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Deep Sea Drilling Project Initial Reports Volume 66 11. PETROLOGY OF MIDDLE AMERICA TRENCH AND TRENCH SLOPE SANDS, GUERRERO MARGIN, MEXICO1 Steven B. Bachman, Department of Geological Sciences, Cornell University, Ithaca, New York and Jeremy K. Leggett, Department of Geology, Imperial College, London, United Kingdom INTRODUCTION son and Suczek (1979). The monocrystalline quartz has both undulatory (>5° extinction) and nonundulatory The sandstone petrology of Leg 66 samples provides extinction in different grains. Subgrain boundaries are insights into changes through time in the geology of the developed in some grains. There is a large variety of source regions along the Guerrero portion of the Middle poly crystalline quartz types; most samples have about America continental margin. This in turn constrains equal percentages of finely crystalline (>5 crystals/ possible models of the evolution of the Middle America grain) and coarsely crystalline (<5 crystals/grain) Trench (e.g., de Czerna, 1971; Malfait and Dinkleman, grains. Many of the finely crystalline quartz grains ap- 1972; Karig, 1974). Primarily medium-grained sands pear to be derived from rocks with a strain and/or meta- and sandstones, representing the widest variety avail- morphic history: polyganized, bimodal (partial recrys- able of trench/trench slope settings and ages, were tallization of the larger set of grains), and tectonite analyzed in both light and heavy mineral studies. Stan- (highly oriented crystals with elongate boundaries) types dard techniques were used as much as possible in order predominate. Plagioclase and potassium feldspar occur to compare results from other margins and from ancient in near equal percentages (Qm-P-K plot, Fig. 1). Micro- rocks. cline is a common accessory in all the samples but is the predominate potassium feldspar in samples in the lower LIGHT MINERALS 100 meters of Site 493 on the upper slope. Only albite twins, which are locally bent or kinked, were observed METHODS in plagioclase crystals. Sand and lithified sandstone samples were prepared for analysis Rock fragments are primarily sedimentary, metamor- such that direct comparisons of their petrology are possible. Medium- grained core samples were used exclusively. When graded beds were phic, and plutonic in origin. Volcanic lithic fragments encountered, the medium-grain-sized intervals were sampled onboard. are uncommon. The sandstone and mudstone clasts Sand samples 10 to 20 cc were split in half, then washed in a stainless look identical to the sediments deposited on the trench steel sieve to separate the fine fraction (< 0.063 mm). The coarser slope and probably represent material ripped up and fraction was then mounted in epoxy in cylindrical molds from which standard thin sections were cut. Thin sections were made directly from redeposited on the slope and in the trench. Common the lithified sandstones. carbonate shell fragments occur even in Quaternary Standard petrographic techniques for point-counting (Dickinson, sands deposited below the CCD. Apparently the mega- 1970; Graham et al., 1976) were used on both the sand and sandstone fossil fragments and some of the foram testes were samples. A discrepancy between the two sampling methods occurs at transported from shallow water settings; the only possi- this point; Dickinson's techniques for point-counting sandstones defines all grains smaller than 0.03 mm as matrix, whereas for the ble in situ megafossil fragments are from the lower part standard sieving technique, the defining fraction is 0.063 mm. We of Site 493, where water depths were relatively shallow used the standard techniques for each sample type because we feel it to (see site chapter). Metamorphic rock fragments are be important for comparison with other data sets. Counts from sand schistose and gneissic. The dominant rock type, quartz- and sandstone samples within several meters of each other gave almost mica schist containing both biotite and white mica, is identical results; thus the difference in grain size limits does not ap- pear to affect the results appreciably. A total of 29 unlithified sands similar to basement rocks cored at Site 489. Rare plu- and 4 sandstones were counted at 160 power, using a 0.5-mm spacing tonic rock fragments contain quartz, potassium feld- between points; 400 to 500 grains were counted in each thin section. spar, plagioclase, and biotite. Feldspars were identified, using the staining techniques of Bailey and Stevens (1960). The only lithified sandstones are of early Miocene age, near the base of Sites 489 and 493. They are ce- Results mented by variably recrystallized micritic carbonate mud, with local extensive patches of recrystallized spar. The sands and sandstones are arkosic to subarkosic Alteration products in most of the sands were ap- (Pettijohn et al., 1972) in composition (Fig. 1; Table 1). parently inherited from source terrains. Feldspars vary The mineralogic components of the samples have been from altered to unaltered within the same sample. How- grouped and plotted on triangular diagrams (Fig. 1), ever, in the lower cores of Sites 489, 490, and 493, car- using the methods of Graham et al. (1976) and Dickin- bonate has partially replaced several feldspar grains. In the lithified sandstones, it is apparent that this replace- Initial Reports of the Deep Sea Drilling Project, Volume 66. ment is diagenetic rather than inherited. 