Sandstone-body structures and ephemeral stream processes in the Dinosaur Canyon Member, Moenave Formation (Lower Jurassic), Utah, U.S.A.

Sedimenta~ Geology, 61 (1989) 207-221 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

207

Sandstone-body structures and ephemeral stream processes in the Dinosaur Canyon Member, Moenave Formation (Lower Jurassic), Utah, U.S.A. HENRIK

OLSEN

*

Institute of General Geology, Oster Voldgade 10, D K-1350 Copenhagen K (Denmark)

Received March 17, 1988; revised version accepted September 20, 1988

Abstract Olsen, H., 1989. Sandstone-body structures and ephemeral stream processes in the Dinosaur Canyon Member, Moenave Formation (Lower Jurassic), Utah, U.S.A. Sediment. Geol., 61: 207-221. Studies of fluvial sandstone-body structures in the Lower Jurassic Dinosaur Canyon Member suggest a threefold subdivision of the ephemeral stream deposits. Sandstone-sheets with interbedded siltstones are less than 1 m thick and laterally extensive for hundreds of metres. They are interpreted as sheetflood deposits. Simple channel sandstone-bodies are a few metres thick and a few tens of metres wide. They reflect solitary channel incision, episodic migration and plugging. Muhistorey channel sandstone-bodies are a few metres thick and laterally extensive for hundreds of metres. They are composed of several channel-shaped storeys and exhibit only local incision. The multistorey sandstone-bodies are interpreted as braided ephemeral stream deposits. Two sandstone-sheet subtypes with grooves and mounds, respectively, are interpreted as intermediate between the sheetflood deposits and solitary incised channel deposits on one hand and between sheetflood deposits and braided stream deposits on the other hand. The solitary channels and braided streams are accordingly interpreted to be initiated from sheetfloods through differential erosion and differential deposition, respectively. This model of channel evolution from sheetfloods is probably applicable to other semiarid and arid fluvial environments dominated by surface runoff.

bedded

Introduction

with

mudstones;

simple

channel

sand-

stone-bodies; and multistorey channel sandstoneA n a l y s i s of fluvial s a n d s t o n e - b o d y

structures

b o d i e s . T h e d i f f e r e n c e s in s a n d s t o n e - b o d y

struc-

h a s b e e n i n t e n s i v e l y u s e d in the s t u d y o f e p h e m e r a l

t u r e s are i n t e r p r e t e d in t e r m s o f d i f f e r e n t e p h e m -

stream

Puigde-

eral s t r e a m p r o c e s s e s . T h e a i m o f the p a p e r is to

f a b r e g a s a n d v a n Vliet, 1978; F r i e n d et al., 1979,

o u t l i n e t h e s e v a r i o u s fluvial p r o c e s s e s a n d r e l a t e

1986; Stear, 1980, 1983; T u n b r i d g e , 1984; O l s e n ,

them

1988). A s i m i l a r a p p r o a c h is u s e d in the p r e s e n t

sheetflooding.

deposits

(Puigdefabregas,

1973;

to

a

model

of

channel

initiation

from

s t u d y of e p h e m e r a l s t r e a m d e p o s i t s f r o m the L o w e r Jurassic

Dinosaur

Canyon

Member

Moenave

Formation

in U t a h .

Three sandstone-

of

the

b o d y t y p e s are r e c o g n i z e d : s a n d s t o n e - s h e e t s i n t e r -

Setting The Lower Jurassic Dinosaur Canyon Member o f t h e M o e n a v e F o r m a t i o n (cf. H a r s h b a r g e r et al., 1957) in s o u t h e r n U t a h a n d n o r t h e r n A r i z o n a is

* Present address: Geological Survey of Greenland, 1Oster Voldgade 10, DK-1350 Copenhagen K, Denmark. 0037-0738/89/$03.50

© 1989 Elsevier Science Publishers B.V.

composed

of aeolian, sabhka,

ephemeral stream

a n d l a c u s t r i n e d e p o s i t s (Fig. 1). It was laid d o w n

208 KANAB

112"

113"

iI/¢

=

sen et al., 1989). The lower part of the Paria section is dominated by aeolian sandstones deposited at the periphery of the Wingate erg and is referred to as the Wingate Sandstone (Clemmensen et al., 1989). .~

,

/

.

