TERRIGENOUS DOLOMITE IN THE MIOCENE OF MENORCA (SPAIN): PROVENANCE AND DIAGENESlS I

TOM FREEMAN AND DAVID ROTHBARD Department of Geology University of Missouri

Columbia, Missouri 65211 AND

ANTONIO OBRADOR Departamento de Estratigrafia

La Universidad Autrnoma Barcelona, Spain

AasTnAc'r: Miocene calcarenites of Menorca (Spain) exhibit abundant terrigenous dolomite, both as me- dium-grained polycrystalline rock fragments and as fine-grained individual abraded crystals. Petrographi- cally, this detritus looks both like neighboring Jurassic dolostones and like Triassic Muschelkalk dolostones of the same general area. In order to determine which stratigraphic unit(s) provided this terrigenous do- lomite, the iron contents of Triassic and Jurassic dolostones were analyzed, with an electron microprobe, and compared with that of the Miocene terrigenous dolomite. As was expected from their present greater surface exposure, Jurrassic dolostones proved to be the principal source. The trace of probable Triassic dolomite detritus in Miocene samples analyzed is proportionate to the small area of Triassic exposure relative to that of the Jurassic.

Diagenetic dolomite in Miocene host calcarenites occurs as l) pervasive crystals of fine- to medium- crystalline dolomite and 2) overgrowths on the terrigenous dolomite crystals. These two varieties commonly occur together, but overgrowths occur in some samples in the absence of the pervasive variety, which indicates that "seeding" by terrigenous dolomite promoted diagenetic dolomitization.

Calcitization of Miocene dolomite favored centers of the pervasive variety of dolomite crystals and the inner margins of dolomite overgrowths. These patterns of dedolomite indicate that the two varieties of dolomite are behaviorly correlative, which suggests that they are temporally correlative as well.

INTRODUCTION

Earlier Reports of Terrigenous Dolomite

Occurrence.--Amsbury's (1962) interpreta- tion of terrigenous dolomite in the Cretaceous of central Texas was based on the abraded shapes of dolomite grains. His examples com- monly exhibit dolomite overgrowths, which are clearly indicated by the fact that the irregular shapes of the nuclei are discordant with those of their rhombic overgrowths. As pointed out by Amsbury, a higher degree of concordance should exist were the more turbid dolomite cores the product of simple crystallographic zona- tion.

Lindholm's (1969) interpreting a terrigenous origin for dolomite in the Devonian of New York was based on reasoning different from

~Manuscript received April 12, 1982; revised Septem- ber 9, 1982.

that of Amsbury. Lindholm documented sim- ilarities, both in abundance and in grain size, between dolomite grains and associated terrig- enous quartz. These similarities were taken to mean that the dolomite, like the quartz, is ter- rigenous. Reed (1968) applied similar reason- ing in his study of the Ordovician Dutchtown Formation of southeast Missouri. Provenance.--ln earlier studies of terrigenous dolomites, authors' interpretations of prove- nance have been based largely on the prox- imity of likely source rocks. A petrographic approach to recognizing provenance, like that applied to silicate sands, is generally pre- cluded by the lithic sameness among dolo- stones of various ages. Amsbury (1962) rea- soned that terrigenous dolomite in the Cretaceous of central Texas was derived from Cretaceous and Ordovician dolostones of the Llano Uplift. He supported his contention by describing terrigenous dolomite from modem sediments in stream courses traversing these

JOURNAL OF SEDIMENTARY PETROLOGY, VOL. 53, NO. 2, JUNE, 1983, P. 0543--0548 Copyright 1983, The Society of Economic Paleontologists and Mineralogists 0022-4472/83/0053-0543/$03.00

544 TOM FREEMAN, DAVID ROTHBARD, AND ANTONIO OBRADOR

MENORCA 45

40

~ 55

|g .

FIG. 1 .--General geology of Menorca (from Mapa Geologico, Ibiza-Mahon, Instituto Geologico y Minero de Es- pafia). Twelve samples (1 through 12) were collected from equally spaced road-cuts along each of five principal highways (A through E).

probable source rocks. Lindholm (1969) was less sure about the source of terrigenous do- lomite in the Devonian of New York but sug- gested that the grains were a mixture of pe- necontemporaneous supratidal dolomite, redistributed by wind, and detritus from older (for example, Silurian) dolostones.

