Dark gray sandstone with planar cross-bedding, cobble conglomerates with clasts partly of a granitoid texture (perhaps syenite), and red claystone or red shale with poor fissility. Sedimentary structures include common raindrop impressions, not so common mudcracks, and load casts. The megaflora in the red beds is dominated by a single seed fern and calamiteans. Lycopods are sphenopteids are not common. Insects (mayflies, brittletails) are rare in the clay drapes on top of the sandstones. Tetrapod footprints are not so rare in the red beds. References would be helpful too. Thank you.
Hello Paul, I was wondering, are these dark gray and red units cyclically repeating? The first thing that popped in my mind based on the color is "banded iron formation".
Hi Abed, Thank you. They are not cyclical, but this is an interesting answer anyway. Paul Lyons.
Hello Paul, do you have any other information about the petrography of both coarse and fine layers (e.g. what provides the dark gray color: oxides, organic carbon, biotite, ...)? Are there any evidences of pedogenesis / are the red clays at the top of a cycle, partially eroded by younger conglomerates/sandstones (e.g. to the latter include clay chips on them) or there are just sharp, planar bedded contacts between them? From the information you gave, I suppose the sedimentation environment may be an alluvial plain system, in a warm, wet climate (tropical?). Laminar cross stratification may be related to meander migration, for instance, whereas clays horizons can be the result of pedogenesis in the swamp around the river banks... but more details are needed to confirm/exclude this hypothesis! References: the assumptions are based on my experience on Late Permian to Early Triassic sediments in the Northern Africa Plate, despite I suppose the conditions I studied were more arid and closer to the sea than in your case.
Hi, Paul. Partly confirming Stefano. These are typical features of an alluvial environment, comprising alluvial plain, and some low?-sloping alluvial fans. You will need to find some cross-stratifications and/or pebble/coble imbrications to find out the direction of paleo-transport and the possible (and needed!) highland source area of the coarse grained material. Many success.
Look after the fossils as well, they can give you a good (wet) paleoclimatic data. And than you can maybe start by some sequence-stratigraphic interpretations (some wells will be useful in the area).
Indeed, thank you Ivan for adding more precise details on the possible environment! Maybe some trace marks in the sandstones may provide additional insights on the paleocurrent directions as well.
By the way (sorry Paul for the off-topic comment) - Ivan I've been at the Štátny Geologický Ústav only a few weeks ago! I've been very impressed by the quality of the research and divulgation your institute is doing! I hope to be back in Bratislava very soon, I like it very much :)
Hello Paul,
I concur with the alluvial setting. In any case, I think that interpretations would go an step further with data about the geometry of the sandstone and conglomerate bodies (tabular or lenticular?), the relative abundance of the sandstone, conglomerate and mudstone (the "clay drapes" you mention mean that mudstones are subordinate?) and the vertical arrangement (though you said there is not any cyclical stacking pattern). Also, the fabric of conglomerates could give insights into the transport and sedimentation mechanisms (fluidal flow deposits, gravity flow deposits?)
To me, a striking point is the absence of root traces and paleosols. I would say that this suggests that high sedimentation rates/channel(?) mobility impeded long term colonization by plants. Maybe this would rather point to an alluvial fan or a braided river system.
Another puzzling point is the monotony of sandstone facies. Just planar cross-bedded units. Maybe a look at Miall´s models of alluvial channels as a first approach would be fine.
Hello Paul,
Sounds like a channel sandstone and floodplain mud association or an alluvial fan association. I am sitting on top of the Newark Supergroup (northern New Jersey) and we see similar conglomerate, sandstone and mudstone facies associations here, including rain drop imprints, mudcracks, and tetrapod tracks. This is an arid system wihout vegetation remains, or root traces.
Hi, Paul, I guess you are in a context of an alluvial environment partly supported by the rain drops impressions and insects in the clay drapes on top of the sandstone. Are these dark grey sandstones ? greywacke in composition. Charles Makoundi, Ph.D Candidate-CODES-UTAS.
hello Paul it seems that everybody agrees that you are dealing with a fluvial environment but the major debate is on the location of this environment i.e. near the mountain side or near the base level. there are some questions about the weather condition I think this will be helpful if you have a more detailed observation to find e.g. weathering evidences and its depth in cobbles if they are sienite so in a wet tropical codition they should be severly weathered unless your sedimentation rate was too high. now if the sedimentatin was too high you should find more evidences for this.what it seems to be clear by your description is that you have great changes in the weather condition(changes from grey shales to red mudstones with raindrop imprints). you have some fossils (insects) that have a short living time so it seems that you are dealing with gradual changing weather condition from more humid to much drier.And the last point is about the source of the sediments especially the cobbles is there any evidence to show you the distance if the distance is not very long so an alluvial fan in a changing weather condition can be plausible especially the distal parts which experiences differenet flow regims in different times of the year.
By the way as a suggestion could you try some geochemical methods to decifer the source and th weather condition.
good luck
Hello Everyone, I appreciate the very thoughtful responses to my question. Yes, I agree it is fluvial. The other aspect not mentioned in my question is that the environment is intermontane with no connection to the sea. The red bed and dark gray sandstone have planar contacts and there is no evidence of paleosols or arid deposits.The abundant raindrop impressions indicate abundant rainfall. Yes, there were dry periods but of short duration. These periods allowed the formation of clay deposits that recorded the presence of insect remains and tetrapod footprints. During heavy rainfall, cobbles from a nearby source hypothesized at a distance of the order of 10 km. To the north, large boulders accumulated right on top of their source in a horst setting. The general drainage was from north to south as hypothesized by another author, and the sandstones took on a red color to the north indicating more upland and more oxidizing conditions. The Triassic-Jurassic Newark Basin is a good analog as suggested by Sandra except there the deposits were of an arid nature. Also, in both basins volcanic deposits accumulated and are testimony to rift basins.
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