¢

km

Fluvial deposits '

w "' ~E Z O >Z < C.)

~

Outcrop

rr <

~ Braided stream I , :':'~'T'I ( m u l t i s t o r e y s s t - b o d y )

O

~

rocks

PARIA

Solitary channel (simple sst-body) •- :

l

a r e a of J u r a s s i c

-

Sheetfloods (sandstone sheets) Lake Sabkha

Aeolian

--

m

0

Fig. 1. Location of the study area at Kanab and Paria in Utah. Simplified sections through the Dinosaur Canyon Member and coeval Wingate Sandstone along with environmental interpretations of the rocks are shown for the two localities (see also Clemmensen et al., 1989).

in an erg margin adjacent to the Wingate erg covering the northeastern Arizona, eastern Utah, western Colorado and western New Mexico (Clemmensen et al., 1989). The fluvial deposits are dominated by very fine- and fine-grained sandstones interbedded with desiccation-cracked mudstones (Olsen, 1987). Laterally extensive exposures around K a n a b and Paria in southern Utah (Fig. 1) provide excellent control on sandstone-body structures. Palaeocurrent indicators show transport mainly towards the north-northwest (Fig. 2) probably indicating a source in the Mogollon Highlands in southeastern Arizona (Clemmensen et al,, 1989). The Dinosaur Canyon Member is unconformably overlain by the Springdale Sandstone Member. The magnitude of the unconformity increases to the east (Clemmen-

Three main fluvial deposits occur in the Dinosaur Canyon Member: interbedded sandstonesheets and siltstones, simple channel sandstonebodies and multistorey channel sandstone-bodies. The deposits are defined on the basis of internal bounding surfaces according to the classification system of Miall (1988a, b). Sandstone-sheets possess internal surfaces of no higher order than second order (coset bounding surfaces). Simple sandstone-bodies exhibit internal surfaces of third order (reactivation) a n d / o r fourth order (upper surfaces of macroforms) but no higher-order surfaces. Multistorey sandstone-bodies are characterized by internal fifth-order surfaces, bounding individual channel-fill complexes. lnterbedded sandstone-sheets and siltstones

Interbedded sandstone-sheets and siltstones are the d o m i n a t i n g fluvial deposits comprising 40-100% of the fluvial facies in individual outcrops. Three subtypes occur. (1) Tabular sandstone-sheets with interbedded siltstones (Figs. 2a, 3) range in thickness from a few centimetres to a m a x i m u m of 1 m. The width of individual thicker sandstone-sheets transverse to local palaeocurrent directions is several hundred metres. Sandstone-sheets occur as isolated beds separated by siltstones and sand-streaked siltstones up to 1 m thick. More commonly, however. sandstone-sheets occur as stacked successions up to 5 m thick with only thin siltstone partings (Figs. 2a, 3). Sandstone-sheets rest on flat eroded and noneroded surfaces and possess horizontal tabular sheets geometries. The sandstones exhibit vertical aggradation and they are dominated by cross-lamination (Figs. 2a. 3). Horizontal lamination is locally abundant but constitutes less than 5% of the sandstones in this facies. The horizontal lamination exhibits upward transition to cross-

209

cm

m

i.

70

f

e

N

m

1

5

~/ 0

,v1~ f

oo

_J

m

Aeol

n =59

'vf ' f

4O

m 30 ~ 20

4

c

~1

d

q

~

[~] 2

--

I0 ~ :: 2 O

Ivf I

1

Ivf~ f

Ivflf

Iml

LEGEND (a e) Cross-lamination

[EE~ Parallel lamination Trough cross-bedding [-~] m

Massive bedding Siltstone & heterolith (desiccation cracks)

Fig. 2. (a-e) Sedimentological logs through ephemeral stream deposits in the Dinosaur Canyon Member. Note the different scale in (a). (a) Tabular sandstone-sheets with interbedded sihstones. See also Fig. 3. (b) Sandstone-sheet with grooved base. See also Fig. 4. (c) Sandstone-mound. See also Fig. 5b. (d) Simple channel sandstone-body. See also Fig. 6c. (e) Multistorey channel sandstone-body. See also Fig. 8. (f) palaeocurrent rose for the ephemeral stream deposits in the Dinosaur Canyon Member between Kanab and Paria. Grain-size of sandstones is indicated by c,f (very fine), f (fine) and m (medium)• The upper symbol of cross-lamination in the legend illustrates ripple-drift cross-lamination.

