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

    To link to this article: http://dx.doi.org/10.1080/00288306.1963.10423625

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  • 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:

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  • 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 predominance ofsmall specimens, in the mudstone are the most significant differences betweensize frequencies in mudstone and sandstone. The smaller sizes cut offsharply in the mudstone, as in the sandstone, but this cut-off is controlled bythe opening size of the sieve used for washing the samples. The larger rangeof sizes in the mudstone does not indicate poorer sorting than in the sand-stone, because the maximum size of shells is less than the maximum size ofclastic grains in the sandstone. The maximum size of shells in the mud-stone is about the same as the maximum size of shells in the sandstone.Viewed as particles that have settled from suspension, the shells in the mud-stone can be interpreted as a poorly sorted fraction, which settled laterthan the sandstone fraction. Adult specimens (and large clastic grains)in the mudstone may have been carried in by the waning turbidity currentafter deposition of the lower part of the rhythm, or may have settled lateowing to their having been carried exceptionally high above the sea floor byeddies generated above the turbidity current.

    DISCUSSION

    Scott (1961), discussing the mode of formation of Upper Miocenegraded-bedded strata at Mangatiti Road, 25 miles north-east of ClelandCreek, gave size-frequency histograms for "Uvigerina miozea", a species

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  • 798 NZ. JOURNAL OF GEOLOGY AND GEOPHYSICS INov.

    Jrijonou va. cfZt>OCU "';"4fa

    m"'clstol\e phase

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    Tr,!onouva. efz~acm"l?aTa.

    S~ndstone pho.se

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  • 1963] VELLA - SIZE-SORTING OF FORAMINIFERA 799

    increases in size. Size frequencies in the mudstone phases of the gradedbeds are similar to those inferred for Elphidium crispum inbeing bimodal,and Scott thought this likely to be due to a high mortality rate of veryyoung individuals and to indicate that the fossil populations in the mudstonephases were not sorted. This conclusion would imply either that the gradedbeds at Mangatiti Road are not turbidity-current deposits, or that the upperpart of each mudstone is a distinct layer not part of the graded bed depositedby each turbidity current.

    The massive mudstone facies of this age in the Wairarapa is generallyan extremely poorly sorted sediment (Orbell, 1961) and probably containsunsorted fossil populations. The size frequencies of "Uvigerina miozea" inthe massive mudstone (Figs. 1 H, 2 H) are not bimodal, but on the contraryshow nearly uniform percentages of nearly all except the largest sizes ofshells. The bimodal size frequencies of Elphidium crispum therefore cannotbe taken as typical of unsorted populations of all species.

    In their extraordinarily high content of juvenile shells the mudstonephases of the graded beds at Mangatiti Road differ from Scott's otherexamples. Although they resemble the mudstone phase at Cleland Creek, thesize range of the shells is larger, the minimum size is smaller, and thepeak-frequency size is smaller (shown most clearly by Fig. 1 E, F)-probablybecause a finer screen was used in preparation of the samples.

    If Scott's samples (which were taken from the topmost parts of tworhythms) were from turbidites they could well represent that final stagewhen the finest material in the cloud formed by the turbidity current wassettling. The secondary peaks of larger sizes of shells (Figs. 1 E, F, 2 E, F),which are not found in the Cleland Creek mudstone phases, are notexplained; they may indicate a somewhat different mode of deposition.Alternatively, these secondary peaks may be subjective, because "Uvigerinamrozea" might include more than one species. Furthermore Figs. 1 F and 2 F(Scott's fig. 3) were based on specimens from two samples.

    Scott stated that juveniles and adults occur in his sample 4 from thesandstone at locality N154/521 (Mangatiti Road), but gave no indicationof relative frequency or of minimum size. The size frequencies of "Uvigerinamiozea" in the sandstone phase of Upper Oligocene graded beds (Figs. 1 G,2 G) are remarkably similar to those in the sandstone phase at ClelandCreek. Therefore, far from representing "an entirely different set of sedi-mentary conditions" as remarked by Scott (1961, p. 87), this kind of size-frequency distribution is probably the complement of that in the mudstonephases at Mangatiti Road, and strongly suggests similar conditions ofdeposition.

    CoNCLUSIONS

    Difference in the average sizes of foraminiferal shells between sandstoneand mudstone phases of a turbidite rhythm at Cleland Creek indicatesmechanical sorting. The sorting is imperfect, like that of the clastic sedi-ment of the rhythm. Further work is required to determine the constancyof size frequencies in different parts of turbidite rhythms, and to determinethe full sequence of vertical changes in a rhythm. Large field samples may berequired to obtain sufficient specimens of a particular species for statisticaltreatment. .

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  • 800 N.Z. JOURNAL OF GEOLOGY AND GEOPHYSICS [Nov.

    A high frequency of adult shells, unimodal size frequency, and absenceof small juvenile shells are probably diagnostic of sandstone phases ofturbidites. A high frequency of juvenile shells and large size range areprobably diagnostic of mudstone phases of turbidites. The Upper Oligocenegraded-bedded strata sampled by Scott at Mangatiti Stream are turbidites.The Upper Miocene graded-bedded strata sampled by Scott at MangatitiRoad are probably also turbidites.

    REFERENCES

    ORBELL, G. E. 1961: The Geology of the Mauriceville Area. (Unpublished M.SC.Thesis, Victoria University of W.ellington Library.)

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