Cretaceous Stratigraphy of Lower Murchison River Area, 43 
AVestern Austjialia. 
sediuicntatioii avoji must have been subsiding, but so slowly that subsid- 
ence lagged behind sedimentation and the delta was gradually pushed 
out to sea. 
As denudation continued, relief was diminished and the supply of 
sediments decreased. Sinking of the sea-lloor continued, deltaic cross- 
bedding disappeared and Hue-grained bedded sandstones were deposited. 
Further deepening of the area of sedimentation led to I he deposition over 
the entire area of a uniform deposit of uuMlium-grained unbedded quartz 
^aud, up to 170 feel thick, which must have* been laid down with great 
ra]>idity. Cross-bedding is practically absent from this deposit so pre- 
sumably it was formed in moderately deej) water at least below the zone 
of Avave action and surfa{*e currents. Continued deepening of the sea 
is indicated liy the apiiearanee of glauconite in the up]Jer l>art of the 
Hutte Sandstone whicli can hai’dly have formed at di^ptlis of less than 
about 2d fathoms. 
The deposition of the following Tlnvindine Sliale indicates further 
reduction of the relief of the adjoining laud or deepening of the sea 
and retreat of the coastline in an luisteily direction. There may have 
been a comhinatiou of both events. 
Throughout all this time, since the hegiuiting of the formation of 
the delta, conditions must have Ijeeu generally unfavourable for most 
types of life. ( ertain tyt)es of saml and ihud burrowers constituted 
])ractica]!y the entii'e fauna, t )c<*asiouaily, especially during the closing 
stages of tile deposition of the Butte Sandstone, logs of wood drifted 
out to sea and AV(‘!'e buried in the sand. Souu* pi<!ces are riddled by 
wood-boj’ers [Teredo^) ami must havf^ driftetl a long time Ix'fore they set- 
tled down on the sea-flooj*. Remains of l)elemnites in the Thiriiuliue Shale 
are so rare that it seems that they must liave drifted there from some 
more favourable euvirouuumt . 
The setlimeuts Avhich were deposited after the Thiriudiiie Shale indi- 
cate clearly a slow, probably more or less continuous deepening of the 
area of sedimentation. The glauconitic Aliuga B(*ds may have been 
formed at de])t!is anywhere between 25 falluuns and UK) fathoms, Avhile 
the ovf'i’lyiug chalk seems to indicate a further deepening of the sea 
below the 100 fatliom line. During the time of the. deposition of the 
Alinga Beds the nearness of the coast in the east, still affected the nature 
of the sediments ‘for there s(‘ems to he a change from gi'eensands in the 
east to glauconitic clays and shales in the western part. Belemmtes w^ere 
abundant at this lime. 
The chalk forms a uniform sheet of sediment and indicates the exi<st- 
eiice in this area of a moderately deej) sea, probably of the order of 100 
or 200 fathoms, where a fairly rich neritic life flourished. Jnocerainus Avas 
common, and also other pelecypods, hrachiopods, stalkless criuoids, and 
other forms of life. The transition from the glauconitic Alinga Beds 
to tlie chalk is mark('d by a l)(*d (tf phosphatic nodules. The chalk Avas 
lormcd daring Middle Sem>nijiu (Snuiouiau) tinu'. This period Avas fol- 
loAved by a time of deposition of glauconitic shale which may indicate 
a slight rising of llu' sca-fioor and a Avestwnrd advanra* of the coastline. 
The ])ost-Cretaceous history of the area will not he discussed in this 
paper. 
