Thanks for starting this thread. I've enjoyed reading it and in particular your layman's questions.
I hope the following will help rather than confuse. I would like you both to go somewhere with me and I'll tell you what I see going on. Use the first picture in post 165 as a reference. Remember, this picture is a snapshot of one moment in time, sometime in the past. Today we have the complete rock sequence of the Dox to look at. I'd like to use this picture to show you how it got there.
Rox, Jar and I are standing in a lagoon just on top of the Shinumo sand. It's pretty muddy but once in a while, because of a storm or heavy rainfall, we see some sand show up. It's coming from that delta we see off landward of us. Problem is the water is getting deeper (or the ground sinking) and our lagoon is slowly moving off to the West(landward,see pix). Our delta's sand and mud deposition can't keep up with the sea level rise so we see it and our lagoon moving off to the West. What this is doing is laying a layer of lagoon mud on top of the Shinumo sand.Where we are the water is getting deeper and deeper. We're going to stay right here,even if we drown, but watch what is happening.(this is the deposition of Unit 1)
Finally sea level stops rising and that delta of ours is starting to prograde, build it's way back towards us. Most of its deposition is close to the delta mouth and stacking up all that mud and sand there creates an unstable condition. Once in a while it slumps and a turbidity current developes which makes it out to us and we are buried in the mud and fine sand. The delta is moving toward us so this is happening more and more. We are being buried by a distal turbidite sequence.(deposition of unit 2)
Holy Molly! This delta won't stop and is getting even closer. We are being buried by more and coarser grained sand from the closer and more powerful proximal turbidites on the delta's depositional slope.(deposition of unit 3)
Don't know about you guys, but I'm getting tired of being beat up by these turbidity currents. The delta is now so close we are on the pro-delta slope. This is cool, except once in a while, because the sediment we are standing on is unstable, it slumps away from under us. It is the beginning of a turbidity curent that will deposit sand and mud seaward of us now, making the sediments of units 2 and 3. (deposition of unit 4)
A note. In the Dox we don't ever quite get to the uppermost and sud-aerially exposed part of this deltaic sequence. The part with the main distributary mouth bars, swamps, marshes, and fluvial (river) deposits. It may exist, but be presently underground where no one can see it, or it may have not been preserved. On the picture, look at the extent of facies unit 4. See it doesn't quite make it to the top part of the delta.
This is a very generalized discription of what occured based on Rox's reference. IT IS NO WAY NEAR GOOD ENOUGH, to say, explore for oil or gas with. I could easily spend a year just figuring out a delta or part of it to be able to predict,where a here to unfound, sand and oil deposit exists.
I hope this explanation has been of some value.
This message has been edited by petrophysics, 04-07-2006 07:19 AM
This message has been edited by petrophysics, 04-07-2006 11:25 AM