The iron content of Menorcan Miocene ter- rigenous dolomite, compared with that of pos- sible source rocks on the island, allows for an interpretation of provenance for the Miocene detritus. Additional petrographic observations reveal a history of dolomitization, favored by the occurence of terrigenous dolomite, and subsequent dedolomitization.

Geologic Setting of Menorca

Menorca, the northernmost of Spain's Bal- eafic Islands, lies approximately 200 km east of the coastal province of Barcelona (Fig. 1). The island is divided subequally into two physiographic/geologic provinces: The north-

ern half, the Tramuntana region, is a moun- tainous area characterized by Paleozoic rocks reflecting intense Hercynian deformation and

FIG. 2.--Photomicrograph of Miocene calcarenitic limestone showing three types of terrigenous grains: A) polycrystalline dolostone, B) individual crystal of dolo- mite, and C) quartz. Plain light.

MIOCENE TERRIGENOUS DOLOMITE 545

FIG. 3.--Photomicrograph of Miocene dolomite limestone showing an abraded terrigenous dolomite grain (A) cloaked with a diagenetic rhombic overgrowth (B). Staining shows that the crystalline material along the inner margin of the overgrowth (C) is calcite. Fine- to medium-crystalline diagenetic dolomite rhombs, some with dedolomitized centers, are abundant in the right half of the photomicrograph. Plain light.

by Mesozoic rocks exhibiting more gentle structural features of Alpine age. The southern half of the island, the Migjorn region, is a cos- tal plain province characterized by Miocene marine carbonate rocks. This costal plain slopes seaward parallel to the dip of underlying rocks, so that the present land surface approximates the Miocene sea floor. Miocene rocks are largely mantled by recent dune deposits, but shallow road-cuts provide exposures across es- sentially isochronous marine strata.

The nearly straight boundary separating the two physiographic regions (Fig. 1) was pro- duced by down-to-the-south faulting of Mio- cene age (Obrador, Mercadal, and Rosell, 1971).

TERRIGENOUS DOLOMITE--THIS REPORT

Sedimentologic Setting Obrador (1972-1973) assigned exposures of

Menorcan Miocene carbonates to various near-

shore environments, including shoreface, fore- shore, and backshore settings, and interpreted local terrigenous sandstones and conglomer- ates marginal to the Tramuntana block as del- taic in origin. His conclusions were based on a combination of textural features and bed forms, supported by fossil faunas and floras. The age of these carbonates, judged from a va- riety of published accounts, is Early Miocene (Burdigalian) and/or Middle Miocene (Langh- ian).

In an earlier study (Obrador and Freeman, 1975) terrigenous dolomite was recognized in calcarenites of Cala St. Esteve, the eastern- most exposure of Menorcan Miocene. The ter- rigenous dolomite occurs both as polycrystal- line dolostone fragments and as individual abraded crystals (Fig. 2). Figure 3 shows an abraded dolomite grain with a diagenetic rhombic overgrowth. Figure 4 shows a similar grain, with overgrowth, that is contiguous with

546 TOM FREEMAN, DAVID ROTHBARD, AND ANTONIO OBRADOR

FIG. 4 . - -Photomicrograph of Miocene l imestone showing an abraded terrigenous dolomite grain (A) cloaked with a diagenetic rhombic overgrowth (B). The dolomite grain is contiguous with a terrigenous quartz grain (C). Plain light.

a terrigenous quartz grain, a result of the de- trital dolomite grain's having been sedimented along with the quartz. (The term detrital merely refers to grains that have been transported and sedimented. Detrital grains can be synsedi- mentary--intrabasina~--in origin. Requiting a higher degree of interpretation is the term ter- rigenous, which means that the detrital grains were eroded from pre-existing rock.)

On the basis of Lindholm's (1969) study, the detfital dolomite of Menorca might be viewed as tertigenous, as suggested by its association with terrigenous quartz (Table l). The detrital dolomite is everywhere accompanied by ter- rigenous quartz, but both dolomite and quartz are absent in some samples. This association (terrigenous quartz/detfital dolomite) suggests that the dctfital dolomite is indeed terrigenous, rather than being intrabasinal detritus like that reported by Sabins (1962).

