Apatite as a paleoenvironmental indicator in the Precambrian-mesozoic clastic sequence of the Middle East

JournalofAfrican EarthSciences,Vol. 6, No. 6, pp. 797-805, 1987

0731-7247/87 $3.00+ 0.00 Pergamon Journals Ltd.

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Apatite as a paleoenvironmental indicator in the PrecambrianMesozoic clastic sequence of the Middle East TUVlA WEISSBROD,* ITHAMAR PERATH* a n d JOSEPH NACHMIASt *The Geological Survey of Israel, 30 Malchei Yisrael Str., 95501 Jerusalem, Israel tNegev Phosphates Ltd., P.O. Box 435, 86103 Dimona, Israel

(Received for publication 16 January 1987) Abstract--Apatite, either detrital or authigenic, or both, occurs in the Precambrian arkoses and Cambrian subarkoses, grits and quartz-arenites in Israel, Sinai, southern Jordan and northwest Arabia. However, no apatite is found in the sandstones that overlie the sub-Carboniferous unconformity (often superseded by a sub-Cretaceous unconformity) throughout the Middle East. Within the Precambrian-Cambrian sequence, apatite distribution is not uniform and varies between 0-80% of transparent heavy minerals. The detrital apatite was derived from the acid igneous terrain that supplied the sands. Authigenic apatite was formed by recrystallization of the detrital apatite. Until its abrupt stratigraphic termination, apatite accompanies the ultrastable minerals zircon-tourmalinerutile without displaying, like the semi- and nonstable heavy minerals, vertical trends of gradual disappearance. Therefore its disappearance cannot be explained by repeated reworking and transportation which, by themselves, are not known to result in the complete elimination of a heavy mineral from an assemblage. Numerous heavy-mineral studies, and especially the experimental work of Nickel (Contr. Sedimentol. 1, 1-68, 1973) have shown that apatite dissolves under conditions of low (<6) pH, which may develop in well-leached humic soils, pedalfers and laterosols, but which are not known in intrastratal groundwaters. The apatite distribution suggests, together with clay-mineral indications, that pedogenesis of this type developed with the spread of terrestrial floras over the Arabo-Nubian land surfaces during the Late Paleozoic. Various lines of evidence, including paleomagnetic data, indicate that the present-day Near East area was in a temperate-humid zone during the Cambrian. After drifting through subpolar latitudes during the Ordovician-Silurian-Devonian, it moved again into a tropical-humid zone toward the Late Paleozoic. This coincided with the appearance of plant fossils in the clastic section, and the disappearance of apatite. Since the dissolution of apatite is inhibited in the presence of carbonate or Ca 2+ ions, its removal must already have been completed before the Permian, when carbonate deposition became gradually dominant.

INTRODUCTION HEAVY MINERALSin the Precambrian-Mesozoic clastic sequence known as the 'Nubian Sandstone' (see Pomeyrol 1970), which overlies the igneous basement over extensive areas in the Near East (see Fig. 1) have been studied by numerous authors (Vroman 1944, Shukri and Said 1946, Shukri and El Ayouty 1953, Bender 1963, 1968, Powers et al. 1966, Greenberg 1968, Lillich 1969, Bisewski 1982, Schneider et al. 1984). Weissbrod and Nachmias (1986), in a recent study, used heavy-mineral distribution patterns as indicators for trends of sedimentary evolution of these clastic terrains, and as a means of regional correlation. They found, however, that the vertical distribution of apatite is not entirely compatible with overall trends displayed by other members of the heavy-mineral suite, and requires further clarification. Apatite occurs as an ubiquitous accessory in the Precambrian and Lower Paleozoic only, as part of the transparent heavy-mineral suite of the arkoses and subarkoses. It appears more irregularly in the Paleozoic quartz-arenites, and is completely absent in sandstone formations from the Carboniferous onward. Looking more closely at the distribution of apatite (see Fig. 2), two peculiarities become apparent: (a) Apatite occurs in two mineral forms, both of which are fluor-apatite: detrital apatite (henceforward D-apatite), and authigenic apatite (henceforward A-apatite).