lamination. The internal structures are commonly partly and locally completely destroyed by bioturbation. The sandstones are very fine-grained and individual beds commonly fine upwards to silty very fine-grained sandstones. Aeolian translatent strata (cf. Hunter, 1977) are present in the

upper part of some sandstone-beds. The interbedded siltstones all exhibit sand-filled desiccation cracks (Figs. 2a, 3). The flat and laterally extensive basal surfaces, the presence of only first- and second-order bounding surfaces internally (cf. Miall, 1988a, bL

Fig. 3. Tabular sandstone-sheets interpreted as sheetflood deposits. The photograph shows typical cross-laminated sandstone-sheets interbedded with desiccation cracked siltstones. Palaeoflow is into the cliff-wall. Scale bar is divided into centimetres. Locality: Paria. See also Figs. 4, 5, 6, 8 and 9 for other outcrop examples of tabular sandstone-sheets.

210

the absence of lateral accretion bedding, and the tabular geometry of the sandstones indicate deposition by sheetfloods. The rocks bear similarities with other ancient deposits postulated to be of sheetflood origin (Steel and Aasheim, 1978; Hubert and Hyde, 1982; Graham, 1983; Sneh, 1983). The paucity of horizontal lamination (upper plane beds) indicates relatively low velocity floods compared to many other modern and ancient ephemeral streams (e.g. McKee et al., 1967; Tunbridge, 1981, 1984; Stear, 1985). The sandstonesheets may either be true sheetflood deposits or overbank deposits associated with channelized flow. The latter is interpreted for a few beds in which the sheets occur as thin sandstone-wings (cf. Friend et al., 1979) associated with simple channel sandstone-bodies (see later section). Usually, however, the sandstone-sheets cannot be traced into channel sandstone-bodies even where lateral control of several hundred metres is available. The majority of sandstone-sheets are therefore interpreted as true sheetflood deposits. The interveening siltstones are interpreted as fallingstage deposits, very low velocity sheetflood deposits and overbank deposits, The numerous desiccation cracks and occurrence of aeolian deposits indicate that subaerial exposure was common.

(2) Sandstone-sheets with grooved bases (Figs. 2b, 4) are similar to the tabular sheets except for the presence of grooves along the bases (Fig. 4). Three examples of this subtype of sandstone-sheet were observed in the study area and one was studied in detail. This sandstone-sheet, shown in Fig. 4, consists of a 50 cm thick tabular body which shows little thinning for 300 m laterally, perpendicular to the local palaeoflow direction. Fourteen parallel grooves occur along the planar base. The grooves are 0.2 2.0 m wide, possess a width/height ratio of 3" 1 and are symmetrical in cross-sections perpendicular to the palaeoflow. The grooves occur in groups of 2-4. The distance between the grooves is constant within individual groups but varies from 0.5 to 3.0 m from group to group. The grooves are filled with either massive or parallel laminated fine-grained sandstone. The groove fillings are overlain by a fining-up sheet of fine-grained to silty very fine-grained sandstone with ripple-drift lamination (Fig. 2b). The palaeocurrent direction obtained from the rippledrift lamination is parallel with the elongation trends of the grooves. This sandstone-body type is interpreted as deposited by sheetfloods with associated groove erosion. The grooves resemble other ancient fluvial and tidal grooves, also called gutter casts (e.g. Allen, 1964, 1974; Friend, 1965;

I

Fig. 4. Sandstone-sheet with grooves interpreted as a sheetflood deposit. Palaeoflow is towards the viewer. A typical tabular sandstone-sheet is seen above. Locality: Kanab.

