NOTES 417 MYERS, R. L. AND VAN SICLEIg, D. C., 1964, Dynamic phenomena of sediment compaction in Matagorda County, Texas: Trans. Gulf Coast Assoc. Geol. Soc., V. 14, p. 241-252. PAINE, WM. R., 1966, Stratigraphy and sedimentation of subsurface Hackberry wedge and associated beds o~ southwestern Louisiana : Trans. Gulf Coast Assoc. Geol. Soc., V. 16, p. 261-274. Roci-ioN, R. W., 1967, Relationship of mineral composition of shales to density: Trans. Gulf Soc., V. 17, p. 135-142. SLOANE, B..T., JR., 1966, The structural history of the Houma Embayment: Trans. Gulf Coast Assoc. Geol. Soc., V. 16, p. 249-260. T1MM, B. C. ANO MARlCELLI,J'. J., 1953, Formation waters in southwestern Louisiana: Bull. Amer. Assoc. Petrol. Geol., 37, No. 2, p. 394-409. DEDOLOMITIZATION FABRIC IN PEELS: A POSSIBLE CLUE TO UNCONFORMITY SURFACES 1 MOSHE BRAUN AND GERALD M. FRIEDMAN Geological Survey of Israel, Jerusalem, Israel and Rensselaer Polytechnic Institute, Troy, N.Y. 12181 ABSTRACT Dedolomitizafion is usually a surface phenomenon related to weathering surfaces and hence is commonly found below unconformities. These unconformities may be ancient ones. Recognition of dedolomite in sub- surface may suggest a possible unconformity. Thin sections and stained peels of dedolomite under plane polarized light may reveal a crystalline lime- stone devoid of ghosts of dolomite rhombs. However, if exactly the same area that has been examined under plane polarized light is viewed under crossed nicols the dolomite ghosts may become evident. These ghosts are diagnostic of dedolomitization. Peels are normally black under crossed nicols because the acetate is isotropic. Etching may bring out solubility differences between calcite crystals making up the matrix of the dedolomite and those in the original rhombs. During peel preparation, the mlcron-sized crystals occupying the original rhombs may become attached to the surface of the peel and hence show up as an anisotropic substance demarcating the dolomite rhombs as ghosts. Not all peels of dedolomite show dolomite rhombs under crossed nicols. Calcitization of dolomite, known by the term of dedolomitization, is common in many carbon- ate terrains. This mineralogical replacement of dolomite by calcite may be complete, but com- monly is only partial. This replacement is usu- ally a surface phenomenon related to weather- ing surfaces. Hence dedolomite is commonly found below unconformities or in areas of karst development; karst surfaces themselves can also be considered unconformities. Dedolomitization has been reported from many areas; Friedman and Sanders (1967, p. 300-302) summarize the subject and cite many examples. Although dedolomitization is related to weathering surfaces, these surfaces may be an- cient ones. This observation is an important one. Ancient unconformities in single drill holes may escape detection, but the presence of dedolomite below a contact may suggest a possible uncon- formity. A method has been worked out which may help in the recognition of unconformities in do- Manuscript received December 3, 1968; revised July 20, 1969. lostones which have been affected by dedolomiti- zation. This method depends on the preparation of stained peels and their examination under the petrographic microscope. The samples employed in this study include dedolomites from the Nezer Formation of Turonian age, the Amina- day Formation of Cenomanian age and the J~ Limestone of Jurassic age; all from Israel. Thin sections and stained acetate peels (Katz and Friedman, 1965) of these samples were pre- pared. In the thin sections, the samples are crys- talline limestones. Individual crystals may range from micron size to 1 and even 3 mm in diame- ter. Under plane polarized light in peels the ap- pearance of the limestones is almost identical to that in thin section. Hence neither in thin sec- tions, nor in peels under plane polarized light, would it be possible to know whether the lime- stone has formed by dedolomitization, or is a product of neomorphism of pre-existing lime- stone in which the depositional fabric has been obliterated. In samples which show only partial dedolomitization, or those in which ghosts of former dolomite crystals remain, this differen- tiation is of course a simple one. This observa-