Intermediate forms of authigenic overgrowths on D-apatite are also found. (b) Apatite displays vertical irregularities (Fig. 3), with great variability between samples only a few metres apart. Samples in which apatite amounts to as much as 80% of all transparent heavy minerals may occur next to samples that are practically devoid of apatite. Unlike the unstable heavy minerals, which disappear upward in the vertical sequence as a result of progressive weathering and maturation, D-apatite, wherever present, shows a close affinity to the group of ultrastable minerals, marked by the dominance of zircon-tourmaline-futile (ZTR). Its stratigraphic termination can therefore not be explained by conventional redeposition and weathering-out. The present paper, in order to arrive at an explanation for these peculiarities of distribution, examines the susceptibility of apatite to various degradation processes, and its value as an environmental indicator.

GEOLOGIC BACKGROUND (Fig. 2) The clastic formations which overlie the Precambrian igneous basement throughout the area of discussion-Egypt and Sinai, southern Israel, southern Jordan and northwest Arabia---span a long period of pericratonic sedimentation, ranging from Late Precambrian to Cretaceous times. Throughout this time the area was un-

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affected by folding or major dislocation, but vertical movements on a regional scale caused notable unconformities and paraconformities, with lacunae that may encompass several periods. The lowermost clastics consist of first-cycle debris of the igneous Arabo--Nubian craton. In Israel they are represented in an arkosic facies by the fairly thick (>2000 m) Zenifim Formation, penetrated only in deep boreholes (Weissbrod 1969a), and in a conglomeratic facies, the Eilat Conglomerate, whose Jordanian equivalent is the Saramuj Conglomerate. These clastics have an immature aspect in both the light- and the heavy-mineral suites. They are truncated by a sub-Cambrian unconformity surface which also planes off extensive areas of the igneous basement. Overlying this regional unconformity is a subarkosic, gritty sandstone of fluviatile facies, known in Israel and Sinai as the 'Amudei Shelomo Formation (Karcz and

Key 1966, Weissbrod 1969b), in Jordan as the Quweira Formation (Wetzel and Morton 1959) or the 'Bedded Arkose Sandstone' (Bender 1963), while in northwest Arabia it forms the lower part of the Saq Formation (the Siq Member) (Powers et al. 1966). Overlying these fluviatile sandstones or separated from them by a thin marine intercalation (the Timna' Formation of IsraelSinai, Burj Formation in Jordan) are subarkosic, variegated, partly shallow marine sandstones: the Shehoret Formation in Israel and Sinai, the Qunaya or 'Massive Brownish Weathered Sandstone' in Jordan, and most of the Saq Formation in northwest Arabia. All these formations are of Early-Middle Cambrian age (based on dating of fossils in the marine intercalations), and form the basal part of a thick series (14001600 m) of Lower Paleozoic, continental to marine subarkoses, arkosic sandstones and quartz-arenites in Jordan and northwest Arabia. In Israel-Sinai, however,

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this sequence goes no further than the Netafim Formation, a quartz-arenite which overlies the Shehoret and older formations with a slight unconformity and which may itself be still of ?Late Cambrian age. The entire Lower-Middle Paleozoic section was differentially truncated toward the end of the Devonian (Gvirtzman and Weissbrod 1985), and subsequently covered by Lower Carboniferous carbonates and quartzarenites (Um Bogma and Abu Thora formations, southwest Sinai). These were overlapped in stages by Upper Carboniferous, Permian and Triassic formations. From regional considerations it appears that the area of study remained largely continental or marginal-marine throughout most of the Late Paleozoic, becoming more marine toward the north during the Early Mesozoic. Clastics of this time are included in the Negev, Ramon and part of the 'Arad groups. A major uplift and truncation between the Jurassic and the Cretaceous caused deep erosion, which throughout the area removed most of the Lower Mesozoic and Upper Paleozoic formations. In many localities the sub-Cretaceous unconformity has superseded the sub-Carboniferous unconformity, and as a result, highly mature quartz-arenites of Early Cretaceous age (the Amir and the Hatira formations of the Kurnub Group) commonly overlie Lower Paleozoic subarkoses or quartz-arenites in southern Israel, southern Jordan or Sinai. Although the primary source of all the clastics, from Late Precambrian times onward, undoubtedly was the craton to the south, several depositional cycles can be