211

Nagtegaal, 1966; von SchriSder, 1966; Bridges, 1972). Several authors propose longitudinal spiral vortices in the groove-cutting process (e.g. Berry, 1961; Bridges, 1972; Whitaker, 1973; Allen, 1982, 1985). Ideally the motion consists of streamwise, oppositely rotating spiral vortices (see later section for details). The regular distance between the grooves in the individual groups indicates that

regular transverse variations occurred locally in the sheetflood and systems of longitudinal vortices seem very likely to have caused these variations. According to Allen (1982), groove spacing is probably 4 times the flow depth and the groove spacing observed in the illustrated sandstone-body (Fig. 4) accordingly reflects variations in the flow depth between 0.1 and 0.8 m. The sedimentary

Fig. 5. Sandstone-mounds interpreted as braid-bar deposits from sheetfloods. Note the horizontal Isolated sandstone-mound. The upper surface of the m o u n d is slightly eroded in the left-hand side side. Palaeoflow is obliquely out of the cliff wall. Locality: Kanab. (b) Sandstone-mounds forming overlying a typical tabular sandstone-sheet. The top of the m o u n d s is noneroded. Base and top sandstones. Palaeoflow is obliquely into the cliff wall. Locality: Paria.

base of the sandstone-bodies. (a) but non-eroded in the right-hand part of a tabular sandstone-body of cliff are composed of aeolian

212 structures and grain-size distribution in the studied

groove

sandstone-body

oc-

l a m i n a t i o n ) , w h e r e a s l o w e r flow r e g i m e c o n d i t i o n s

during

were achieved on the recession limb of the flood

curred

at upper

indicate

that

sheetflooding

flow regime conditions

erosion

(massive

bedding

and

parallel

Fig. 6. Simple channel sandstone-bodies interpreted as solitary incised channel deposits. (a) Simple sandstone-body iwith lateral wings) composed of three bedsets separated by fourth-order bounding surfaces. The sandstone-body reflects one major episode ol channel migration and unilateral (point bar) deposition followed by two episodes of channel plugging with only minor erosion. See also Fig. 7a. Palaeoflow is towards the viewer. Immediately above the simple sandstone-body braided river deposits of the Spnngdale Sandstone Member is seen. Locality: Kanab. (b) Close-up of a simple channel sandstone-body with lateral wings. The bedsets ~ / 2 3) are separated by third-order surfaces. Lateral accretion exhibited by first-order surfaces is shown by dashed lines in bedset 1. The surfaces extend unbroken into the lateral wing exhibiting vertical aggradation (overbank deposits). Bedsets 2 and 3 are also laterally accreted. Bedset 3 has no lateral wings. Palaeoflow is obliquely out of the cliff wall. Locality: Kanab. (c) Lateral accretion bedding in a simple channel sandstone-body recording point bar deposinon and lateral channel nugration towards the left. Two third-order bounding surfaces are visible. Palaeoflow is directly out of the cliff wall. Locality: Kanab. See also Fig. 7b.

213

Fig. 6. (continued).

(cross-lamination and fining-upwards in the sandstone-sheet). (3) Sandstone-mounds (Figs. 2c, 5) are convexup very fine- to fine-grained sandstone-bodies resting on horizontal eroded or non-eroded surfaces. The mounds are either isolated (Fig. 5a) or form part of tabular sandstone-sheets (Fig. 5b). Individual mounds are up to 60 cm thick and 3.5

m wide, perpendicular to the palaeoflow direction. The spacing between the mounds, perpendicular to the palaeoflow direction, varies from 2 to 4 m. Internal structures include cross-lamination and parallel lamination. The set boundaries and laminations are horizontal at the bases of the mounds and arch up towards the tops where they parallel the mound surfaces. The mounds fine

a

J 5m

2 __

t

f

J

1

J

Fig

6c

J

Fig. 7. Simple channel sandstone-bodies from Kanab interpreted as solitary incised channel deposits. (a) Simple sandstone-body composed of three bedsets separated by fourth-order surfaces. Lateral wings (overbank deposits) are visible. Drawn from photo (Fig. 6a). Palaeoflow towards the viewer. (b) Simple sandstone-body with seven laterally accreted bedsets exposed. The bedsets are separated by third-order surfaces and bedset 7 is overlain by a fourthrorder surface. The sandstone-body reflects repeated episodes of lateral channel migration and associated point bar deposition followed by channel plugging with mud. Palaeoflow towards the viewer. See also Fig. 6c.

