Primary (sedimentary) structures in Kangaroo Island, South Australia

 

Coastal outcrops on the north coast of Kangaroo Island expose numerous examples of soft sediment and primary structures, such as slump, diapirism, dewatering, ripples, cross-bedding, erosional channels and general loss of layering. These features occur in all three main rock sequences investigated here: the Kanmantoo Group, and the Mt McDonnel and Stokes Bay Formations of the Kangaroo Island Group. All photographs were taken at high angle to bedding.

 

Slump structures

Diapirs

Dewatering structures

Primary structures

Bioturbation

Ripples

Erosion and Channels

Cross bedding

Shortened boudins at Harvey's Return

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Part I. Soft Sediment Structures
Slump

Figure 1A: Slump, almost rootless pair of recumbent, intralayer, isoclinal folds. Stokes Bay Fm at NE side of Snelling's Beach.	Figure 1B: Slump, chaotic folds. Kanmantoo Fm, Morrisons Point.
Figure 1C: Dismembered sandstone layer by slump, forming ellipsoidal blocks. Mt McDonnel Fm NE of Stokes Bay, NE of Stokes Bay.	Figure 1D: A complexly folded ellipsoid of sandstone isolated inside mudstone.is part of the dismembered layer on the left. Mt McDonnel Fm NE of Stokes Bay.

 

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Diapir

Figure 2A. Quartz-rich sand layer rising into a clay-rich layer.	Figure 2A. Complex diapiric structure between denser clay-rich sandstone and more pure layers.
Figure 2C. Complex instability between mud rich and clean sandstone.	Figure 2D. Intrafolial diapirism in Stokes Bay Fm.

Figure 2E Diapirs of pink, clean sandstone into a greenish denser sandston, Stokes Bay Fm.

 

 

 

 

 

Figure 2F. Downgoing diapirs of biotite-rich layers, developed from an initially cross-bedded layer as indicated by truncation of layers on the top left hand-side.	Figure 2G. Diapir detail. Kanmantoo Gp at Harvey's Return.
Figure 2H. Downgoing diapirs of biotite-rich layers. Kanmantoo Gp at Harvey's Return.	Figure 2I. Diapir. Kanmantoo Gp at Morrisons Point.
Figure 2J. Downgoing diapirs of biotite-rich layers. Kanmantoo Gp at Harvey's Return.	Figure 2K. Diapir. Kanmantoo Gp at Morrisons Point.
Figure 2L. Blob of mud-rich sediments with complex internal folding, inside sandstone. Interpretation: negative diapir that sunk through liquefied sand. NE Snellings Beach: Stokes Bay Fm.	Figure 2M. Blob of mud-rich sediments with complex internal folding, inside sandstone. Note layering in sandstone wraps around the mudstone blob. Interpretation: negative diapir that sunk through liquefied sand. NE Snellings Beach: Stokes Bay Fm.
Figure 2N. Doming of massive orange sandy layer below a pinkish weakly laminated layer. Wavelength measured from dome top to dome top is ~ 2m. SE Stokes Bay: Stokes Bay Fm.	Figure 2O. Same coastal outcrop as Fig. 2N, 2O metres away, dome with an amplitude of more than 1m. SE Stokes Bay: Stokes Bay Fm.
Figure 2P. Doming of massive orange sandy layer below a pinkish weakly laminated layer. Two walls a right angles. SE Stokes Bay: Stokes Bay Fm.	Figure 2Q. Detail of Fig. 2P. Pink laminated overburden is folded wth layering nealy vertical at the margin of dome and layering is destroyed upwards. Note camera lens for scale in the upper left. SE Stokes Bay: Stokes Bay Fm.

Figure 2R Elliptical, sheath, sunk diapir of dirty sand into clear sand. Stokes Bay Fm.

 

 

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Dewatering Structures

 

Figure 3A. Dewatering structures. Kanmantoo Gp at Harvey's Return.	Figure 3B. Dewatering structures. Kanmantoo Gp at Harvey's Return
Figure 3C. Dewatering structure: a cuspate upright fold, truncated by overlying layers, indicat ing younging upwards in the direction of the upward flow of liquefied sandstone. Stokes Bay Fm.	Figure 3D. Same features south of Stokes Bay. Stokes Bay Fm.
Figure 3E. Dewatering structure in deeper orange and lacking bedding, cutting across disrupting and tilting slightly, layered sandstone. Stokes Bay Fm.	Figure 3F. Dewatering structure showing loss of layering and associated with two recumbent isoclinal folds on the left hand side. These folds are interpreted as syn-sedimentary and indicative of upward drag by the upwards flowing liquefied layer. Notice that at the base of the photo , sandstone layers are continuous. Stokes Bay Fm.

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Part II. Primary Structures.

 

 

 

 

Figure 4A. Bioturbation? Stokes Bay Fm.	Figure 4B. Ripples on surface. McDonnel Fm, NE of Stokes Bay Beach.
Figure 4C. Massive greenish sandstone in erosional contact with layered pink sandstone. Sediment-filled scar. Note on top part of the photo, upward pointing cuspate folds indicative of dewatering. Stokes Bay Fm.	Figure 4D. Erosional unconformity between layered sandstones, Kanmantoo Fm. Harvey's Return.
Figure 4E. Erosional channel. Kanmantoo Fm. Harvey's Return.	Figure 4F. Kanmantoo Fm. Harvey's Return.
Figure 4G. Cross bedding. Stokes Bay Fm.	Figure 4H. Cross bedding. Stokes Bay Fm.
Figure 4I. Cross bedding. Disruption of bedding in the top layer. Stokes Bay Fm.	Figure 4J. Cross bedding, subsequently folded. Stokes Bay Fm.

 

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Part III. Deformed boudins from Harvey's Return.

 

 

 

Figure 5A. Boudins developed in massive, clear sandstone. Vertical outcrop, note quartz veins on the upper part and thicken upwards.	Figure 5B. Boudins in massive sandstone. Vertical outcrop, note quartz veins on the upper part and thicken upwards.
Figure 5C. Boudins in massive sandstone. Vertical outcrop. Kanmantoo Fm. Harvey's Return.	Figure 5D. Lens of massive sandstone, initially boudinaged, with quartz veins on boudin necks, and later folded. Kanmantoo Fm. Harvey's Return.
Figure 5E. Boudins: note quartz vein are more numerous and thicker upwards. Note also erosional unconformity on sand beds on lower part. Vertical outcrop, Kanmantoo Fm. Harvey's Return.	Figure 5F. Boudins tightened by subsequent shortening. Kanmantoo Fm. Harvey's Return.
Figure 5G. Cross bedding truncation involved in boudin neck with quartz vein. Kanmantoo Fm. Harvey's Return.	Figure 5H. Complex features associated with boudinage, in block. Kanmantoo Fm. Harvey's Return.

 

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