recognized by means of mineral assemblages. Three associations were recognized (Weissbrod 1980, Weissbrod and Nachmias 1986) on the basis of heavy minerals: an unstable chlorite-hornblende-staurolite association which occurs only in Precambrian arkoses and conglomerates; a zircon-tourmaline-rutile (ZTR) plus apatite-barite association which characterizes all Lower Paleozoic sandstones, and an ultrastable, solely ZTR association, which marks all continental sandstones from the Upper Paleozoic upward, with interspersed admixtures of semi- and nonstable minerals of local derivation. A temporary reversal of transport after the Late Devonian uplift is indicated by paleocurrent data from Egypt (Klitzsch 1984) and by the onlap pattern of Upper Paleozoic-Lower Mesozoic formations in Israel and southwest Sinai. This may have contributed to the intensified reworking and overall maturation of the sandstones in this belt, which is intermediate between the cratonic mass and the northern pericratonic margin.

APATITE DISTRIBUTION IN THE PRECAMBRIAN-PALEOZOIC CLASTIC SEQUENCE Following is a brief description of the vertical apatite distribution in the sandstone sequence of Israel and nearby countries (after Weissbrod and Nachmias 1986).

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TUVIA WEISSBROD, ITHAMAR PERATH a n d JOSEPH NACHMIAS

Upper Precambrian. Apatite occurs in the _Zenifim arkoses in variable amounts (up to 7% of the transparent heavy minerals), averaging 2%. This formation, known only in deep boreholes in southern Israel, is stratigraphically equivalent to the Saramuj Conglomerate in Jordan, from which no apatite data were available. Lower Paleozoic (see Fig. 3). Samples were examined from southern Israel (Timna'; Nahal Shehoret) and Sinai (Um Bogma), and from southern Jordan (Wadi Qunaya-Wadi Kerak). Apatite occurs in about 90 out of 140 samples, and consists dominantly of D-apatite. Quantitative results from Arabian sections were not available, though apatite was identified in most Lower to Middle Paleozoic sandstones (Powers et al. 1966) which, unlike in Israel and Sinai, include thick Ordovician to Devonian sediments. Bender (1963, 1968), who examined the Cambro-Silurian sections in southeastern Jordan, did not specify apatite distribution. Apatite in the Lower Cambrian 'Amudei Shelomo Formation (grits and subarkoses) consists mainly of D-apatite, though in places A-apatite is common as a cement (Karcz et al. 1971). In the marine Lower Cambrian Timna' Formation, authigenic apatite occurs in veinlets and in irregular lenses (reputed to be syngenetic--Bar-Matthews 1984) varying from a few cm to 50 cm in thickness and up to 50 m in length. This occurrence is confined to the sandy facies of the section. D-apatite may exceed 20%. In the ?Middle Cambrian Shehoret Formation, which is a variegated subarkose, apatite ranges from 0 up to 85% of the transparent heavy minerals, averaging 8% to 50% in the formation's separate members, with a tendency to decrease upward. In the Timna' Valley section practically all the apatite is authigenic, most likely from the replacement of D-apatite. The range is 0--44%, averaging 4%. In the equivalent Jordanian samples the average apatite content is about 9%. In the ?Upper Cambrian, quartz-arenitic Netafim Formation practically all the apatite is of the A-apatite type, ranging from 0 to 85%, but averaging 2-13% at the various localities. In the equivalent section in Jordan (the upper Qunaya-lower Ram sandstones) the common form is D-apatite, heavily overgrown by A-apatite. As already mentioned, apatite shows no sign of weathering-out in the successive formations of the Lower Paleozoic. On the contrary, at many localities in southern Israel the Shehoret Formation shows significant local enrichments (up to 80% and more--see Fig. 3) of the transparent heavy-mineral suite. In the higher parts of the section, however, it shows notable reduction, disappearing definitively in the Lower Carboniferous Abu Thora Formation. Moreover, a closer look at the individual distribution of A- and D-apatite (see Fig. 2) shows that D-apatite disappears at lower levels than A-apatite, which continues to be present, whether as homogeneous grains or as overgrowths, till this form also disappears. It appears therefore that apatite, either detritic or authigenic, does not fit into the conventional scheme of

