Ok. First of all, we often do find that the surface below an unconformity is uneven, with the next layer filling in the hollows and then extending over the top. This is called ‘buried relief’ and occurs at many scales, from centimeters to hundreds of meters. It does indicate erosion of the lower unit before the deposition of the upper one. Such erosion can be subaerial but can also take place on the sea bed, although usually it will then be of smaller lateral extent.
A year or two ago I visited a site in South Wales where you could see this right in close up:
This is evidence of exposure and erosion of the lower unit (Carboniferous Limestone), followed by deposition of the upper unit, in this case Triassic (?) conglomerates. These particular conglomerates are more likely terrestrial deposits (alluvial fans or flash flood mass flows) than submarine sediments, but the principle is the same.
Now, this doesn’t always happen. As you point out, very often the layers are deposited quite parallel to each other. It is important to realise that sedimentary layers are like individual frames of a film. There is layering because sedimentation is often interrupted for some duration, that can come with subtle changes in the influx of sediment (changes in source area for instance, or simply changes in grain size or composition, climatic influence perhaps). Such breaks result in the bedding planes we see. In a way, each bedding plane can be a mini-unconformity of limited duration.
Now, it is very important to realise that unconformities and breaks can form without the need for exposure. Over time, influx varies for numerous reasons, and also there are constant variations in sea level that mean that an area finds itself at times in deeper or shallower water, and further away or closer to the coast. All of these variations will leave their imprint on the eventual rocks and can cause breaks in the sedimentation that manifest themselves as bedding planes or overall changes in the character of the rocks.
Some areas can be stable over very long periods of time. In such conditions we don’t expect much in the way of visible deformation (folds and/or faults).
So although we often do see buried topography at unconformities, in many cases we do not. All these things are factors of the ever present and ever-variable interplay of movements of the earth’s crust, sea level, climate and sediment source areas.
Sediment source areas will vary in size. Just like today, where we have land masses ranging from tiny islands to enormous continents. Look at a map of the USA and draw the drainage area of the Mississippi. That is one enormous sediment source area, most of which is going to end up in the Gulf of Mexico. There have been loads of wells drilled in the GoM, and masses of seismic data has been gathered.
Look at this one:
Here you see a seismic line through some part of the Gulf of Mexico. The top half consists of generally flat-lying, parallel sedimentary layers, just like what you see in your geological model above. This entire succession was laid down under the sea, without uplift and exposure. Yet you can clearly see that the layers are discrete and individual, different from each other, giving rise to the stripiness. Changes over time as I discussed above are the cause of this.
We know all this because there are hundreds of wells drilled in the Gulf, and samples are taken throughout these layers and analysed. They are full of marine microfossils (that show systematic vertical changes in their assemblages, by the way)
So, here we have parallel layering in a pile of sediments all laid down in the sea. No buried topography here, nor would we expect any.
Ha, this is new to me, I hadn’t seen these before (I haven’t really done much work inland USA). Looks intriguing. I will need to read up on them before I can comment.