yes it is. The thermocline depth depends on how deep the lake and where the thermocline sets up. This is from lake Whitney last year. The thermocline set up at just about 40' and will go all the way to the bottom when it sets up. Soooo when in water less than 40' there will not be a thermocline. When fishing water deeper than 40' the thermocline will go all the to the bottom. Now the thermocline can sometimes vary from different parts of the lake but it will be pretty close.
Diesel, the lake I fish on does have some 40' + water but I see a very similar pattern even in 25-30 ft water, what could be causing that pattern. Thanks
Each lake has its own personality. Not all lakes will develop a thermocline and the ones that do will vary from one lake to another and from year to year. Rivers will not develop a thermocline due to the water flow. Your lake could very well be setting up a thermocline at 25-30' . It could also be what Clay pointed out above especially if you saw it latter in the day.
The water below the thermocline is considered the dead zone but it is cooler and more sense than the water above it. I have heard from folks that fish will swim in the water to cool off and quickly return to the warmer more oxygenated water. I have never seen this so I really can't say one way or another.