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stable and unstable minerals. The ambiguity appears to be a result of apatite's differential susceptibility to single weathering factors. From the following review it becomes evident that soil acidity plays a critical role in apatite weathering, and that the peculiarities of its vertical distribution may therefore be subject to paleoclimatic-paleopedological interpretation--which in turn may add knowledge to the crustal history of the region.

PREVIOUS INVESTIGATIONS OF DETRITAL APATITE

The many heavy-mineral studies that have analysed and assessed the role of mineral stability in the history of a sediment often do not mention apatite. Where mentioned, it occupies a dubious position, even on scales of stability that compare the same set of minerals. Apatite has been variously classified as 'stable' (Smithson 1941), 'semistable' (Freise 1931, Hubert 1971), 'unstable' (Sindowski 1949) and even 'extremely unstable' (Friis 1974). This difficulty did not go unnoticed. Smithson (1941) described apatite as 'stable within the earth's crust, decomposed in weathered rocks'. Pettijohn (1957) placed apatite sixth on a 22-species list of minerals, ranked according to their order of persistence through

Apatite as a paleoenvironmental indicator in the Middle East

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geologic time, yet he mentions that apatite 'seems to be of the chemical environment. However, whereas some unstable in soils'. Several authors mention a similar authors ascribe great importance to weathering (Dryden equivocality with regard to kyanite, and especially gar- and Dryden 1946, Van der Marel 1948, 1949, Hubert net (Smithson 1941, Allen 1948, Piller 1951, Nickel 1962, 1971), others propose post-depositional alteration 1973). by groundwater ('intrastratal solutions') as the major One reason for the vagueness is the non-uniform chemical influence on the heavy-mineral assemblage, meaning ascribed to the concept 'stability'. Although it including apatite (Smithson 1941, Pettijohn 1957). is clear that by 'ultrastable' one refers to minerals that In most weathered sequences, apatite is ranked are highly resistant to both abrasional and chemical among the less stable minerals. Friis (1974), adding his degradation, and that unstable minerals are those that own observations to those of 20 previous authors, ranked are highly vulnerable to both, there is an intermediate 13 of the common heavy minerals into four stability spectrum of minerals with differential resistivities to groups, classifying apatite among the 'extremely unstachemical conditions and to attrition by transport. To ble'. Jackson and Sherman (1953), classifying the <5 these minerals may be accorded different ranks of stabil- micron fraction of 42 minerals into 13 stability ity, depending on the agent to which they have been categories, placed apatite in the 12th category, as did Graham (1940) on the basis of H ÷ reactivity with clay, most exposed. Regarding apatite from this point of view, it may be placing apatite in the 'lowest stability group'. Grimm seen that its behaviour is not as inconsistent as might (1957), ranking 14 heavy minerals according to chemical appear. Studies on artificially or naturally abraded sedi- stability, even placed apatite in the 8th group out of eight ments show that although not ultrastable, apatite groups. remains a persistent member of the heavy-mineral Several authors, however, have remarked on apatite's assemblage under prolonged abrasion. Freise (1931), apparent stability where it is not in the weathering doing dry abrasion experiments, rated apatite as fourth environment. Smithson (1941), remarking on the diffein the order of abrasion resistance of seven minerals, rential post-depositional alteration of various minerals, coming even before quartz. Marsland and Woodruff noticed that the distribution of apatite 'is controlled by (1937), experimenting with