Textural Maturity

 

JOURNAL OF SEDI SEDIMENTARY MENTARYPETROLOGY, PETROLOGY,VOL. VOL. 21, 21, NO . 3 PP. 127-130 FIG. 1, SEPTEMBER 1951 S T A G ES O F T EX T U R A L M A T U R I T Y

I N S E D IM E N T A R Y

ROCKS

ROBERT L. FOLK The Pennsylvania State College

ABSTRACT Four stages of textural maturity in sediments are defined by the occurrence of three sequential events (1) removal of clays, (2) sorting of the sand fraction, and (3) attainment of high roundness roun dness.. A binomial binomial terminolog y is is introduced introduced wherein the term de noting textural m aturity is prefixed to the rock-composition name, obtained from the mineralogy of the silt-sand-gravel portion (e. (e.g., g., snbm atnre graywack e ).

I NTRODUCTION

Sediments are characterized by two fundamental properties, mineral comp o s i t i o n a n d t e x t u r e ( K r y n i n e , 1 9 48 48 ) . I t is desirable to include both these propert ie ie s w h e n d e t e r m i n i n g a n a m e f o r a p a r ticular rock. Texture may be considered f r o m t w o p o i n ts ts o f v i e w : ( l ) t h e v a r i a t i o n i n g r a i n s i z e ( a v e r a g e a n d r a n g e ) ; a n d ( 2) 2) the textural m aturi ty of the rock as expressed in clay content, sorting, and r o u n d n e ss ss . T h e b r o a d c o n c e p t o f m a turity was introduced by Pettijohn ( 1 94 94 9 ) a n d P l u m l e y ( 1 9 4 8 ) t o d e s c r i b e the approac h of a sediment to the mos t i n e r t e n d s t a t e p o s s i b l e , th th r o u g h t h e o p eration of both physical processes (tending to produce a perfectly sorted and rounded sediment) and chemical processes (tending to produce a sediment containing only the most stable minerals). T h e p r e s e n t p a p e r c o n c e r n s o n l y lex rural m a t u r i t y ; f o u r d is i s t i n c t s t a g es es a r e d e f i n e d , w i t h t h e s u i ta ta b l e t e r m e x p r e s s i n g textural m atur ity being prefixed to the compositional name of the rock (e.g., submature graywaeke ). Th e need for such a binomial term inology arises due to the fact that textural maturity and mineral composition are capable of much independent variation, although, of course, ultimately affec ting one another. For example, an arkose with a composition of 70 per cent quartz and 30 per cent feldspar may show all degrees of textural matu rity from an angular,

poorly sorted fanglomerate to the perfectly rounded an d sorted sands of the Cambro-O rdovlc~an of Pennsylvania. It i s o b v i o u s l y u n s a t i s f a c t o r y t o c a ll ll t h e s e two contrasting rocks by the unqualified term arkose. Similarly, perfectly rounded and sorted sediments may consist entirely of quartz, or may contain in addition sizeable quantities of feldspar, er et cs .i s tTa on t c imr ec ut ammvoernpth i tch e rsoe c kd i fffriacgu m lt ieenst sa, lt twofold terminology, embracing both m i n e r M c o m p o s i ti ti o n a n d t e x t u r a l m a t u r it3', is here introduced. DEF INITION OF THE F OUR S TAGES OF TEXTURAL MATURITY

The passage from an initial clayey, p o o r l y s o r t e d, d, a n g u l a r s e d i m e n t t o a c o m pletely matured, rounded and sorted s a n d i s m a r k e d b y t h r e e e a s i l y re re c o g n i z able steps, which take place in constant sequen tial order in response to the total i n p u t o f m o d i f y in in g e n e r g y . T h e s e t h r e e steps are, in the order of their occurr e n c e : ( 1 ) r e m o v a l o f c la la y , r e q u i r i n g l e a s t energ y; (2) atta inm ent of good sorting in the non-clay portion, requiring more e n e r g y ; a n d ( 3) 3) r o u n d i n g o f t h e g r a in in s , r e q u i r i n g m o s t e n e r g y . T h e f o u r s ta ta g e s of textural m aturity, described below, are delim ited by the occurre nce of these three steps. I. Immature stage. Sediment contains considerable clay and fine mica, the non-clay portion is itself itself poorly sorted,

 

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I M MA MA T U R E

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