Sea. (3) A megabreccia interpreted as forming by solution collapse of evaporites on the basin margin of the reef complex occurs at the base of the terminal carbonate complex. Updip, a major subaerial unconformity separates the reef complex and terminal carbonate complex. Evaporite deposition likely occurred during this exposure event and has been dated at 5.82 & 0.02 Ma near the base of chron C3r. We contend that these evaporites, restricted to the shallow Melilla Basin, are related to a continuation of an initial stage of the major drawdown of the Paleo-Mediterranean Sea. (4) Results of comparing chronostratigraphic and lithostratigraphic results from the Melilla Basin and Sorbas Basin suggests that a major sequence boundary (subaerial exposure surface) between the fringing reefs complexes and terminal carbonate complexes of the Melilla and Sorbas Basins are coincidental events (lower chron C3r). This suggests a regional or, possibly, eustatic control. (5) The age (-5.78 Ma or lower chron C3r) of the major drawdown of the Paleo-Mediterranean Sea during the Messinian Salinity Crisis has been inferred by combining new stratigraphic, paleontologic, magnetostratigraphic, and radiometric results from the Melilla Basin, NE Morocco. The evidence from the Melilla Basin is approximately coeval with an increase in benthic foraminiferal Al80 values from Bou Regreg, strongly suggesting, at least in part, that a glacio-eustatic lowering of sea level induced the final closure of the Mediterranean during the Messinian Salinity Crisis.

162

K.J. Cunningham

et al./Sedimentan

Geology 107 (1997) 147-165

Sorbas Basin

Melilla Basin PLIOCENE

..:;. ._.:.

..‘.

. . . . . .. . .. .

ignh. tratlgraph 1\--_.

\

\

\

\ \

\

\

-

\

\

\

\

\

‘7

3Cr

\;

Magne stratlgra

?

.

to- : PM --

-s-’ /

/’

/

I

3Cr

5’ F

I

-----

--

3An.l n

/

3An.lr

3An

3An.2n -

S _ _

3Ar /

-----

--_

’ /

3Bn _--

_

_’ /

--_

40

40Ar/39Ar dates @

6.68

20.02

Ma& 6.72

@

7.0~0.14Mo&7.0~0.14Ma

kO.02

Ma

@

5.79

50.02

Ma

@

5.82

20.02

Ma

@

5.95

iO.lOMa

@

P

5.80

K/

20.29

40

Ar dates

Ma & 5.87

+0.29

Ma

No magnetostratigraphic

data

K.J. Cunningham

et al./Sedimentary

Geology 107 (1997) 147-165

Conglomerates, sandstones. gralnstones. 8 bioherms

Shallow

Reef blocks I3 breccias, silts, marls, & diatomites

Sandstones

Evaporites

Bioclastic grainstones & packstones/Ha//meda-rich

Calcareous sandstones & grainstones

Fringing

C&n~lomerates,

sands,

Lacustrine

reefs

Mollusc-rich packstones

carbonates

Conglomerates

Volcanic

Marls,

Lime mudstones megabreccias

lime & grainstones

ashes

Bryozoan &red algal lime packstones & grainstones

&

Patch reefs

Subaerial

TCC

Marls

Shallow-marine peloidal grainstones stromatolites

I,’ 3D

dolomitized

Marls & diatomites

Basalts

m

163

&

siltstones,

Metamorphic

unconformities

Erosional

sandstones

rocks

unconformities

Fig. 8. Proposed correlation of sequence boundaries from the Melilla and Sorbas Basins. The data from the Sorbas Basin is the result of correlating lithostratigraphic, paleontologic, and magnetostratigraphic conclusions of Gautier et al. (1994) to lithostratigraphic results Rl and R2 represent depositional sequences within the ramp, and BPI, BP2 and reported by Martin and Braga (1994). Abbreviations BP3 within the bioclastic platform of the Melilla Basin. The TCC represents the terminal carbonate complex.

Acknowledgements This paper has been revised from part of a dissertation completed by the senior author and co-directed by E.K. Franseen and Paul Enos. Their reviews improved early versions of the manuscript. M.R. Farr, R.H. Goldstein, and E. Gischler also provided useful comments. Without the logistical support of the Office National de Recherches et d’Exploitations Petroliers, Rabat, this project would not have been possible. Bouslouk Moha and Taib Mahmoud were of great assistance in the field. M.T. Heizler and W.C. McIntosh completed 40Ar/39Ar analyses at the New Mexico Geochronological Research Laboratory. We thank referees T. Aigner and I.M. Villa for their useful reviews, and editor K.A.W. Crook for his edifications. The costs of the project were sustained by the following: Exxon Production Research, Texaco, Walcott Fund of the Smithsonian Institution,

Geological Society of America, Sigma Xi Scientific Society Grants-in-Aid of Research, American Association of Petroleum Geologists Grants-in-Aid Fund, Phillips Petroleum Research Fellowships, University of Kansas Geology Associates Fund, Haas Professor Fund, D.A. McGee Scholarships in Geology, Union Pacific Resources Scholarship, and H.A. Ireland Scholarship. References Arias, C., Bigazzi, G., Bonadonna, F.P., Morlotti, E., Racidati di Brozolo, F., Rio, D., Torelli, L., Brigatti, M.F., Giuliani, 0. and Tirelli, Cl., 1976. Chronostratigraphy of the Izarorene section in the Melilla basin (northeastern Morocco). Boll. Sot. Geol. Ital., 95: 1681-1694. Baksi, A.J., 1993. A geomagnetic polarity time scale for the period O-17 Ma, based on 40Ar/39Ar plateau ages for selected field reversals. Geophys. Res. Lett., 20: 1607-1610. Baksi, A.J., 1995. Fine tuning the radiometrically derived geo-

164

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et al. /SedimentaT

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