windblowing of artificially factors quite different from those which produced the crushed monomineralic aliquots, ranked apatite 5th-- corrosion of garnet'. Piller (1951) ranked apatite as less together with calcite----on a list of seven, by applying stable than garnet, but Fuechtbauer (1974), citing Piller criteria of rounding. Thiel (1940), by wet-tumbling artifi- (1951), noted that this is the case for carbonate-free soils cially crushed and screened minerals in mixtures of two only. Lemcke et al. (1953), who ranked apatite 4th in a or three species, found apatite to be the least resistant of sequence of five, noted that its solubility is lower in the a group of six minerals, noting that 'it is also the most presence of Ca ions. Wieseneder (1953) and Wieseneder soluble'. In his later experiments, wet-abrading crushed and Maurer (1958) found apatite to be among the more and screened mineral grains with equal volumes of stable minerals in intrastratal solutions, placing it in the quartz grains (Thiel 1945), apatite ranked 14th in order second of six groups, after the ultrastable ZTR. Much of the ambiguity concerning apatite stability has of resistance on a list of 23 mineral species. No data on apatite are given in more recent experimental abrasion been clarified thanks to the experimental work of Nickel (1973). Examining the solubility of numerous detrital studies. Natural transport effects on heavy minerals were minerals at various degrees of acidity, he found that at studied by Russel (1936), Rittenhouse (1943) and Van pH values from 0.2 to 5.6 apatite is highly soluble--rankAndel (1950), who presented data from the Mississippi, ing highest among the 12 examined minerals. At pH Rio Grande and Rhine rivers, respectively. Abrasion 10.6, however, apatite ranks only 5th place out of 10 tests notwithstanding, it is the consensus of these authors rankings. The pH values of 5.6 and lower correspond to that fluvial transport, even over great distances, has little those found in the more acid weathering environments, effect either on the frequency or on the size reduction where apatite indeed ranks as an unstable mineral, while of the common heavy minerals, nor has prolonged pH values of 10.6 correspond well to the intrastratal hydraulic reworking in the surf zone, equivalent to solution environment (Fuechtbauer and Mueller 1970, thousands of miles of transport (Van Andel 1959). Fuechtbauer 1974). The rather straightforward conAlthough no author denies the role of transport and clusion, that apatite can be leached out of a sediment reworking as factors of comminution, there is no evi- only through a process of acid weathering, unbuffered dence that this process by itself leads to the complete by carbonate, but will persist during transport and under neutral to alkaline groundwater regimes, may well be elimination of any heavy mineral from the assemblage. Altogether, apatite appears among the less resistant the answer to such apparent bafflements as expressed by minerals on most scales of abrasion resistance, and is Smithson (1941, p. 109): 'The positions assigned to occasionally even used as an index of an assemblage's apatite and kyanite [stable within the earth's crust, non-maturity (Stanley 1965). Two kinds of explanation unstable in weathered rocks] may cause some surprise, are usually offered when a mineral displays erratic pre- but they appear to be valid for the sediments which the sence in a heavy-mineral sequence: the dominant influ- writer has examined', or by Stanley (1965, p. 37), writing ence of source material and provenance terrain (Krynine about the French Alps: 'How non-resistant apatite was 1942, Van Andel 1950, Stanley 1965), and the influence preserved in Permian rocks (and in the later flysch and

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TUVIA WEISSBROD, ITHAMARPERATH and JOSEPH NACHMIAS

molasse) while more resistant species were selectively removed, has yet to be explained.'