214

upwards in places associated with a change from parallel lamination to cross-lamination. The mounds are either draped by finer-gained deposits or slightly eroded. The five occurrences of this sandstone-sheet type were exposed perpendicular or obliquely to the local palaeoflow direction. The width/length ratio is therefore uncertain. though the mounds always appear several times longer than wide. The horizontal bases of the sandstone-mounds and the association with tabular sandstone-sheets indicate that they were deposited by unchannelized flows, probably sheetfloods. The convex-up morphology and arching up of internal laminations indicate a depositional origin of the mounds, as opposed to being erosional remnants. Subsequent erosion, however, modified some of the sand-mounds after deposition (Fig. 5a). The sandstone-mounds resemble larger-scale inferred crevasse splay lobes (Tyler and Ethridge, 1983) and sandflats (Allen, 1983). The lateral amalgamation of sandstone-mounds seems, however, to disqualify a crevasse splay origin. The mounds probably developed in sheetfloods as sandflat-tike small braid-bars. Comparable modem sand-mounds (longitudinal ridges) are reported from intertidal flats (Allen, 1982) and rivers (Coleman, 1969). They were interpreted as the result of differential deposition due to longitudinal spiral vortices in the streaming water. The spacing of the sandstone mounds between 2 and 4 m reflects a flow depth of the sheetfloods in the order of 0.5 to 1 m (cf. Allen, 1982).

Simple channel sandstone-bodies This sandstone-body type (Figs. 2d, 6, 7) forms generally less than 10% of the fluvial deposits in individual outcrops. The sandstone-bodies are 0.5-3 m thick and a few metres to a few tens of metres wide, perpendicular to the local palaeoflow direction. The sandstone-bodies are bounded below by concave-up erosion surfaces (Figs. 6a, b, 7a), which may appear planar when the exposures are small compared to the width of the sandstonebodies (Figs. 6c, 7b). Individual sandstone-bodies are composed of several bedsets separated by thin desiccated siltstone layers a n d / o r erosion surfaces.

These internal surfaces are generally inclined up to 15 o and parallel to one side of the sandstonebody. Palaeocurrents are perpendicular to the dip direction of the internal surfaces. The sandstonebodies are composed of very fine- and fine-grained sandstones. Parallel lamination and cross-larmnation dominates, whereas trough cross-bedding is locally present. The sandstone-bodies occasionally fine upwards, in places associated with a change from trough cross-bedding to cross-lamination. More commonly individual bedsets exhibit an oblique-up decrease in mean grain size perpendicular to the internal surfaces (Fig. 2d). The internal bedding of individual bedsets is inclined parallel with the internal surfaces (Fig. 6b). The sandstone-bodies are commonly associated with thin sandstone-sheets or -wings which extend laterally away from the central bodies (Figs. 6a. b. 7a L The simple (or single-storey) channel sandstone-bodies resemble ancient solitary-channel ephemeral stream deposits interpreted by Puigdefabregas (1973), Puigdefabregas and van Vliet (1978), Friend et al. (1979, 1986) and Stear (1983). The presence of internal desiccated siltstone layers and close association to sheetflood deposits seem to justify a similar interpretation for these sandstone-bodies. The basal erosion surfaces are fifthorder bounding surfaces (cf. Miall, 1988a. b). The internal parallel bounding surfaces (third-order surfaces of Miall, 1988a, b) and associated bedding with palaeocurrent indications perpendicular to the dip direction of the surfaces (lateral accretion bedding) indicate that each episode of channel-flow was followed by asymmetrical infilling and lateral shifting of the depositing stream. Deposition probably occurred on point bars in sinuous channels. Each bedset lying between internal surfaces was probably deposited during a single major flood event (see also Bridge and Diemer, 1983). The oblique-up decrease in mean grain size within individual bedsets records falling stage deposition (cf. Bridge and Diemer, 1983), whereas the upward fining of the entire storey (sandstonebody) resembles classic fining-upwards in point bars due to the helicoidal flow pattern (e.g. Allen, 1963; Jackson, 1976; Bridge and Jarvis, 1982). Individual sandstone-bodies indicate either that the point bars migrated laterally in association

215

5 - - - ' - - - -

1

~

3

.I-"

~

J

4m

,/

Fig. 8. Multistorey channel sandstone-body. Notice the horizontal base of the sandstone-body. Five storeys bounded by concave-up fifth-order surfaces (internal bold lines) are shown. Each storey represents the deposition of a braid-bar (1, 4) or in a braid-channel (2, 3, 5). Thin lines within individual storeys represent third- and fourth-order surfaces indicating interruptions of the flow. The sandstone-body was deposited by a braided stream characterized by ephemeral flow. Palaeoflow is obliquely into the cliffwall. Locality: Kanab.

with the cut-banks (Figs. 6b, c, 7b) or that an initial erosional episode was followed by deposition in a diminished channel (Figs. 6a, 7a). Vertical aggradation of sand is commonly observed in the last deposited bedset of a sandstone-body (Figs. 6a, 7a). The vertical aggradation deposits are interpreted as channel-fill plugs and thus overlie fourth-order bounding surfaces (cf. Miall, 1988a, b). The lateral wings associated with some of the sandstone-bodies are interpreted as overbank deposition during floods (cf. Friend et al., 1979).

