Non deposition or erosion ?

In Bachelor’s Geology class,? when my Professor of Structural Geology , in a Central University of ?India? was teaching about “Unconformities” , I wondered, “sounds good, but where will be the usage ?”. At that time I never knew, I would be working in the Petroleum industry and that my professional life would be spent on unconformities, chasing one,? looking for one, defining one on the data and staring at them on the outcrops.

?As understood, in simple words, Unconformity is a surface of erosion or non deposition recorded on the rocks ?and that a time gap is implied before the next formation of rock was deposited. Missing record of deposited rocks and no deposition at all implied respectively.

?Looking back, while defining sequences on seismic, the endeavour was always to look for a surface which is erosive , usually in basin margins ?with equivalent correlative conformities inside the basin/ subsurface, and that it took two unconformities/ correlative conformities to? define a sequence. We now know, that the tie up of the unconformity with the equivalent correlative conformity is not simple and what we think ( induced in my head through articles on sequence stratigraphy). The same was also translated on well logs. Sometimes the biostrat data came to the rescue. The emphasis was always on “erosion” and never on “ non deposition”. The point is, how does one define non deposition in the subsurface?

?While looking for erosion, basin margins were always a favourite spot, also advocated and emphasized by the academia through articles on “Sequence Stratigraphy” . Sub aerial unconformities, yes but what about subaqueous ones? Is it possible to have a surface or non deposition in the shallow , marine shelf?? What mattered is, besides the basin margin, the basin centre could be a site of erosion with axial drainage and the lateral drainage with erosive channels being very strong through alluvial fans, fan delta(?) in rift basins/ lateral and axial channels in the subduction zones in convergent margins. Leaving out the fluvial channels and their cut, the deep water clastics deposition, particularly on an exposed shelf? due to sea level fall, always brought out the extremely erosive channels? with IVF’s( incised valley fills) on the shelf and strongly erosive channels on the continental slope and even to much distance on the basin floor. Shelf collapses on the slopes, emphasizing the debris flow runs included. Remembering the sea level curves and great falls of sea level near the? Early Oligocene- Late Oligocene boundary and Upper Miocene-Mid Miocene boundary from published sources.

?In all this matrix of thought, non deposition never came to my mind. Paleosols and paleo karsts on outcrops imply non deposition as understood. So does non conformities. Paleosols imply lengthy episodes of landscape stability which may mark a stratigraphic diastem or unconformity.

?Giving my thoughts a spin, non deposition can certainly happen , but is the entire time gap there, having no erosion? What I mean is, it could be loess ( again eroded and derived from somewhere and deposited elsewhere/ allochthonous) or in situ soil formation disaggregated from the underlying rocks .Or , is it a case of long durations of non deposition punctuated by small periods of erosion ( wind, weather, water, whatever)? Or is it that theoretically true, on a practical plane to get one is difficult ? Or is it that non deposition is possible ?to define on a present day topography or looking at ancient paleosols only from outcrops?

?In all this thinking , and contemplating, I came across the term “ blended unconformity”. But then, that is another story! Ha ha ha!

Please give your thoughts a free flow.

?Bouquets and brickbats , openly accepted.