PALEOENVIRONMENTAL INFLUENCE ON THE DISTRIBUTION OF APATITE

In the light of the above-described peculiarities of apatite stability, it may be concluded that the apatitebearing sands cannot have spent long time in low-pH weathering environments, or been affected by acid intrastratal solutions. Conversely, it can be stated that the absence of apatite from a sandstone indicates either that it was derived from an apatite-less parent material, or that it was affected--either during transport or at the site of deposition--by acid weathering. The acidic conditions that result in apatite leaching are attained in nature in humus-bearing soils, preferably well-leached laterosols, pedalfers and podzolic soils (Carroll 1970, Fig. 26). No such soils, indeed any soils at all, appear to have developed on the areas that supplied the arkosic clastics of the Precambrian Zenifim, Eilat and Saamuj formations. Their light- and heavy-mineral suites, as mentioned above, are immature (Weissbrod 1980, Weissbrod and Nachmias 1986), and their clay fraction consists for the greater part of detrital, well crystallized illite (degraded micas) with minor amounts of authigenic illite-smectite mixed-layer (symmetrically ordered) minerals (Heller-Kallai and Kalman 1972, Heller-Kallai et al. 1973), typical of regosols. Kaolinite is rare or lacking. The Lower to ?Middle Cambrian clastics ('Amudei Shelomo and Shehoret formations and their JordanianArabian equivalents), consisting of subarkosic grits and sandstones, also bear evidence of poor soil development. These sediments were derived from Precambrian igneous rocks and arkoses, but neither weathering at the site of provenance nor attrition by transport were intensive enough to cause complete elimination of the feldspars, nor was the apatite dissolved. Most of the accompanying clay minerals consist, as in the Precambrian arkoses, of illite, and some illite-smectite mixed-layer minerals, randomly interstratified. Kaolinite is present in small amounts, increasing notably upward (Heller-Kallai et al. 1973). Such clays are typically found today in tundras, red deserts, gray-brown podzolic and noncalcic brown (shantung) soils, planosols and loess (Toth 1964), all of which occur in zones ranging from cold to humidtemperate (Carroll 1970). In Cambrian times these zones carried no plant life as they did from the Devonian onward (Seward 1933, Yaalon 1963), and rock weathering never achieved the pH conditions required to dissolve apatite. (If such conditions had somehow been attained, the apatite would doubtlessly have disappeared from the section, since no carbonate terrains are known at the time whose ionic contribution could have inhibited the dissolution process.) Thus, the presence of non-leached D-apatite together with cold- to

temperate-zone clay assemblages, and the facial evidence of a strong fluviatile system, support the conclusion of Heller-Kallai et al. (1973) about a non-vegetated, temperate, fairly humid continent as the source of the Cambrian clastic sediments. The quartz-arenites of the Netafim Formation and its equivalents, representing mostly second-cycle weathering products of the ?Late Cambrian land surface, are evidently derived from areas where most of the unstable minerals, though not the apatite, had been weathered out. Nevertheless, the upward replacement of D-apatite by A-apatite, sometimes observable (as in the upper part of the Qunaya Sandstone and the lower part of the Ram Sandstone in Jordan, see Fig. 2) as an arrested in situ process, indicates that these strata were affected post-diagenetically by groundwaters of relatively low acidities (pH 5-6), not acid enough to dissolve and flush the apatite but sufficient to favour its in situ or nearby recrystallization. No such acidities are likely to develop in phreatic groundwaters. The origin of these groundwaters was most probably in the weathering zone that developed on the uplifted Late Devonian land surface (Gvirtzman and Weissbrod 1985). The paleosols themselves were subsequently eroded, and it was not before the Early Carboniferous (Visean) that clastic sedimentation in the pericratonic belt was resumed. In northwest Arabia, the Paleozoic clastic sequence continues up to the Devonian with apatite present up to the Late Devonian unconformity (Powers et al. 1966). With regard to the mode of genesis of A-apatite it should be noted, however, that authigenic mineralizations have taken place in the Rift Valley and its vicinity during several post-Paleozoic phases (Segev 1986), and that D-apatite may have been locally affected by these events. The formations that unconformably overlie the Lower-Middle Paleozoic sandstones today are all mature quartz-arenites devoid of apatite. Since they bear no signs of post-depositional weathering or alteration, it is evident that they were derived from areas where apatite was already leached out and where, moreover, soil clays represent a more advanced state of weathering (Bentor et al. 1963). The Lower Carboniferous Abu Thora Sandstone in Sinai--the earliest clastic formation to overlie the Late Devonian unconformity-contains remnants of a Lepidodendropsis-type flora, and clay minerals dominated by kaolinite. This points to a provenance area of vegetated uplands with wellleached humic soils, typical of warm to temperate zones (Dixon 1977), in which apatite weathered to virtual disappearance and the illite clays evolved into kaolinite. The Abu Thora Formation's generally fluviatile character points to active transport and reworking of this parent material, while the presence of clay and coal streaks indicates deposition in humid lowlands. It seems that humic soils of this type, developing from the Devonian onward, were the environment in which not only the apatite of the source areas was dissolved, but also the above-postulated groundwaters were generated which percolated through the sub-Carboniferous