429 S. B. BACHMAN, J. K. LEGGETT Qm L F Trench Environment Qm E. Plio. L. Mio. Slope Environment E. Mio. Upper Slope Lv Lsm P K Figure 1. Triangular diagrams of detrital compositional modes of sands and sandstones from the Guer- rero subduction complex. See Dickinson and Suczek (1979) for discussion of end members. Q = total quartz; F = total feldspar; L = total lithics; Qm = monocrystalline quartz; Lt = total lithics plus polycrystalline quartz; Lv = volcanic lithics; Qp = polycrystalline quartz; Lsm = sedimentary and metamorphic lithics; P = plagioclase; and K = potassium feldspar.) The triangular diagrams in Figure 1 indicate the ferent size fractions are mounted on each disc. The mounted grains relative fractions of major components of sands of are then ground and polished on one side. With careful choice of grain concentration and monitoring of the thickness of the mount, a large varying ages and depositional settings. Sediments older enough number (> 200) of grains, if available in the original separate, than Quaternary have been classified in a trench or will be exposed at the surface and may be examined using both re- slope setting according to their inferred depositional flected and transmitted light microscopy. In the microanalyzer the sites (see discussion in site chapters). The lower Miocene grains are moved sequentially under the electron beam, and an energy- dispersive spectrum is acquired. If a composite grain is suspected from upper slope sediments generally have a higher percent- prior optical inspection, the procedure is repeated several times to en- age of feldspar (particularly potassium feldspar) and sure reproducibility. If this is not reasonably good (bearmE m mind, metamorphic rock fragments. The Pliocene sediments, for example, compositional variations across garnet grains), the grain both from trench and slope settings, have larger frac- is rejected. In many cases the observer is able to identify the mineral, tions of quartz, particularly monocrystalline quartz. or at least classify it into a group in a time as short as ten seconds. In more difficult cases fully quantitative analyses may be required, and in these cases spectra may be stored on disc and processed later to be HEAVY MINERALS compared with mineral standards and analyses. On average, up to 100 grains per hour may be easily characterized. METHODS A specific advantage of the technique, aside from speed and utility Table 2 contains grain size and sorting statistics for 18 samples to the inexperienced petrographer, is that it can be used to subdivide from the fine-medium or coarser sands, dried and sieved to 0.300 groups of minerals according to their chemistry. For example, garnet mm, 0.210 mm, 0.125 mm, 0.063 mm, 0.053 mm, and finer divisions. species are relatively easy to identify, whereas it is very difficult using Heavy minerals were separated, using traditional techniques, with tet- the traditional combination of optics and X-ray diffraction. Opaque rabromoethane (specific gravity 2.97) as the separating medium. minerals, not normally dealt with in heavy mineral studies, can be A technique for heavy mineral characterization, using the electron easily included in the analysis and indeed in this case have proved very microprobe with E.D.S. (energy-dispersive spectrometer), was em- useful. ployed to count grains. In many respects this method has advantages Some minerals, however, cannot be distinguished by chemistry both in speed and accuracy over traditional methods. Different size alone, particularly when short counting times and semi-quantitative fractions of the heavy mineral concentrates are mounted in shallow analyses are used. The examples relevant to this study are the Al- grooves cut across circular epoxy or Perspex discs; normally three dif- silicate polymorphs and certain clinopyroxenes and amphiboles ap- 430 SAND PETROLOGY OF THE MIDDLE AMERICA TRENCH Table 1. Modal percentages of detrital components as percent volume, Leg 66 sands. Sample Qm QP Qp 5 Qp > 5 Qpch Qp bimodal Qp polyg Qp tect P K Lv Lsm Carb. Glauc. Mafics 486-3-2, 66-68 43.0 13.4 5.9 7.5 14.9 12.2 0.5 2.9 5.1 7.8 486-3-6, 58-60 40.0 15.7 8.6 6.9 0.2 18.4 12.0 1.7 2.7 2.7 6.9 486-5-2, 40-42 43.0 16.8 8.1 8.7 1.3 1.7 16.1 12.8 0.3 4.0 4.7 2.3 486A-1-2, 90-92 41.4 8.0 4.0 4.0 0.5 16.8 14.9 0.9 2.4 4.5 11.1 488-26-6, 13-15 50.2 4.9 3.5 1.4 0.3 0.3 25.1 11.1 0.7 0.7 2.4 4.5 488-35-1,88-90 43.8 11.4 6.7 4.7 1.3 17.0 12.2 0.9 13.3 0.2 1.3 488-35-1, 130-132 40.7 26.1 16.9 9.2 3.2 11.9 9.9 3.2 6.7 0.7 0.7 488^5-1,61-63 53.4 12.7 6.8 5.9 1.2 17.8 12.9 1.0 0.7 0.5 1.0 488-45-2, 42-44 50.7 19.5 10.2 9.3 1.0 1.0 10.2 16.4 1.2 1.4 1.0 0.2 489A-25-5, 37-39 47.0 3.2 1.4 1.8 19.2 25.8 0.7 1.8 0.5 1.8 490-5-7, 9-11 22.4 9.7 3.8 5.9 0.2 0.7 5.7 7.3 0.7 2.1 tr.
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