Multistorey channel sandstone-bodies This sandstone-body type (Figs. 2e, 8, 9) forms up to 50% but generally less than 20% of individual outcrops. The sandstones are 1-9 m thick and occur as sheet-shaped bodies which exhibit no thinning for several hundred metres laterally. The sandstone-bodies are bounded downwards by horizontal eroded and noneroded surfaces (Figs. 8, 9) locally cut by concave-up erosion surfaces. Internally the sandstone-bodies are characterized by concave-up erosion surfaces which divide the

sandstone-bodies into storeys a few metres to tens of metres wide and 0.5-2.5 m thick. Individual storeys commonly contain internal erosion surfaces a n d / o r desiccated siltstone layers separating the storeys into bedsets. The sandstones are very fineto medium-grained and exhibit parallel lamination (most common), trough cross-bedding and crosslamination. Palaeocurrents vary up to 120 ° from one storey to another in individual multistorey sandstone-bodies. The upper few centimetres of individual storeys and bedsets within the storeys are commonly finer grained or composed of aeolian translatent stratification (cf. Hunter, 1977). Multistorey sandstone-bodies are locally, through a zone of interbedding, traced into cross-bedded sandstone-bodies of equal thickness, entirely composed of aeolian strata. At one locality the multistorey sandstone-bodies form part of coarseningand thickening-up sequences (Fig. 9). In some of the sequences an upward transition from thin tabular sandstone-sheets to thicker sandstone mounds and finally a multistorey sandstone-sheet is evident. The lowermost storeys in one of the multistorey sandstone-bodies are convex-up and resemble the sandstone mounds below (Fig. 9b).

216

Fig. 9. Coarsening upward sequences in ephemeral stream deposits. The top of each sequence is composed of a multistorey sandstone-body. Notice in (a) the smooth horizontal base of a multistorey sandstone-body (indicated by bold arrows~ and internal fifth-order surfaces, channel scours (fine arrows). In (b) a close-up of one of the coarsening upward sequences is shown (indicated in (a)). The sequence consists of tabular sandstone-sheets (A) overlain by a sandstone-mound (B) and a multistorey sandstone-body (C). Notice the lowermost convex-up storey in the multistorey sandstone-body; the original shape of the sand mound ~sandflat-like bar) is only weakly modified by erosion. The sequence reflects initial sheetfloodings (A) followed by bar development ( B ) and finally true braiding of the streams (C). Palaeoflow is into the cliff-wall. Man for scale in (a). Locality: Paria.

I n general outline the multistorey s a n d s t o n e bodies resemble the inferred b r a i d e d stream deposits depicted b y A l l e n (1983), i.e. sheet geometry, multistorey c o m p o s i t i o n a n d d o m i n a n c e of parallel l a m i n a t i o n . However, i n contrast to A l l e n ' s s a n d s t o n e - b o d i e s these multistorey c h a n n e l sand-

stone-bodies rest o n planar, horizontal sixth-order surfaces (cf. Miall, 1988a, b) only locally cut b y c h a n n e l scours. The basal scour surfaces, described b y Allen, are very irregular a n d interpreted to reflect episodic lateral erosion of the braided river system. T h e smooth, horizontal, locally non-