Apatite as a paleoenvironmental indicator in the Middle East unconformity and caused some recrystallization of apatite in the vadose zone of the underlying Netafim or Shehoret formations or their equivalents. In any case, in the absence of Late Devonian sediments the Carboniferous is the earliest date for which evidence exists of conditions that could provide an acid soil environment. The apatite-free continental sandstones from the Carboniferous onward (the Negev, Ramon, 'Arad and Kurnub groups) are all mature quartz-arenites with a dominantly kaolinitic clay-mineral content. This, together with remains of a Klukia-Sagenopteris flora (Lower Mesozoic) and a Weichselia (Middle Mesozoic) flora, indicates the spread of an equitable, warm to tropical climate (Barnard 1973), under which humic or even lateritic soils could have developed. Apatite does not reappear until the Cretaceous, where autochthonous types of carbonate apatite occur with smectite clays within the marine section.

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OTHER PALEOGEOGRAPHIC EVIDENCE

Beyond mineral differences, the entire Late Precambrian-Middle Mesozoic sequence shows no significant change of sedimentary facies. Throughout 470 million years it remained part of a pericratonic sedimentary blanket, ranging between piedmontal to nearshore marine. No evidence of significant tectonism or volcanism is found at the sites of deposition, nor are such events indicated for the areas of provenance, except for slow, vertical epeiric movements. Thus, the paleoenvironments that influenced weathering, transport and soil conditions during the long time span under consideration, appear to have been controlled by climate rather than by tectonic events, although the change of paleoclimates by itself was evidently the result of continental drift. It should be noted that Bentor et al. (1963), writing before the days of paleomagnetism and plate tectonics, regarded the temporal changes in the clay spectrum of southern Israel as a manifestation of slow evolution from continental-clastic (initially non-vegetated) to increasingly marine environments, without evidence of major climatic change. Paleogeographical reconstructions (Fig. 4) for the Paleozoic-Early Mesozoic, based on integrated paleomagnetic and paleoclimatic data outside the area of study (King 1961, McElhinny 1973, Smith et al. 1973, Scotese et al. 1979, Ziegler et al. 1979), show the presentday Arabo-Nubian area to have been at south temperate latitudes (45-55 °) during the Middle-Late Cambrian. Through the Ordovician-Silurian this crustal area drifted, as part of the African paleocontinent, along a high-latitude southern trajectory which took it to temperate-subpolar regions, and then again northward. This time span is represented by alternating shallow marine and continental clastics in Arabia and southern Jordan. Due to the Late Devonian uplift and truncation in Sinai and Israel we have no evidence of contemporary continental climates there, but it is doubtful whether, at the postulated latitudes of 70° south lat. in the Ordovician

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Fig. 4. Apatite and clay assemblages in the Near East clastic sequence, as a function of continental drift and vegetation. (1) Based on King (1961), McEIhinny (1973), Smith etal. (1979) and Ziegleretal. (1979). (2) Based on Barnard (1973) and Chaloner and Lacey (1973). (3) Based on Bentor et al. (1963), HeUer-Kallai and Kalman (1972), and Heller-Kallai et al. (1973). (4) Based on Weissbrod and Nachmias (1986) and Powers etal. (1966).

