Page 30 - Atlas of Sedimentary Rocks Under The Microscope

Terrigenous clastic rocks

Sandstone classification Sediment maturity

Modern sandstone classifications require the estimation of the propor­ There arc two type� of sediment maturity mineralogical and textural.
tions of the principal grain types and thus thin section study is Mineralogically mature sediments are those containing a high propor­
required. tion of the most chemically stable and most physically resistant
Of the different sandstone classifications proposed. we present a minerals such as quartL, chert and ultrastable heavy minerals, such as
widely-used example, that of Folk ( 1974). Figure B shows the Lircon and tourmaline. Mineralogically immature sediments contain
classification of those rocks containing less than 15% fine-grained the less stable grains. such as feldspars. and those rock fragments not
matrix in terms of the three principal components: quartz, feldspar consisting principally of quarll.
(plus granite and gneiss fragments) and other rock fragments. Those The textural maturity of a sediment depends on the content of fine­
sandstones containing more than 15% fine-grained matrix are the grained material. the sorting and the roundness of the grains. A scale
greywackcs, and arc subdivided according to Fig. C. We refer readers of textural maturity proposed by Folk ( 1951) is presented below.
to Pettijohn ( 197 5) for dctai Is of other sandstone classifications and for lmmnture stage Sediment contains > 5% clay matrix. Grains
the classification of conglomerates and mudrocks, where studies using poorly-sorted and not well-rounded.
the petrographic microscope arc less important. Submnturc stn,::c sediment contains < 5% clay matrix. Grains
poorly-sorted and not well-rounded.
Mature stage - sediment contains lillie or no clay. Grains well­
sorted. hut 110t well-rounded.
(All quartz. but not chert) Supcrmnturc stugc sediment contains no clay. Grains well-sorted and
Q well-rounded.
Sub litharenite Diagrams illustrating visual estimation of sorting sediments using thin
sections arc shown in Fig. D.

F 1 : 1
(Feldspar + granite (All other
+ gneiss fragments) rock fragments) Very well-sorted Well-sorted

Sedimentary RF

arenite
Phyllarenite

Vo1can1c
RF
Fig. B Cla.lsific(l/icm c1/ swui.IIOIIC'.\. T h e upper lriangle shows a
sands! one c/assificcllinnfor sedimenls ll'ith less than 15% fine-grained Moderately-sorted Poorly-sorted
ma!rix. C/assi/imlion illl'oll'e.\ the remora/ c?(malrix, cemenl, micas
etc. and reca/cu/(1/ion f!{ compmwnt.\' to 100%. The lower 1riangle Fig. D Soning Ill in sections ( a{ter Peuijolln et al.. 197 3)
shmn luJII litlwrenite.\ IIIli\' be /itrther classified. (From Folk. 1974)
Quartz
95 o Ouartzwacke
Feldspath1c Lithic
greywacke greywacke

Feldspar 1 : 1 Rock fragments

Fig. C Cla.H(fication of'swulstone will/ more than 15% f ine-grained
marrix ( gn'\'u·ac/..c•., J

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