Soft Sediment Recumbent Folds in Cross-Bedded Sandstones Orford and Port Arthur, Tasmania, Australia
Roberto
Weinberg |
Soft Sediment Recumbent Folds in Cross-Bedded Sandstones Orford and Port Arthur, Tasmania, Australia
Roberto
Weinberg |
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This page covers a set of observations on sandstones from the Stapleton Beach, 2 km south of Orford, Eastern Tasmania and sandstone blocks used as building stones in Port Arthur's Convict Site. These photographs record the evolution of folding of cross-beds in the sandstones to form a series of isolated, recumbent, isoclinal folds due to slumping. In these rocks there is no evidence for relative vertical movement of sediments induced by gravity inversion. This contrasts with the soft-sediment deformation in Kangaroo Island where there is considerable movement induced by gravity due to density difference. |
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| Figure 1a) Cross-bedded strata at Stapleton Beach (block). | Figure 1b) Cross-bedded strata at Stapleton Beach (block). |
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| Figure 1c) Port Arthur's building stone, top arch of hospital entrance. Cross-bedded sandstone showing younging upwards and water flow generally to the right. |
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| Figure 2a) Cross-bedded strata, two layers of which with incipient folding towards the basin (same direction of flow that deposited the strata- to the left). The top folded layer has caused layer thickening, and thinning of the layer above indicating syn- depositional layer slumping. | Figure 2b) Three layers with different stages of fold development. Bottom layer: fully developed recumbent fold with closure to the left. Middle layer: asymmetric fold pair with closure well-developed both to the left and right. Notice the truncated top. Top layer: incipient folding of cross beds. |
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| Figure 2c) Three layers showing different stages of fold development. Bottom layer: fully developed recumbent fold closing to the left, in the direction of flow. Top two layers detailed in Fig. 2d. | Figure 2d) Details of (c) Bottom layer, incipient asymmetric folding of cross beds in the direction of flow. Top layer, asymmetric folds formed by slump in the direction of flow. |
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| Figure 2e) | Figure 2f) . |
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| Figure 2g) Building stone on facade wall of Port Arthur's hospital. Bottom layer shows a fully developed isoclinal, recumbent fold with fold closure against the direction of flow indicated by the upper two cross-bedded layers (flow to the left). | Figure 2h) Building stone on facade wall of Port Arthur's hospital. Several stages of development of syn-sedimenatary recumbent folds. Younging in this block is downwards as indicated by well-preserved cross-beds. Folds amplify to the left, and do not form a fold pair, but a single fold-closure to the left, against the direction of water flow during sedimentation of all layers (to the right). One of the layers become fully disaggregated by the slump that produced the recumbent folds becoming a homogenized black mass towards the lower left of the photograph with some mixing with the top of the layer below at the time of sedimentation (layer above in the photograph, marked with a white arrow) |
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| Figure 3a) Four recumbent isoclinal fold closures to the left in different cross-bedded layers. Unfolded, think layers in between folds indicate, flow to the right. | Figure 3b) Three recumbent isoclinal fold closures in different layers. |
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| Figure 3c) Two isoclinal, recumbent folds, separated by a massive sandstone layer. | Figure 3d) CHANGE TO LATER Sedimentary layering truncated by erosion and renewed deposition of cross-bedded sandstone. |
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| Figure 3e) Cross-bedded layer folded and truncated by another layer with an isoclinal, recumbent fold. Both folds closures are to the right. The asymptoptic curvature of the cross-bedded layers indicates younging upwards and water flow to the left. Building stone in Port Arthur's hospital facade. | Figure 3f) Same as (e) possibly even same fold in a different block. Building stone on bottom floor wall of Port Arthur's hospital. |