size-sorting of foraminifera in graded beds, wairarapa, new zealand

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  • This article was downloaded by: [Fondren Library, Rice University ]On: 13 November 2014, At: 04:28Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH,UK

    New Zealand Journal ofGeology and GeophysicsPublication details, including instructions forauthors and subscription information:http://www.tandfonline.com/loi/tnzg20

    Size-sorting of foraminifera ingraded beds, Wairarapa, NewZealandPaul Vella aa Geology Department , Victoria University ofWellington , WellingtonPublished online: 12 Jan 2012.

    To cite this article: Paul Vella (1963) Size-sorting of foraminifera in graded beds,Wairarapa, New Zealand, New Zealand Journal of Geology and Geophysics, 6:5,794-800, DOI: 10.1080/00288306.1963.10423625

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    http://www.tandfonline.com/page/terms-and-conditions

  • 794 [Nov.

    SIZE-SORTING OF FORAMINIFERA IN GRADED BEDS,W AIRARAPA, NEW ZEALAND

    PAUL VELLA

    Geology Department, Victoria University of Wellington

    (Received for publicatiou, 14 AuguJt 1962; as modified, 20 March 1963)

    ABSTRACT

    From two samples, one from sandstone and one from mudstone, from a gradedbed in Upper Miocene turbidites at Cleland Creek all the specimens of two speciesof Foraminifera were measured. Small specimens predominate in the mudstone andlarge specimens in the sandstone. The maximum size is about the same in bothsandstone and mudstone, but the minimum size is much smaller in the mudstone. Thesize-frequency distributions appear to be nearly typical for sandstones and mudstonesof turbidites.

    INTRODUCTION

    A previous paper (Vella, 1963) lists Foraminifera in Upper Mioceneturbidites at Cleland Creek, Wairarapa, and discusses vertical changes intaxonomic composition of faunas within individual rhythms. The followingaccount describes a preliminary investigation of vertical change in averagesize of foraminiferal shells within one rhythm (graded bed) at ClelandCreek. The location is shown in the earlier paper (Vella, 1963, fig. 1).

    Orbell (1962) collected samples from the sandstone and mudstonephases of several rhythms. During examination for age determination itwas noticed that the average size of the Foraminifera in the sandstonesamples was greater than in the mudstone samples. This applies to samplesas a whole as can be easily seen by comparing mounted slides, and toindividual species. For a more quantitative investigation of the size distribu-tion of foraminiferal shells, two of Orbell's samples from one rhythm wereused. They were not prepared especially for the purpose, and the resultsserve merely to indicate that the shells are definitely size-sorted and thata more thorough investigation is warranted.

    SIZE-FREQUENCY DISTRIBUTIONS OF INDIVIDUAL SPECIES

    Only a few samples contain enough specimens of an individual speciesto show statistically significant differences in size-frequency distribution.A useful species must have a reasonably large adult shell and a smalljuvenile shell, and must be abundant in both the sandstone phase and themudstone phase of one rhythm. Generally only a small proportion of speciesare abundant in any non-turbidite sample, and any species is generally lesscommon in turbidite samples than in non-turbidite samples. Furthermore,species that are common or abundant in one phase of a turbidite rhythmmay be infrequent or rare in the other phase.

    NZ. J. Geol. Geopbys. 6 : 794-800

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  • 19631 VELLA -. SIZE-SORTING OF FORAMINIFERA 795

    The species that shows most obviously the size difference between sand-stone and mudstone phases is Karreriella cylindrica Finlay, a large arenaceousspecies with distinctive juvenile and adult stages, which is generally repre-sented mostly by juveniles in the mudstones and mostly by adults in thesandstones. Unfortunately it is too infrequent at Cleland Creek for statisticalstudy without much larger samples than those available.

    The most satisfactory examples found were Hojeeruo (Trigonouva) d.zeacuminata Vella and Notorotalia d. depress Vella in sandstone and mud-stone samples of a rhythm at locality NI58/625. The two samples were ofthe size usually collected as spot samples, weighing approximately one pound,and were washed by a routine method over a 240 mesh screen. All speci-mens of both species were picked, and reasonably large numbers wereobtained from the sandstone sample, but rather small numbers from themudstone sample. Using an ocular scale, specimens of Hojkeruva weremeasured between the apex and the apertural end, and specimens ofNotorotalia were measured along the maximum diameter. Measurementsand numbers of specimens are given in Table 1, and are plotted as histo-grams showing percentage of specimens against linear increases in sizein Fig. 1 A-D.

    BNoforotttha. cf depr-essamudstone phase

    NotoYotalia cf201% de.pre.ss a:

    sa~5to"e. pha.se.10 n= 64-o

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    10 n=/8

    -~~---

  • 796 NZ. JOURNAL OF GEOLOGY AND GEOPHYSICS [Nov.

    TABLE I-Dimensions of Two Species of Foraminifera in Sandstone and Mudstone ofa Turbidite Rhythm

    (Locality NI58/625, Cleland Creek; grid. ref. 278817)

    Number of Specimens

    Trigonouva N otorotalia

    Size cf. zeacuminata cf. depressa

    (mm) Sandstone Mudstone Sandstone Mudstone

    0'260'280'300'32 20'34 10'36 3o37 40'39 2 3041043 2045 1 1047 1 2 1049 10'51 1 2 10'52 1 2 10'54 1 2 10'56 2 5 3O 58 2 6060 3 1 3 1062 3 2 2 2064 2 2065 2 3067 3 6069 3 1 20'71 4 1 5 10'73 20'75 2 4 20'77 3 60'79 2 10'80 4 10'82 1 2084 30'86 2 10'88 40'900'92 20'94 40'96 30'97 10'991'011'031'05 11'07118

    Totals 58 26 64 18

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  • 1963] VELLA - SIZE-SORTING OF FORAMINIFERA 797

    The three main features of the size-frequency distributions are: (1)juveniles predominate in the mudstone whereas adults predominate in thesandstone; (2) specimens with a measured dimension less than 0'45 mmoccur only in the mudstone phase; (3) a greater range of sizes occurs inthe mudstone than in the sandstone, the maximum size being about thesame in both mudstone and sandstone. Larger samples from the mudstonecould not upset these conclusions.

    In general, all sizes of shells, from proloculum stage to adult, are repre-sented in any unsorted sample of Foraminifera. The absence of specimensless than 0'45 mm (which is considerably larger than the proloculum ofeither species) in the sandstone phase therefore indicates mechanical sortingof the shells. The high proportion of small specimens in the mudstone isalmost certainly to be correlated with the high proportion of large speci-mens in the sandstone, and is due to mechanical sorting.

    In their sorted condition the foraminiferal shells are not representativesamples of the original living populations and should be regarded assedimentary particles. Histograms showing percentages of specimens plottedlinearly against size are appropriate for studying foraminiferal populationsbut not for studying sediments. The data were therefore replotted to showpercentages of specimens against a (logarithmic) Wentworth Scale withdouble the usual number of divisions (Fig. 2 A-D). This has the effect ofcompressing the larger sizes into a smaller number of classes, making theright-hand side of each histogram more compact.

    The strongly unimodal size-frequency distribution and sharp cut-off ofthe small sizes suggests fairly good sorting in the sandstone, but clasticparticles of much larger size than the largest measured foraminifer alsooccur abundantly in the sandstone. The lack of small specimens is the mostsignificant feature.

    The occurrence of much smaller specimens, and the predomina

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