217 eroded surfaces at the base of the multistorey sandstone-bodies more closely resemble the basal surfaces of the tabular sandstone-sheets and thus probably record initiation of the braided streams by sheetfloods. Each storey (i.e. complex of Allen, 1983) is interpreted as a channel-fill complex (in-channel deposit or braid-bar (macroform) deposit). Only rarely lateral accretion bedding was observed in the storeys implying that channel scouring and plugging dominated over lateral channel migration and deposition. The presence of translatent stratification, indicative of aeolian reworking, and desiccated siltstone layers suggest an ephemeral stream origin of these multistorey sandstone-bodies (cf. Glennie, 1970). The lateral transitions into aeolian sandstone-bodies indicate considerable aeolian activity and the braided ephemeral streams are thought to be the main source of aeolian sand in the area (Clemmensen et al., 1989). The presence of multistorey sandstone-bodies as the upper parts of thickening-up sequences composed of tabular sandstone-sheets, sandstone-mounds and multistorey sandstone-bodies implies a close genetic association to the sandstone-mounds. The convex-up storeys in the lower part of multistorey sandstone-bodies may accordingly be relict sandmounds deposited as sandflat-like bars in sheetfloods.

a

Fluvial styles The three types of sandstone-body in the Dinosaur Canyon Member are the product of three different ephemeral stream types. The interbedded sandstone- and siltstone-sheets reflect repeated sheet-flooding. The simple channel sandstone-bodies record solitary channel incision and episodic lateral migration associated with deposition on point bars. The multistorey sandstone-bodies indicate deposition in braided ephemeral streams with only local initial incision of the channels. In the following sections the development of grooves and sand-mounds in sheetfloods is discussed in the context of longitudinal spiral vortex flow and a model for initiation of the incised solitary channels (simple channel sandstone=bodies) and the braided streams (multistorey channel sandstone-bodies) is suggested.

The role of spiral vortices in sheetfloods The role of longitudinal spiral vortices in fluid flows has been discussed in detail by Allen (1982, 1985), and a short summary is given here. Longitudinal spiral vortices occur as oppositely rotating vortices lying side by side on the bed (Fig. 10). Amongst the bottom currents alternate zones of convergence and divergence occur. Two contrasting cases are considered.

b

Fig. 10. Implications of spiral vortex flow for groove erosion and sand-mound deposition in sheetfloods. (a) Differential erosion due to small amounts of transported sand and corrasion along zones of convergence (groove erosion). (b) Differential deposition due to large volume of sand and deposition along zones of convergence(tabular sand-sheet with sand-mounds). Differential deposition also occurs if the surface is resistant to erosion by corrasion (isolated sand-mounds). (Modified after Allen, 1982).

218 In the first case (Fig. lOa) the a m o u n t of transported sand grains is small. If the mode of erosion is corrasion the rate must be greatest along zones of convergence, because the areal concentration of grains moving at the bed is here a maximum. Grooves now develop beneath each zone of convergent flow lines. In the second case (Fig. lOb) the amount of sand provided is sufficient to cover the whole bed and thereby protect it from erosion by corrasion. Sand grains will move from zones of divergent flow lines towards zones of convergence. The surface will become shaped into mounds of sand below each line of convergence. A similar effect of differential deposition is obtained if the bed is resistent to erosion by corrasion.

Spiral vortices applied to sandstone-bodies The grooves and the sandstone-mounds in the interbedded sandstone and siltstone facies may thus be applied to longitudinal spiral vortices: The grooves probably formed by differential corrasional erosion. Grooves were, however, not developed along the entire bed. This feature probably reflects initial irregularities. The deeper the irregularities, the more the local flow depth and the local flow velocity will be (Allen, 1985). As the rate of erosion by corrasion is proportional to the flow velocity (cf. Allen, 1971), the grooves will develop preferently in the deeper irregularities. This may explain the grouping of grooves along the base of these sandstone-sheets. Groups of grooves developed at the deeper initial irregularities. The sandstone-mounds are interpreted as formed by differential deposition below zones of flow line convergence. Sandstone-mounds forming part of a tabular sheet are the result of excess sediment supply. Isolated sandstone-mounds are the result of beds resistant to erosion by corrasion. The two-dimensional exposures of these sandstone-bodies prohibit detailed investigations of the three-dimensional geometry of the mounds. They may either be flow-parallel ridges or more restricted three-dimensional bar forms following each other in flow-parallel rows.