Provenance

Terrigenous dolomite in the Lower Miocene of Menorca must be indigenous to the island. Some studies have suggested rotation of the Balearic block away from peninsular Spain (for example, Freeman, 1975), but such rotation occurred before Early Miocene, as evidenced by the occurrence of Burdigalian dacites drilled at D.S.D.P. site 123 (Ferrara Bigazzi, and Bonadonna, 1973). In addition, the 0.1- to 0.2- mm size of the terrigenous dolomite precludes wind transport from peninsular Spain. More-

over, the occurrence of the terrigenous dolo- mite is clearly proximal to the probable Tra- muntana source to the north (Table 1).

The only two possible dolostone source rocks in the Tramuntana region are the Triassic Mus- chelkalk Formation and unnamed Jurassic do- lostones. The two are petrographically identi- cal to each other (Fig. 5) and to the Miocene terrigenous dolomite as well (Figs. 2, 3, and 4). The greater surface area of the Jurassic (Fig. 1) suggests that it was the principal source, but in an effort to test this assumption we at- tempted to match minor-element composition of the terrigenous dolomite with that of each of the two possible source rocks. The iron con- tents of Trassic and Jurassic dolostones were analyzed, with an electron microprobe, and compared with that of Miocene terrigenous do- lomite (Table 2). Triassic dolostone proved to be ferroan, whereas Jurassic samples were found to be nonferroan. Miocene terrigenous dolo- mite is, like that of the Menorcan Jurassic, nonferroan. (A single sample of Miocene ter- rigenous dolomite contains 1.2% FeCO3, so it could reasonably be ascribed to a Triassic source.) Ferroan dolomite is probably less sta- ble in the weathering environment, so this fea- ture combines with the relatively small surface area of the Triassic to explain the near absence of a Triassic fraction in the Miocene terrige- nous dolomite.

DIAGENESIS

Dolomitization

Lindholm speculated (1969, p. 1038) that detrital dolomite should promote later diage-

TABLE 1 .--Occurence of terrigenous grains in the Miocene of nenorca

Traverse

Sample Number

A B C D 1 qDd qdd q 2 qd qd q q 3 qd qd q q 4 qDd q q q 5 q q q 6 q q q 7 q q 8 q q 9 q q

10 q q q 11 q q 12 q q

E qd q qDd qd qd qDd qd

Note: Distribution of traverses and samples are shown in Figure 1. q = quartz, D = polycrystalline dolostone, and d = single- crystal dolomite.

MIOCENE TERRIGENOUS DOLOMITE 547

netic dolomitization because "the kinetics needed to initiate crystallization of dolomite are more rigorous than those required to con- tinue growth on previously formed nuclei." Diagenetic dolomite in the Miocene of Men- orea occurs as 1) pervasive fine- to medium- crystalline rhombic crystals and 2) rhombic overgrowths on terrigenous dolomite grains (Fig. 3). In some samples, dolomite over- growths occur in the absence of the pervasive variety, which demonstrates that terrigenous dolomite grains can indeed promote diagenetic dolomitization by acting as "seeds" for dolo- mite nucleation.

The two varieties of diagenetic dolomite are believed to be cogenetic, as suggested by com- mon patterns of dedolomite. Partial to total de- dolomitization of both the overgrowths and the finely crystalline variety have occurred. (Cases of thorough dedolomitization of rhomb-shaped

TABLE 2.--Weight percent FeCO~ in possible dolostone source rocks (Triassic and Jurassic) and in terrigenous dolomite of the

Miocene, measured with the electron microprobe.

