LETHAIA SEMINAR

LETHAIA 10 (1977) 144

Genesis of beds rich in phosphate nodules - a historical note WERNER D. BRUCKNER

From a detailed study of phosphate nodules in the Cenomanian Glauconitic Marl of southeast England, Kennedy & Garrison (1975) concluded that their forma- tion involved: (1) infilling of shells, (2) burial, (3) pre- fossilization-cementation of fossil infillings, probably by high magnesian calcite, (4) dissolution of aragonitic shell material, (5) disinterment and exposure of moulds on the sea floor, followed by (6) phosphatization, boring, and encrustation by various organisms. Many nodules showed evidence of having passed through several cycles of this type. To those authors’ knowledge ‘the literature on phosphate nodules contains no de- scriptions of complex burial-exhumation cycles of the kind we have described for nodules in the Glauconitic Marl’, but they suggest ‘that phosphate nodules pro- duced and modified in this manner are common’ and give some examples from the Cretaceous of western Europe.

Yet Kennedy & Garrison have overlooked pertinent work written about 40 years ago - understandably per- haps because of the unrevealing titles of these publica- tions: they dealt with phosphate beds in the Helvetic sequence of the Swiss Alps and contained conclusions very similar to their own. The phosphate beds con- cerned - known for their fossils for more than a cen- tury - were studied in much detail in areas of differing Helvetic facies by Fichter, Schaub and Briickner, then doctorates of the University of Basle. Their results formed parts of dissertations published in 1934, 1936, and 1937, respectively, and they were dealt with again in papers by Schaub in 1948 and Bruckner in 1951.

All three authors recognized that simple to multiple reworking, more or less in situ, coupled with simple to multiple phosphatization of calcareous sediment mat- ter, had played a significant role in the formation of these ‘horizons of condensation’, as they were called at that time. Fichter’s work (1934:53-84) dealt with a sequence of late Aptian to Turonian age, in which rock fragments and large quartz grains derived from clearly identifiable sources nearby, as well as mixing of phosphatized fossils of different ages in the same bed, were proof of reworking. Bruckner (1937) re- corded twelve phosphate-bearing horizons ranging in age from Valanginian to Cenomanian, ten of which were beds with phosphatized fossil moulds and nodules and two were instances of hard-ground phosphatization. Reworking was recognized clearly, in several cases from contrasting lithologies of phosphatized bodies and enclosing matrix, and in the uppermost bed also from chance mixing of Upper Albian and Cenomanian fos- sils (Briickner 195 1 : 199).

The most remarkable study, however, was that of Schaub (1936). Dealing with a thin bed full of phos-

phate nodules and mixed fossils representing six Albian ammonite zones, he thin-sectioned identified phosphate fossils and found that the phosphatized sediment of each ammonite zone was characterized by a specific lithology distinct from those of the others and that of the matrix. This indicated a cyclical history of forma- tion, each cycle leading from slow, mainly calcareous, deposition (differing in character from cycle to cycle) to phosphatization with reworking in situ, a genesis identical in principle to that inferred by Kennedy & Garrison (1975) for the Glauconitic Marl. In his paper of 1948, Schaub reiterated his results more concisely, emphasizing the repetition of phosphatization and re- working. In 1965, he died suddenly, only 53 years old, at the peak of a successful career as a petroleum geo- logist (Mohler 1965). It is the main purpose of this note to remind present-day students of phosphate beds of his ingenious pioneer work on their complex genesis.

The author wishes to thank Professor M. M. An- derson for editorial assistance.

References Briickner, W. 1937: Stratigraphie des autochthonen

Sedimentmantels und der Griesstockdecke im oberen Schachental (Kanton Uri), eine Studie der nord- helvetischen Schichtfolge. Verh. natf. Ges. Basel 48, 77-181.

Bruckner, W. 1951: Lithologische Studien und zyklische Sedimentation in der helvetischen Zone der Scbwei- zeralpen. Geol. Rrrndsch. 39, 196-212.

Fichter, H. J. 1934: Geologie der Bauen-Brisen-Kette am Vierwaldstattersee und die zyklische Gliederung der Kreide und des Malm der helvetischen Decken. Beitr. geol. Karte Schweiz N.F. 69. 128 pp.

Kennedy, W. J. & Garrison, R. E. 1975: Morphology and genesis of nodular phosphates in the Cenoma- nian Glauconitic Marl of south-east England. Lethaia 8, 339-360.

Mohler, W. 1965: Dr. Hanspeter Schaub 1912-1965. Bull. Ver. scltweiz. Petroleum-Geol. u. -Ing. 32, 15- 76.

Schaub, H. P. 1936: Geologie des Rawilgebietes. Eclo- gae geol. Helv. 29, 337-407.

Schaub, H. P. 1948: Uber Aufarbeitung und Kondensa- tion. Eclogae geol. Helv. 41, 89-94.

Werner D. Briickrter, Departntent o f Geology, Memo- rial University of Newfoundland, St. John’s, Newfound- land, Canada, A IC 5S7: 10th January, 1977.