I will bet all of my bitcoins that it is a JPEG artifact. In fact, I just got my kit lens and camera and took a shot that shows the JPEG artifact - but only when I create it myself in processing. My shot is with the K-7, 12 bit RAW, kit lens at the same focal length as the OP, f11, 1/180 because my camera is set for half stops, and ISO 100 because my sky is brighter. I imported the shot into Lightroom 4 but all settings are zero or neutral: WB uses the camera AWB setting, no exposure changes, lens corrections or sharpening. Then I used Photoshop CS5 to reduce the size to 1000x664. No other processing was done. If I save that file as a JPEG with quality level 12, the highest, I get this:
If I follow the same exact steps as above, but save as quality level 4, I get this:
That's not from repeated saving of a JPEG either, because I'm saving a DNG.
Remember that your shot is in RAW, but to see an image on a screen, that data has to be interpreted by software. Some software uses the preview image in the RAW file, because it's easy. Some software doesn't know enough about your camera. You can't just say "I shot in RAW and here is my terrible image" without checking all those software steps in the middle.