TRIASSIC JURASSIC MIOCENE 16 samples) (6 samples) (6 samples)

Four sites, same sample

Four sites, same sample

2.1 0.1 3,4 0.1 1.8 0.1 2,9 0.1

4.3 0.1 3.8 0.1 3.5 0.1 3,7 0.1

4.8 0.3 2.7 0.1 2.6 0.1 0.6 0.1

0.1 0.1 0.1 0.1

0.1 0.2

Note: The 0.1 values are taken to indicate nonmeasurable amounts.

dolomite overgrowths are good evidence that the calcite within overgrowths is in fact de- dolomite, rather than a discontinuous genera- tion of calcite cement.) In cases of partial de- dolomitization the innermost parts of both the overgrowths and the finely crystalline rhombs have been selectively dedolomitized. In the lower right quadrant of Figure 3 there occur scattered crystals of finely crystalline dolomite with dedolomitization centers. The "plumbing system" for this centrifugal dedolomitized was most likely along cleavage traces in the dolo- mite rhombohedra. The reality of such a sys- tem has recently been demonstrated by Frank (1981) in his study of giant rhombohedra from the Cambrian Taum Limestone of southeast Missouri.

CONCLUSIONS

FIG. 5.--Photomicrographs of Triassic Muschelkalk dolostone (A) and unnamed Jurassic dolostone (B) of Menorca. Plain light.

1) Detrital dolomite in the Miocene of Men- orca, as indicated by its association with ter- rigenous quartz, is terrigenous.

2) Considerations of the age of this terrige- nous dolomite and the time of possible rotation of Menorca from peninsular Spain show that the terrigenous dolomite is indigenous to Men- orca.

3) A Jurassic source for Miocene terrige- nous dolomite is indicated by their both being nonferroan. (The only other possible source, the Triassic Muschelkalk, is ferroan.)

4) Terrigenous dolomite in the Miocene of Menorca promoted diagenetic dolomitization.

5) Patterns of dedolomite in dolomite ov- ergrowths and in finely crystalline diagenetic

548 TOM FREEMAN, DAVID ROTHBARD, AND ANTONIO OBRADOR

dolomite suggest that the two var iet ies are co- genet ic .

6) The exchange o f f lu ids resu l t ing in cen- tr i fugal dedo lomi t i za t ion is be l ieved to have been a long c leavage traces w i th in do lomi te rhombohedra .

REFERENCES

AMSatmY, D. L., 1962, Detrital dolomite in central Texas: Jour. Sed. Petrology, v. 32, p. 5-14.

FERRARA, G., BIGAZZI, G., BONADONNA, F. P., AND GtU- LIANI, O., 1973, Radiometric dating of the Valencia Trough volcanic rocks, in Ryan, W. B. F., and Hsii, K. J., et al., Initial Reports of the Deep Sea Drilling Project, Volume XIII: Washington, D. C., U.S. Gov- ernment Printing Office, p. 773.

FRANK, J. R., 198l, Dedolomitization in the Taum Sauk Limestone (Upper Cambrian), southeast Missouri: Jour. Sed. Petrology, v. 51, p. 7-18.

FREEMAN, Z., 1975, Dispersal patterns in the Spanish Buntsandstein suggest clockwise rotation of the Bal- earic block: Ninth International Congress of Sedimen-

tologists, Theme V, p. 11. LINDHOLM, R. C., 1969, Detrital dolomite in Onondaga

Limestone (Middle Devonian) of New York: its impli- cations to the "dolomite question": Am. Assoc. Petro- leum Geologist Bull., v. 52, p. 1035-1042.

OBRADOR, A., 1972-1973, Estudio Estratigrhfico y Sedi- mentol6gico de los Materiales Mioc6nicos de la Isla de Menorca: Talleres Grhficos Coll., Mahon, 182 p.

OBRADOR, A., AND FREEMAN, T., 1975, Erosional features and multiple generations of dolomite in the Miocene of Cala St. Esteve (Menorca, Baleares): Ninth Interna- tional Congress of Sedemintologists, Theme VII, p. 159- 164.

OBRADOR, A., MERCADAL, B., AND ROSELL, J., 1971, Ge- ology of Menorca, in Freeman, T. and Simancas, R., eds., Tenth International Field Institute Guidebook: Washington, D. C., Am. Geological Institute, p. 139- 148.

REED, B. E., Jr., 1968, Petrology of the Dutchtown For- mation, Southeast Missouri [unpub. Masters thesis]: Univ. Missouri-Columbia, 32 p.

SABINS, F. F., Jr., 1962, Grains of detrital, secondary, and primary dolomite from Cretaceous strata of the western interior: Geol. Soc. America Bull., v. 73p. 1183-1196.