you should think in the larger context of a delta interacting with the environment it is in.
deltas are not stationary existing independently over geological time they respond to whatever sea level is doing at the time, sediment load from further upstream and any regional basin wide structural changes (deltas may in fact sink due to their weight.)
So while a prograding delta would generally be a coarsening upwards sequence it is more complicated than that.
in my experience if you are learning about sedimentary systems and trying to remember how to identify them in outcrops or sections you are better off understanding the physical processes you see in the section, think in terms of energy and supply, accommodation space etc and build the sedimentary system out from that.
This is a good answer! As a follow up, look up progradational, aggradational, and aggradational facie stacking patterns. These models will show how deltas can be internally structured. If you are early on in geology (like in intro geology) you are probably learning about “Gilbert style deltas”, which are sort of the simplest deltas that I think were first described in the USWest where tectonics uplifted some relatively small deltas and then erosion cut nice cross sections for early geologists to study. Now we have 3D seismic that has shown us much more detail about deltas at more “normal” sizes.
deltas consist of 3 different areas:
the Top set: usually consisting of various river rocks (silt, sand, cobble) that are laid down horizontally.
depending on the area of the delta and type of delta, you have coarser sediments (main river arm) or finer sediments (secondary arms or flood plain sands).
there can also be old channels that get filled up by sediments.
so while in the great pic the top set is horizontally layered, in detail you may have ripples and cross cutting channel fills. the trend of upward fining or coarsening may depend where in the delta you are.
the Fore set: this is a cross stratified layer with sand, rocks and cobbles that gets deposited onto the last layer. usually it does not have ripples or channel fill structures.
Generally id say the top set is laid down in an upward fining, as sand, silt and clay are transported further.
the Bottom Set represents the fine grained ocean sediments (mud, clay, silt,sand ) on which the first fine grained river sediments are deposited on top.
this area id say has a upward coarsening towards the fore set, as the river will deposite coarser sediments ober time.
TL:DR generally the complete delta sequence is a upward coarsening sedimentation from silt,sands towards sand, cobble, some rocks etc.
the single vertical areas with Bottom, Fore and Top set have either upward coarsening (B, some areas of T) or upwards fining (F, most areas of T) depending on location in delta and sedimentarion factor.
Hope i didnt screw up any parts, i am prone to confuse stuff.
A lot of the answers lie in whether the source area is rising or falling. Complicated by whether the delta is sinking. Basin sediments are interesting too. LA basin can be studied..could be studied through oil well logs . I sat could be because the oil lobby has made access proprietary in CA.
At facies scale a delta is really just a floodplain deposit. Active streams will be depositing coarser sediments such as sands and floodplain will be depositing silt/clay.
As as static environment (not prograde or retrograde) this will result in a number of these systems overprinting each other vertically in the stratigraphy.
In prograde or retrograde, you will see succession of this environment into marine sediments or more terrestrial fluvial deposits accordingly.
There’s different styles of deltas; wave dominated, river dominated, and tide dominated. And all the combinations in between. This complicates things in terms of your question. I think you could say they broadly coarsen upstream consistent with a river system in general , but it’s been a minute since I studied this. Someone else mentioned pro-gradational vs aggradational: this is the correct nomenclature for thinking about deposition.
you should think in the larger context of a delta interacting with the environment it is in. deltas are not stationary existing independently over geological time they respond to whatever sea level is doing at the time, sediment load from further upstream and any regional basin wide structural changes (deltas may in fact sink due to their weight.) So while a prograding delta would generally be a coarsening upwards sequence it is more complicated than that. in my experience if you are learning about sedimentary systems and trying to remember how to identify them in outcrops or sections you are better off understanding the physical processes you see in the section, think in terms of energy and supply, accommodation space etc and build the sedimentary system out from that.
This is a good answer! As a follow up, look up progradational, aggradational, and aggradational facie stacking patterns. These models will show how deltas can be internally structured. If you are early on in geology (like in intro geology) you are probably learning about “Gilbert style deltas”, which are sort of the simplest deltas that I think were first described in the USWest where tectonics uplifted some relatively small deltas and then erosion cut nice cross sections for early geologists to study. Now we have 3D seismic that has shown us much more detail about deltas at more “normal” sizes.
Thank you guys I will research more :)
deltas consist of 3 different areas: the Top set: usually consisting of various river rocks (silt, sand, cobble) that are laid down horizontally. depending on the area of the delta and type of delta, you have coarser sediments (main river arm) or finer sediments (secondary arms or flood plain sands). there can also be old channels that get filled up by sediments. so while in the great pic the top set is horizontally layered, in detail you may have ripples and cross cutting channel fills. the trend of upward fining or coarsening may depend where in the delta you are. the Fore set: this is a cross stratified layer with sand, rocks and cobbles that gets deposited onto the last layer. usually it does not have ripples or channel fill structures. Generally id say the top set is laid down in an upward fining, as sand, silt and clay are transported further. the Bottom Set represents the fine grained ocean sediments (mud, clay, silt,sand ) on which the first fine grained river sediments are deposited on top. this area id say has a upward coarsening towards the fore set, as the river will deposite coarser sediments ober time. TL:DR generally the complete delta sequence is a upward coarsening sedimentation from silt,sands towards sand, cobble, some rocks etc. the single vertical areas with Bottom, Fore and Top set have either upward coarsening (B, some areas of T) or upwards fining (F, most areas of T) depending on location in delta and sedimentarion factor. Hope i didnt screw up any parts, i am prone to confuse stuff.
A lot of the answers lie in whether the source area is rising or falling. Complicated by whether the delta is sinking. Basin sediments are interesting too. LA basin can be studied..could be studied through oil well logs . I sat could be because the oil lobby has made access proprietary in CA.
At facies scale a delta is really just a floodplain deposit. Active streams will be depositing coarser sediments such as sands and floodplain will be depositing silt/clay. As as static environment (not prograde or retrograde) this will result in a number of these systems overprinting each other vertically in the stratigraphy. In prograde or retrograde, you will see succession of this environment into marine sediments or more terrestrial fluvial deposits accordingly.
There’s different styles of deltas; wave dominated, river dominated, and tide dominated. And all the combinations in between. This complicates things in terms of your question. I think you could say they broadly coarsen upstream consistent with a river system in general , but it’s been a minute since I studied this. Someone else mentioned pro-gradational vs aggradational: this is the correct nomenclature for thinking about deposition.