(glacial till was found in central Arabia; McLure 1978), 45-50 ° south lat. in the Silurian, and still over 40° south lat. in the Devonian, any soil of high acidity could develop on the still-unvegetated uplands. In any case, no plant fossils older than Carboniferous have been found in the area. As the supercontinent of Gondwana assembled during the Devonian and a uniform land flora developed over the southern hemisphere (Chaloner and Lacey 1973), the Arabo-Nubian area moved back into subtropical latitudes (25-20 ° south lat.), at the same time undergoing mild uplifting and truncation during Late Devonian and Permian times (Gvirtzman and Weissbrod 1985) along its northern margin. Lower Carboniferous faunas of the marine phase between these events, found in southwest Sinai, are of a subtropical character with corrected oxygen isotope values suggesting water temperatures of about 25-28°C (Kora 1984). The development of a subtropical highland vegetation under humid conditions (throughout the Upper Paleozoic no part of the AraboNubian craton is more than 10° from the continental edge), and the fluvial-floodplain character of the Lower Carboniferous sandstones with their Lepidodendropsis floral remains, all indicate that the climate and the geographic setting were favourable for the development of well-leached podzols or even laterosols, with accompanying development of kaolinite-type clays (Bentor et al. 1963) and the local dissolution of bedrock apatite. Apparently, those conditions persisted throughout the Early Carboniferous, when low-pH groundwaters from

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TUVIA WEISSBROD, ITHAMAR PERATH a n d JOSEPH NACHMIAS

the Soil environment penetrated the Lower Paleozoic sandstones of the substrate, effecting D-A apatite transformation in the vadose zone. It must be noted that, although these processes could take place under a tropical climate, there is no evidence of the 'ferralization' (Millot 1970) that produces lateritic soils. CONCLUSIONS (1) Apatite is irregularly present in all Precambrian and Lower Paleozoic clastics of Sinai, Israel, northwest Arabia and southern Jordan, accompanying the ultrastable heavy-mineral assemblage and showing no trends of weathering. (2) From the Lower Carboniferous upward, apatite is totally Jacking in the heavy-mineral fraction. (3) The apatite in the section underlying the unconformity that truncates the Lower Paleozoic (formed in Late Devonian and Permian times and rejuvenated in the Early Cretaceous) is either authigenic (A-apatite), or partly recrystallized D-apatite. (4) Since apatite is fairly resistant to mechanical wear, but vulnerable to acidic weathering, it would appear that no acidic soil conditions existed throughout the provenance area of the clastics, from Late Precambrian times up to the Carboniferous, nor were the deposits subjected to diagenesis by acid (<6 pH) intrastratal solutions. (5) During the Late Devonian uplift, acidic soil conditions developed in the provenance areas of the Upper Paleozoic (Carboniferous and onward) clastics, causing dissolution of the apatite. Similar conditions at the sites of sandstone deposition caused the formation of acidic soil fluids, which locally effected partial or complete recrystallization of apatite in the immediately underlying formations. (6) Previous investigators found that the fine-grained fraction accompanying the apatite-bearing section consists of illite and mixed-layer type clays, with some kaolinite, evolving upward into a dominantly kaolinitic clay assemblage. This is the change one would expect if a non-vegetated regosol evolved into a well-leached podzolic soil, or even further into a laterosol. (7) During all this time, from the Late Precambrian to the Middle Mesozoic, neither tectonism nor volcanism affected the Arabo-Nubian craton. The mineralogic changes that evolved during this time are to be regarded as a sequence of maturation, and not in the least influenced by climate, as linked to continental drift. (8) Assembled data of various authors, expressed on their respective Paleozoic world maps, show that during this time span the Arabo-Nubian craton, as part of the African paleocontinent, moved from temperate to subpolar latitudes, then back again to subtropical and eventually equatorial latitudes. The latter stages coincided with the Paleozoic development of a Gondwanan Lepidodendropsis flora, followed during the Mesozoic by Eurasian Klukia-Sagenopteris and Weichselia equatorial floras. These stages agree well with the evolution of soil conditions as reflected in both clay- and heavy-mineral compositions.

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