Channel-initiation from sheeffloods Two different evolutions of channetized flow are here interpreted to result from sheetfloods associated with groove erosion and differential deposition, respectively. The erosion of groups of grooves at the deeper irregularities in the surface possesses a self reinforcing effect. The local lowering of the bed will increase the local flow velocity. Because erosion by corrasion increases with the flow velocity, the grooves will deepen and widen and finally amalgamate to a larger incised channel form. If this channel form is not plugged by the same flood it will be a potential locus for subsequent floods. This sequence of groove erosion and subsequent channel development by amalgamation of the grooves has been experimentally produced by Shepherd and Schumm (1974), but has not been reported from modern fluvial environments. Differential deposition of bars in a sheetflood will result in a positive relief after flood recession. If the bars are large enough, later flood streams will be split into a system of interconnected channels forming a braided pattern. This sequence has, however, not been observed in either modern fluvial environments or experimental flows.

Sandstone-bodies and fluvial sffle The majority of fluvial sandstone-bodies in the Dinosaur Canyon Member is of the tabular sandstone-sheet type. The absence of grooves and mounds in these sandstone-bodies indicates that spiral vortices usually were not developed in the sheetfloods or that spiral vortex flow was incapable of causing differential erosion or deposition. The sandstone-sheets with grooves and mounds have been discussed above. They are interpreted as sheetflood deposits associated with longitudinal spiral vortices. The incision of channels (simple channel sandstone-bodies) is interpreted in terms of a sequence of sheetflooding, groove erosion and grove amalgamation. The channels were the loci for several floods and associated deposition of sand. The braided fluvial style evolved around sandmounds initiated in sheetfloods. Later floods were

219

: ' : . ) T < : I : ¸ :~.5

: = ; ,

/." TABULAR

\

/

SANDSTONE SHEETS

MOUNDS

GROOVES

CHANNEL -

SANDSTONE BODIES

SIMPLE

MULTISTOREY

Fig. 11. Sandstone-bodytypes and their genetic relations in the Dinosaur Canyon Member. Simple sandstone-bodies (solitary incised channels) are formed through a process of sheetflooding (tabular sandstone-sheets) and associated groove cutting (sandstone-sheets with grooves).Multistorey sandstone-bodies(braided streams) are formed through a process of sheetflooding(tabular sandstone-sheets) and associated bar development (sandstone-sheets with mounds). The sandstone-sheets are accordingly arrested stages in the processes leading to channelized ephemeral flows.

split by the mounds (braid-bars) into interconnected (braided) channels and deposition of sand in the channels resulted in a multistorey sandstone-body. The fluvial sandstone-bodies in the Dinosaur Canyon Member thus reflect two different evolutions of channels from sheetfloods (Fig. 11). Tabular sandstone-sheets represent the initial sheetfloods without effective longitudinal spiral vortices. Sandstone-sheets with grooves represent the intermediate stage in the development of incised channels. Here longitudinal spiral vortices were responsible for groove erosion. Sandstonemounds represent the intermediate stage in the development of braided streams. Longitudinal spiral vortices were responsible for deposition of mounds. The model assumes that surface runoff is dominant and solely responsible for channel-initiation. This assumption is justified for the present semiarid depositional environment because surface runoff is a response to most rainfalls in dry climates (Yair and Klein, 1973). The model is, however, not applicable to humid environments where runoff is a combination of surface runoff, infiltration, subsurface flow and return flow.

Conclusions Three sandstone-body types are recognized in the Lower Jurassic Dinosaur Canyon Member in Utah: sandstone-sheets, simple channel sandstone-bodies and multistorey channel sandstonebodies. The sandstone-sheets are interpreted as sheetflood deposits. The simple channel sandstone-bodies are interpreted as solitary incised channel deposits. They are dominated by lateral accretion deposits and reflect deposition on point bars in sinuous channels. The multistorey channel sandstone-bodies are interpreted as braided stream deposits. Deposition occurred mainly by channel aggradation. The solitary channels were initiated by differential erosion, whereas the braided channel systems were initiated by differential deposition in sheetfloods.

Acknowledgements The study is part of a Lic. Scient. (Ph.D) thesis and funded by the Danish Ministry for Energy, which is gratefully acknowledged. Thanks are directed to my supervisor, L. Clemmensen, whose many useful comments to early versions improved

220

the manuscript. Reviews of early versions of the manuscript by D.A. Barnes, G. Dam, F. Surlyk and two unknown referees are gratefully acknowledged. The field support by R. Blakey is also greatly appreciated. N. Turner typed the manuscript and J. Lautrup and J. Halskov completed the figures.

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