Facies

Lithologic description

Sedimentary structures

Ichnological/fossil content

Interpretation

F-1: Organic

rich siltstone

・ Dark to black colored organic-rich, dolomitic siltstone.

・ The grains are subangular to subrounded, and well sorted.

・ Mineralology comprises quartz, dolomite, mica, and clay.

・ Preserved sedimentary structures appears as horizontal to wavy lamination.

・ The thickness of individual fine-silt lamina within F-1 is approximately 2 to 3 mm.

・ The basal contact between lithofacies F-1 and lithofacies F-2 is abrupt.

・ Trace fossils and bioturbation are not present in lithofacies F-1.

・ There are no body fossils preserved in F-1.

・ The dark colored organic rich content of lithofacies F-1 is related to high nutrient rich sediment source and rapid sedimentation, which enhanced the preservation of organic matter in oxygen-depleted depositional environment. The organic carbon richness of F-1 makes it a potential hydrocarbon source rock.

・ Mechanism of deposition of silt and very fine-grained sediment of F-1 is related to mobilization of fines across shelf or transported through processes, which may include sediments deposition from suspension or hyperpycnal plumes. Suspended silty-clay sediments are transported primarily as flocculated material.

・ Conclusion: Distal offshore (outer shelf) depositional environment.

F-2: Very fine-grained

sandstone and siltstone

・ F-2 is composed of interbedded, laminated, very fine-grained silty sandstone.

・ The ratio of sand to silt in F-2 is ~3:1 in some successions and ~2:1 ratio in some beds.

・ Petrographic examination of F-2 further shows that the grains sorting are variable, from moderately well sorted to poorly sorted, and texturally matured, but mineralogically submature as evidenced by the present of feldspar and clay.

・ Petrographic observation of mineralogy of F-2 lithofacies shows evidence of “clot” pyrite that replaces organic matter due to dissolution of kerogen during transformation of organic matter.

・ F-2 has well-preserved plane parallel laminae.

・ The sand laminae exhibit fractures along sandstone bedding planes.

・ F-2 has well preserved dewatering structures, soft sediment deformation structures, convolute bedding/lamination, and small-scale current ripples.

・ The current ripples are dominantly straight-crested, and rarely bifurcating.

・ The basal contact of lithofacies F-2 with lithofacies F-3 is sharp, and abruptly ungradational.

・ Trace fossils and bioturbation are not present in lithofacies F-2.

・ There are no body fossils preserved in F-2.

・ Fugichnia are present as evidence of episodic, event, or rapid deposition in F-2.

・ The present of preserved interbedded sandy-siltstone, and thinly laminated layers (<10 mm) in F-2 reflect variation in sediment texture.

・ Laminae form due to transcurrent migration of grains due to low relief in topography, and bed waves.

・ The absence of sedimentary structures characteristic of upper shoreface environment, such as hummocky cross stratification, wave or oscillatory ripples, infer quiescent depositional mechanism.

・ The present of current ripples in F-2 probably correspond to waning flow conditions.

・ Soft sediment deformation structures in F-2 are due to a deformational event at or near the contemporary surface of unconsolidated sediments either before, or soon after burial. The silty grains in F-2, in part suggest input from aeolian sediment source.

・ Pyrite in F-2 is interpreted as post-depositional emplacement of organic matter due to diagenesis.

・ Conclusion: Lower shoreface through proximal offshore depositional environment.

F-3A: Bioturbated

very fine-grained

sandy-siltstone

composed of

Skolithos ichnofacies

・ Very fine-grained sandy-siltstone.

・ Thin-section shows that the grain sizes are moderately well sorted.

・ Vuggy porosity resulting from bioturbation are present.

・ F-3A has preserved wavy to parallel lamination (~10 mm to 15 mm in thickness), wave ripples, and escape structures such as fugichnia indicates rapid, episodic, or event deposition.

・ The trace fossils in F-3A comprises Planolites, Phycosiphon, Cruziana, Skolithos, Teichichnus, and Palaeophycus.

・ There are no body fossils pre-served in F-3A.

・ F-3A is characteristic of a diverse but low abundance of ichnological assemblage, typical of the Skolithos ichnofacies.

・ The Skolithos ichnofacies is indicative of relatively high energy environment such as the shoreface, and they are associated with wave, current energy and typically developed in slightly muddy to clean, moderately sorted sediments.

・ The Skolithos ichnofacies can also develop in a loose or shifting particulate (sand-prone) substrates in marine environments. The present of escape traces in F-3A correspond to disturbances made by organisms entrained in, or buried by the event bed during sedimentation.

・ Conclusion: Lower shoreface depositional environment.